Table of Contents
- Diminishing Returns of Insulation
- Return on Investment
- Performance of Thermal Envelopes
- Case Study
- How can I understand the law and principle of diminishing returns?
- What are the primary diminishing returns issues in building envelopes?
- What misconceptions persist about wall/insulation thickness and upgrades?
- What misconceptions persist about utility penetrations in the thermal envelope?
- What misconceptions persist about mismatched elements in the thermal envelope?
- What misconceptions persist about building envelope tightness and indoor air quality?
- What misconceptions persist about passive solar, in light of weak links and net benefits?
- What are the implications of a full understanding of diminishing returns of building envelopes?
Even though knowledge is the only instrument of production that is not subject to diminishing returns, the human mind too often ignores the concept in assumptions about scale. This deficiency leads to poor choices in many life circumstances, and it leads to notably egregious outcomes in the design and construction of buildings. Understanding the two related concepts of marginal analysis and opportunity costs would help grasp and effectively utilize the law of diminishing returns, but these are also too often ignored or misunderstood. It is impossible to evaluate diminishing returns without utilizing marginal analysis to calculate per-unit costs and benefits, at the margin, while scaling up. When applied to the building envelope, people embrace the logic that more is better; a heavier structure is stronger, and more insulation will reduce heat loss and energy use. Unfortunately, we too often fail to compare diminishing benefits against the cost of additional units, even when they approach negligible or negative benefit. Without marginal analysis, homeowners and designers keep bulking-up the building envelope until budget runs out, making no reasoned calculation of whether each additional unit to the envelope was worth adding.
If the concern is for long-term structural integrity, it would be very difficult to quantify with precision the additional benefit for each step up in structural heft. Fortunately, we do not need to consider those complexities, because building codes in the U.S. have evolved over time with practice, cases, and research to now mandate a minimum inspected standard that achieves indefinite life. As we outlined in Chapter 5, and supported with objective research and data, the code-compliant house is expected to serve an indefinite life, if constructed with quality and maintained effectively. Whether homeowners are concerned about outliving their homes, passing a valuable asset to their heirs, or assuring good stewardship in the use of all the materials and energy that went into constructing the home, they may rest assured that the code-compliant house structure in the U.S. meets or exceeds those objectives. An important exception is for homes that are located in areas of high risk to natural disasters, such as hurricane, tornado, or earthquake. In those regions, local building codes may still be evolving with what seems like new weather patterns and more fierce and frequent storms. For most Americans, however, the typical standard construction of 2×4 wood stud walls is adequate for long-term structural integrity, and adding further structure will be more costly (in dollars and embodied energy) than beneficial (in longevity); it will also quickly escalate costs that are unlikely to be appraised at constructed cost and included in financing. Given the realities of consumer demand, an upgraded structure is more likely to result in a real (inflation-adjusted) financial loss at resale, or a lower return on investment compared with a standard structure. In other words, there are decreasing returns to scale, and building codes mandate a minimum structure along the scaling-up arc that already exceeds a cost-benefit optimum.
Diminishing Returns of Insulation
If the concern about the building envelope is reducing heat loss and energy use, the following paired graphics are instructive on the diminishing returns of insulation. The image on the left is the effectiveness profile of insulation in resisting heat transfer, or heat loss. The image on the right takes the data from the performance profile and shows more intuitively how insulation resists heat transfer/loss across the full thickness of material. The right-side image could be considered a cross-section of insulation in a wall cavity during heating season, with the warmth of interior air from the left being lost or transfered through the insulation to cooler outside temperatures to the right.
The second issue that makes the concept of diminishing returns challenging to grasp and utilize with rationality is opportunity costs, defined as the next best alternative forgone. Most people understand this concept intuitively and utilize it effectively–if subconsciously–for small purchases with short durable use. If I really want a burger and fries and soda, but I only want to spend $2, I know that I can’t have all three, so I prioritize and choose one. The opportunity cost of choosing a burger is the benefit I would have derived from the fries and soda I didn’t choose. For larger purchases, and especially those that have long durable use, the human mind struggles to organize and understand this concept of opportunity costs. Building a house and investing in solar PV both fit that description; both are big purchases with expected long lives. A failure to consider opportunity costs on purchases this big often leads to suboptimal choices for the individual, and outcomes for society, and we discuss these later in this chapter. But where it connects to diminishing returns and marginal analysis is more subtle. We already established that increasing both structure and insulation have diminishing returns. The first unit returns the most benefit; this is sometimes referred to as the biggest bang for the buck. But as each additional unit is added, even when marginal benefits exceed marginal costs, the bang for the buck ratio declines. Assuming that there are many other competing interests for available dollars or budget, diminishing returns should be changing opportunity cost calculations even when returns are still positive, yet evidence of choices in the residential building industry suggest they are not. This becomes even more perplexing when we look at the poor economic returns for nearly all building envelope upgrades beyond code compliance.
Return on Investment
The Department of Energy reports that the average U.S. household spends $1,945 annually on energy, from all uses (DOE, 2018a), and that 48% on average is used for conditioning indoor air (DOE, 2018b). Consequently, the average American household spends approximately $934 annually for heating, air conditioning and ventilation (HVAC). If building envelope upgrades are selected primarily to reduce heat loss, and reduce energy bills, those savings would come from reduced need for–and operation of–HVAC equipment. If it were possible to positively match each building envelope upgrade to the reduced energy use commensurate with that upgrade, it would be a simple process to calculate the financial return on investment (ROI) and determine whether that upgrade should be added for economic reasons. However, since direct matching of these elements is impossible because of the wide variability of factors, another way to consider ROI is to build hypothetical scenarios. The following chart displays that data for four levels of upgrade-savings possibilities. This analysis uses the energy data for the average American household and works in reverse to identify the largest expenditure for an upgrade to break even with commensurate energy/cost savings over 30 years.
Financial Model Assumptions (no inclusion of environmental cost of energy production):
Average annual American household cost of energy for HVAC operations ($934)
Cost of funds: 4.5%, proxy rate for 30-year mortgage (higher COFs, lower max. cost)
Rate of energy inflation 3.0%: conservative annual escalator, given historical trends
30-year period for break-even ROI: common mortgage length and long-term analysis
What this analysis demonstrates, for the first scenario, is that if a particular envelope upgrade successfully achieved 5% reduction in energy need and cost from reduced HVAC use ($47), the maximum initial investment, for that upgrade to simply break even over 30 years, is $1,050. That is a very small allowance against the cost of almost any envelope upgrade. The other end of this scale is even more enlightening. Even if it were possible to reduce HVAC use and costs by 50%, which we will show is an unlikely achievement in any case, the maximum initial investment in upgrades to attain that reduction, and simply break even over 30 years, is $10,500. In short, $10,500 in upgrades will never come close to reducing HVAC energy use and cost by 50%; it is a miniscule amount compared to the cost of upgraded wall systems, insulation, windows, and doors. Stated differently, $10,500 will not purchase much in thermal envelope upgrades, and it certainly will not buy enough to cut HVAC costs in half. This analysis is based on the average American household, but we find that it scales quite well in either direction from the mean, both in envelope size and in the energy behaviors of occupants. While we do not have data for cases that would represent extreme outliers, such as for mansions or tiny homes, the physical limits and current economics of the analysis suggest similar conclusions even at those extremes.
Accounting for Environmental Externalities
Environmentalists may chafe at these findings, at least in part because they fail to account for the environmental externalities of energy production from fossil fuel sources. Doing so for all possible scenarios would add nearly infinite variability, but the final verdict remains the same. As an example, the environmental externality of electricity produced in the SERC Virginia/Carolina region would add $0.0378/kWh to the cost of grid-distributed energy, at $80 per metric ton of CO2e. This is based on the actual fuel mix for the region, which is close to the national portfolio, and converting nitrogen oxides and carbon dioxide emissions to CO2-equivalent (CO2e). Internalizing the externality would add a 37.8% cost premium to regional grid rates, which we can add to HVAC costs for a comparative analysis.
- Environmental externality included (37.8%); represents $80 per metric ton for CO2e
- Average annual American household cost of energy for HVAC, with CO2e ($1,287)
- Cost of funds: 4.5%, proxy rate for 30-year mortgage (higher COFs, lower max. cost)
- Rate of energy inflation: 3.0%, conservative annual escalator, given historical trends
- 30-year period for break-even analysis, common mortgage length and long-term anal.
A couple of broad conclusions can quickly be drawn from this comparative analysis. First, even at these higher allowances for thermal envelope upgrades, the dollar amounts still pale in comparison to most upgrade materials and systems. More detail is offered in the Case Study section of this chapter, but for a sense of magnitude, envelope upgrades to the case house cost in excess of $100,000 and didn’t come anywhere close to offsetting half of the HVAC energy needs and cost. Second, and for wholistic perspective, environmentalists should also be concerned about the increased embodied energy sunk into upgrades to the thermal envelope; the case analysis at the end of this chapter will discuss that more thoroughly. And finally, it has already been established that solar PV is the better choice for household energy; a strong financial investment even if we do not consider its enormous environmental benefits, and the embodied energy of a PV system pales in comparison to any envelope upgrade that has meaningful impact on reducing energy need and cost.
The charts above provide analysis for just four specific scenarios of reduced HVAC need and cost, with the highest reduction rate of 50%. Readers may be wondering, like we did, how different thermal envelopes actually perform in reducing household energy needs. The industry literature is crowded with theoretical claims of energy savings from specific materials, methods, and wall systems, but we were able to find very little evidence-based impact from whole-house lived experience. Part of this may be due to the complexity and integration of many component parts of both the thermal envelope and HVAC systems. For example, there are many different types of structures, windows, and doors, and houses have a wide range of permutations in the proportional coverage of each of those elements. Weather variations and broad choices of HVAC system-type and efficiency rating add further permutations, and that is all before there is any account for number of inhabitants and their unique energy behaviors. Perhaps because of these challenges at the whole-house level, manufacturers and policy interests have focused on lab-based research of component parts. While we did not question those research findings, we found it an impossible task to piece it all together with any level of confidence or magnitude.
Performance of Thermal Envelopes
Because our research team included seasoned industry professionals with years of experience in residential design and building, we realized that we could build a sizeable database of existing homes of various building envelope system and HVAC type. If we could also obtain energy use data and a measure of conditioned space to proxy HVAC energy needs/use, the analysis would provide a rough sense of impact by system, and help answer the question of magnitude of impact. There were many factors that we could not control for, a challenge indicative of this task, but at least with all the households in relative close proximity, this analysis would control for weather variation and energy rates (all uses). Knowing that we also could not statistically isolate individual systems and impacts with precision, we expected that the blunt measures we would obtain would still provide a sense of magnitude of impact of building envelope upgrades.
As the raw data accumulated, most individual cases fell into three wall system types, 2×4 wood stud (code base), 2×6 wood stud (common mid-range option), and insulated concrete forms (ICF; premium system). Since ICF is often considered the most robust and premium building envelope type, these three classifications would provide information on the purported best system (ICF), the most basic application of building code (2×4), and a mid-range system between (2×6). The results of our analysis were shocking. Not only did the purported best thermal envelope fail to deliver energy savings, households in ICF structures averaged more energy use per square foot of living space than either of the two lesser structures.
- Data includes averages across wall system classification of more than 40 cases
- The differences in energy costs are not statistically significant (small sample size)
- Some data screening to account for anomalies such as well pump and EV charging
- Volume of conditioned space rather than square foot would provide a more direct link between HVAC energy use and building envelope type; however, volume data were difficult to obtain in retrospect, and for the smaller sample for which we had volume measures, it did not appreciably alter overall placement or magnitude outcomes.
We need to be clear that our analysis is not a precise and direct measure of the impact of thermal envelope system on HVAC energy needs and use. However, across averages of over 40 cases, one would expect less overall energy use in premium envelope structures due to reduced HVAC need and operations; that is why environmentally-conscious homeowners accept an enormous cost premium in construction. In the region of our study and case home, where there is little known threat of catastrophic structural damage from natural disasters, why would homeowners pay such a high premium in construction cost or purchase price if not for an expected reduction in energy use and cost, or reduced environmental impact? All of the ICF homeowners in our sample selected the more expensive wall system because they thought it was the more responsible environmental choice. Lab-based research and component-specific benefits of upgraded wall systems paint a convincing story in the industry, even as advocates struggle to claim overall magnitude of impact, and almost no one is utilizing marginal analysis, opportunity costs, diminishing returns, and cost of funds to an overall value assessment.
Controlling for Variables
The complexity of integrated systems and plethora of permutations is noted above as one reason for a lack of whole-house, whole-systems impact analysis of building envelopes types. Another reason is the human element. Two same-sized families living in identical side-by-side homes will not demand the same amount of energy. Individual and collective interests and behaviors can mean that two families could have vastly different energy demands. Skeptics could argue that homeowners of the ICF structures in our sample must be more wealthy to be able to afford the much more expensive construction or purchase price, and maybe that explains a more voracious use of energy in the home. On the other hand, each of the families in our sample that chose to build or buy an ICF home did so for supposed environmental responsibility, and one could argue that would show up in thrift for energy in all areas of life. This lack of a perfect control group mechanism blurs and confuses envelope system impact assessments; fortunately, our case project would present a revealing comparison both on impact and on cost comparison.
Reducing environmental impact was the preeminent priority in design of the case home and selection of building materials and systems; every element was considered through that filter. Careful and long-term study of the literature in the building industry, best practices, and research led our team to a premium thermal envelope, and insulated concrete form (ICF) walls specifically. The homeowner had been living in a new house with many similar characteristics, except that the thermal envelope was code-minimum. The square footage and conditioned volume were nearly the same, both had partially submerged (walk-out) basements, and they are located in the same neighborhood, which controlled for weather. Perhaps most notably, the inhabitants would move from the code-minimum house to the premium thermal envelope house with the same people and energy patterns and behaviors. In addition to the premium envelope, which would be expected to reduce operational energy, the new case home was equipped with a geothermal heat pump and more energy-efficient appliances. The code-minimum, home utilized a basic 13 SEER air-to-air heat pump and standard appliances. Our research team expected reduced energy use in the new home, but we were left with the question of overall magnitude.
The case home would have a limited period where the human conditions would be the same as in the code-compliant house. Fortunately, the six month timeframe would span across cooling, swing, and heating seasons, and we compared month-to-month to control for temperature and other weather variability. By the time monthly energy data began arriving for the new home, our research had already introduced a measure of skepticism on the energy efficacy of a premium thermal envelope. Still, we were surprised to discover that the case home had used nearly 10% more energy than the code-minimum home through the first six months of operation. We cross-checked energy use with average monthly temperatures across the two successive years to ensure that weather variation was not so extreme as to skew operational demand. The side-by-side data is provided below:
- Electricity is the only energy source in both homes
- Thermostats set to same readings for both cooling and heating seasons
- People and use patterns intentionally kept the same through comparison period
Average monthly temperatures: mean of means, from Weather Underground URL
This two-home comparison provided one of the best available cases of control group (same people and patterns) testing, and this study was independent of the broader dataset of regional homes categorized by thermal envelope type. The results and findings of both studies were conclusive; that premium thermal envelope homes (at least those represented by ICF wall systems) do not in reality result in lower operational energy demand and use. How could this be? The following chart highlights select features of the two homes that should have impact on overall operational energy use.
While the list of features in the chart above is not meant to be all-inclusive, the first four are known to be significant factors that in isolation should translate to lower energy use in the case home. The fifth feature, mechanical ventilation with an ERV, would clearly tip operational energy use in the opposing direction, but we did not encounter in the research much energy analysis on that element and its trade-offs. Further, we expected the energy impact of an ERV to be more than offset by the combination of the other factors. Among the data we collected on regional homes and energy use, we noted that all of the ICF homes in the sample employ mechanical ventilation except one; that one ICF (non-ERV) exception was among the best performing on operational energy use, but there were also wood-framed homes in a similar performance range. Could the use of an ERV wipe out operational energy savings from a premium thermal envelope? We will return to this question in the energy analysis chapter, but first we need greater understanding of the impact of weak links and mismatched elements in the building envelope.
Even a structure without windows, doors, vents, and utilities would not achieve code-required R-values for the entire building envelope because the structural members not only displace insulation, but their thermal bridging properties work at opposing purposes by transferring heat through the structure. And since people do not want to live in homes without windows and doors, and electricity and plumbing, we need to accept still further compromise of whole-house heat loss through those weakest elements. As weak(er) links increase in size and number, they become responsible for a larger proportion of the overall heat loss from the thermal envelope and progressively diminish the proportional value of insulation in structural elements.
It should be clear by now that thermal envelopes have many weak points and mismatched components in resistance to heat loss, and the strongest element by far is the insulation across structural sections. Now consider the diminishing returns on insulation thickness explained earlier in this chapter, and the performance profiles arcs that show most of the value in heat loss resistance is already achieved by code-minimum requirements. Wall systems are most often upgraded to reduce heat loss, even though there is negligible value gained from that isolated element, and nothing gained if weak links are not strengthened from independent processes. Furthermore, the opportunity costs of very expensive wall system upgrades are often the weak(er) links that do not get addressed. Priority effort and home building budgets should go toward improving the weakest links; here are a few strategies for addressing the same elements noted above.
An extreme example helps illustrate this lesson. Consider two houses side-by-side in winter heating season, having identical designs, but one with a premium thermal envelope and the other with a standard code-compliant structure. Now consider that a hole the size of a dinner plate is cut into each front door and left open for the free flow of air; this would be an extremely weak link. We quickly realize that nearly all the heat produced inside these two homes would escape out the front door breach, and if both houses had the same heat source, their energy use and cost for HVAC operations will be nearly identical. In other words, the very expensive premium thermal envelope house will not return savings in operational energy from reduced heat loss. Now consider that a small hole in the front door may not actually be extreme in comparison to all of the non-obvious list of compromises noted above. The holes that we do not often think about could be several 3-4 inch diameter pipes through the envelope for dryer, plumbing vent, range hood, and several bath fans. It could also include two 6-inch pipes for the fresh and exhaust sides of an ERV. Most of these will have dampers or gravity louvers, but none are airtight, most will allow an outside wind to pass through, all of these pipes displace insulation from the thermal envelope, and many are metal with high thermal bridging properties. And this list of “holes” does not even include compromised insulation from utility incursions in exterior planes, or poorly installed insulation, or thermal bridging through structural members, or windows and doors with vastly lower insulating value than walls or ceilings.
Even if all of these strategies are employed, these weak links remain significantly weaker in resistance to heat loss than wall system structures across insulated sections. Given these physical realities, and the insulation performance profile at code requirements, it is not surprising that upgraded wall systems do not significantly reduce operational energy; this is consistent with the findings of the regional database of homes and also with the case home comparison. The simple conclusion is that upgrading wall systems beyond code minimum, without strengthening the weakest links, is like throwing away money and using more resources than necessary. With priority first given to the worst offenders, which are also the least-obvious, the next step is to consider the more obvious weak links of windows and doors.
Because code-minimum construction in the U.S. requires more insulating value in walls and other structural planes than is possible with even the best windows and doors, the closest match of whole wall components is code-minimum structure and premium windows and doors. Other combinations progressively widen the gap between the relative strength of structural elements and the relative weakness of windows and doors, with wider gaps further compromising the desired benefits of structural elements in the thermal envelope. This bears repeating. Investing in more robust structural elements (as in thicker and better-insulated walls) returns less benefit to offset the higher cost, as the gap widens between the insulating value of the wall structure and the windows and doors mounted within. Very wide gaps will eliminate most or all of the benefits of the thicker wall; this leaves a homeowner having spent significant treasure and embodied energy in a robust structure, but not able to receive its intended benefits. Here is a matrix that addresses nine broad combinations of matched and mismatched elements:
We have returned to our three-part classification of homes in relation to their suitability or availability for onsite clean energy generation. The cells shaded in red highlight scenarios with highly mismatched elements; these should be avoided unless the structure is required by code (as in hurricane or tornado areas), and the budget does not allow upgraded windows and doors. There is only one combination with these elements closely matched (yellow shaded), which could be recommend for SORTA and SNAIL homes with goals of reducing operational energy because onsite clean energy generation is either limited or not available. Even though the green-shaded scenario has mismatched elements, that represents the best combination of cost, value, resource-use, and environmental responsibility, when onsite clean energy generation can meet 100% of household energy demand, as in a SOAR home.
The green and yellow cells also provide a fantastic example of opportunity costs informing choices in homebuilding. Selecting premium windows and doors for a typical average-sized home can cost $20,000 or more, and the benefits in lower HVAC costs will never break even on the investment (when including cost of funds; see previous chapter). Meanwhile, installing enough solar PV to generate total net annual household energy needs will cost less than $20,000 initially (net after ITC; see Chapter 3), and that investment will do far better than break even (see chapter 3), while directly eliminating a significant climate footprint. Furthermore, if any spending on premium windows and doors uses budget that would otherwise go toward onsite solar PV, the outcome will be worse for the homeowner and society. The homeowner would be selecting the choice that provides the worst return on the original investment that is also least likely to be valued in appraisal and financing. That choice would also be worse for the planet, with greater use of resources for much less environmental benefit, and an opportunity will have been lost to eliminate household climate emissions (and possibly also from transportation).
Mid-range structures require a bit more nuance. There are a number of options between a base code-minimum structure and a premium envelope, but the most common mid-range choice is the upgrade of wood stud walls from 2×4 to 2×6. This adds more structural material (timber), more insulation, and more finish materials, as in window and door extensions. The classification averages in our regional database indicated promising efficacy in reducing overall energy use from an upgrade to 2×6 wall structure, but without more research we would like to temper any enthusiasm for that wall type. First, this structural envelope included the fewest cases in the database, and too few for us to draw broader conclusions. The second reason has to do with utility incursions in exterior walls. Electrical boxes are particularly egregious, and the common practice across most of the U.S. is to recess these into walls were required by code (for outlets and switches), regardless of wall thickness. In code-compliant 2×4 wood stud walls, such boxes displace more than half of the cavity insulation, creating weak points in an otherwise unbroken plane. Additionally, the practical challenge of installing insulation around these boxes almost always results in imprecise fill or notching, leading to air infiltration. One member of our team has performed energy audits on hundreds of homes, where he commonly finds breaches around and through recessed electrical boxes; see images below.
Recessing electrical boxes in exterior walls invites significant compromise through and around those features, and it adds weak links that widen the mismatch of elements and further reduces the proportional value of the strongest elements of the thermal envelope. The 2×4 wood stud wall is adequate for structural integrity and longevity. That base wall is also thick enough to provide optimal levels of insulation in stud cavities, but not if holes are created every 10-12 feet for electrical boxes. In the previous chapter we appeal for design work that minimizes utilities in exterior planes, and attractive surface-mount fittings where codes require placement along the thermal envelope (see examples in previous chapter). If the homeowner or builder is not willing to make these accommodations, than we recommend upgrading to 2×6 stud walls simply to reduce the impact of these weak links.
An upgrade to thicker walls requires sacrifice or cost in one of two ways. Either living space is reduced with the same exterior footprint, or the footprint is enlarged to retain the same living space. This can be significant in both dollar cost and environmental impact. The case home has three levels with linear wall footage of 144, 144, and 92 respectively. Since it was built with thick ICF walls, the total 380 linear feet amount to 250 additional square feet, just for the additional wall thickness, compared to basic code minimum. At the constructed square-foot cost of $160 for the case home, the extra square footage alone might represent $40,000 for what amounts to unusable living space. What makes this even more troubling are the findings of our study that suggest premium envelopes do not appear to render operational energy benefits; in fact, our data show ICF homes using more energy, both for the two-house comparison and in the larger regional dataset.
This case provides another excellent example of opportunity costs in building systems that also helps place these choices in perspective. Using the example of the case home, we will consider that the opportunity cost of enlarging the footprint to accommodate thicker ICF walls as installation of solar PV and switching to EV transportation fueled (charged) by the onsite clean energy generation.
As if this comparison is not convincing enough, consider that the $40,000 in extra costs associated with an enlarged footprint does not even account for the cost premium of the more expensive upgraded wall system, which can run into six figures for moderate-large homes! Once again, even if the premium envelope showed success in reducing operational energy demand (which our findings do not support), these additional premiums are far more in initial investment than any rate of benefit could return for financial break even. Stated another way, premiums paid for thicker envelope sections will be many times more than the cost of an onsite solar PV installation that will not only fully offset household operational and transportation energy, but also render an attractive financial return on investment. And the latter will have sequestered much less embodied energy!
Mechanical Ventilation (need for)
Earlier in this chapter we noted that all of the ICF cases in our regional dataset–except one–had an ERV installed and in use. Another claim by premium envelope advocates is that they are so airtight that mechanical ventilation must be added to maintain healthy indoor air quality. In the recent past indoor air quality concerns have been largely in the areas of carbon monoxide, volatile organic compounds (VOCs), and molds. Carbon monoxide poisoning can be fatal in high concentrations, but risks have declined significantly in modern homes with fewer open flames and code-required CO sensors/alarms. VOCs off-gas from building materials and adhesives, can accumulate to high concentrations in enclosed spaces, and may create health risk to inhabitants. However, VOC off-gassing levels diminish significantly with time, and an initial flush regimen for newly-constructed homes will minimize the threat; VOCs occur in nature and are ubiquitous in low concentrations that do not threaten human health. Further, manufacturers have also found ways reduce VOCs in building materials. Most contractors now purchase and install no/low VOC products; we recommend addressing this in design and building specifications. Mold also occurs in nature and is always present; low concentrations do not threaten human health, but homes without sufficient ventilation and humidity control can permit mold colonies to thrive and threaten human health. ERVs provide ventilation, but indiscriminate use can make indoor humidity levels worse. If envelope systems are designed and constructed properly, they will dry effectively in multiple directions, and HVAC use in tightly enclosed spaces can effectively control humidity.
Carbon dioxide (CO2) is a relatively new concern for indoor air quality and human health. CO2 receives a lot of attention in climate science, where general atmospheric concentrations are now typically above 400 parts per million (ppm). CO2 at those levels is not known to be harmful for human respiration, though climatologists document many current harms and significant future threats from a warming planet occurring primarily from rising atmospheric CO2. Indoor concentrations of CO2 have long been used as a proxy for the aesthetic quality of indoor air (ASHRAE 2013), but only recently has it emerged as a direct pollutant and threat to human cognition (Satish, et al. 2012). A recent controlled study led by a Harvard environmental health researcher (Allen, et al. 2015) found that from a base CO2 rate of 550 ppm, “cognitive function scores were 15% lower for the moderate CO2 day (~945 ppm) and 50% lower on the day with CO2 concentrations around 1400 ppm.” These statistically significant findings add an entirely new variable to indoor air quality and health and, based on our small sample of readings, it appears that most homes in the U.S. have an indoor CO2 problem if they are not ventilated in some way (ERV or open windows).
Even homes that employ mechanical ventilation are likely to see unhealthy concentrations of CO2 from extended human (and possibly pet) respiration in enclosed rooms, unless effective and sufficient distribution of fresh air is designed into the system. Bedrooms, especially, are susceptible to this problem, because people spend many hours sleeping, in relatively small spaces, and typically behind closed doors. ERV manufacturers specify that fresh air input from the ERV must be dumped into the return side of a central air ducted system. ERV specs warn against connection on the supply side because the much stronger HVAC blower fan would compete against the weaker ERV fan; this could damage the ERV fan, and possibly even push HVAC-conditioned air back through the ERV and out of the building. Most residential central air systems rely on just one or a few central returns; often one on each floor of a multi-story building. These central returns are also placed in the largest open areas; it is very rare to find effective returns directly out of bedrooms. When fresh air from the ERV is dumped into return side ductwork, it will flow to the path of least resistance, which is out the central return air grille(s). HVAC units place an air filter where the return air enters the blower fan, and the filter offers resistance to ERV-freshened air to flow through the air handler and out the supply vents. When the ERV runs at the same time as the HVAC system, the fresh air will be distributed to all spaces with supply vents; however, the conditions for both to operate in tandem are infrequent. Since bedrooms rarely have direct returns, and since ERV operation together with the HVAC is only by chance and infrequent, the rooms most in need of continuous fresh air (bedrooms) get very little, and none if the HVAC does not operate.
Case Project Example
The case project offers a practical example to highlight this problem. The home has three levels and, as typically designed, the HVAC contractor placed one large return in the open space of each floor. The ERV fresh air input was ducted to the return air plenum just ahead of the filter, as required by ERV specs. Predictably, this pushed the fresh air out the three central return air grilles when the HVAC system was not operating. This problem was positively determined, with no fresh air flowing through supply vents, and all of it flowing out of the central return grilles; that fed the large common areas with fresh air, but not individual rooms. Bedrooms with doors closed overnight, encounter CO2 levels close to 2,000 ppm with two sleeping occupants, and nearly 1,600 for one, even with the ERV running continuously (24/7). If the HVAC runs during the night as required periodically to maintain thermostat setting, some of the fresh air is supplied to the bedrooms, but in dilute concentrations, and the infrequency of operation still allows CO2 to concentrate to unhealthy levels in occupied enclosed bedrooms. After documenting this phenomenon, which we have since learned is common in nearly every ERV installation, we wired the HVAC blower fan to operate whenever the ERV runs. This unconventional fix solved the problem by drawing the ERV-freshened air through the filter and out the supply vents, more evenly distributing fresh air into all rooms. It solved the distribution problem, but at the cost of additional energy demand, which we will discuss further in the next chapter on energy.
Readers may at this point be wondering about CO2 concentrations in their own homes, and especially in bedrooms. Fortunately carbon dioxide monitoring equipment has recently become affordable to homeowners; small CO2 meters can be purchased for about $100 as of this writing. This is allowing homeowners for the first time to measure and track indoor CO2 levels, and we have discovered that every home we’ve been able to test in our region exhibits concentrations high enough to cause cognitive impairment. The severity obviously depends on size of space, number of humans (and pets) respirating, and whether (and frequency) windows and doors are used. ERVs have historically been recommended only for premium envelope homes that were thought to be so tight as to require mechanical ventilation. Ironically, we have found unhealthy levels of CO2 in all house types, even in very old buildings, and we contend that even code-minimum structures can be tight of air infiltration with careful design, quality craftsmanship, and a recommended blower-door test during construction (see chapter 5). In other words, every home in the U.S. should consider this indoor CO2 problem and design systems to address it. Houses without forced-air central HVAC systems are at particular risk because they have no distribution network for fresh air delivery.
We expect fresh air design will eventually work into residential building codes as the prevalence and health implications of indoor carbon dioxide become more widely understood. In any case, indoor air highly-laden with CO2 should be replaced, as needed, with outdoor air; this could occur through leaky windows and doors, intentionally opening/cracking windows, or by mechanical ventilation. Any of these scenarios represents weak links in the thermal envelope, which further diminish the proportional value of the most robust thermal elements and further erodes rationale for investments in wall systems beyond code requirements.
Passive SolarOne of the misconceptions that persists today about passive solar is how much solar heat gain has become compromised with better insulated windows. Years ago, passive solar offered net benefits at a time when the best available windows were plain single-pane glazing. Those units had very poor insulating properties, but since people prefer living in homes with windows, and there were no better options available, at least a few were added in strategic locations. Placing these windows on the south elevation to receive the winter sun offered desirable benefits in passive solar heat gain, and the costs were accepted as necessary for simply having windows. However, as designs and technologies evolved over time with coatings, multi-paned glazing, and vacuum or gas-filled gaps, windows improved dramatically in insulating value, but they also filtered out more of the sun’s incident solar radiation. How has this tradeoff affected passive solar? Let’s apply these principles–in concept–to the matching elements chart from above, but now only with the recommended code-minimum structure:
As with the overall impact and value proposition from earlier in this chapter, the code-compliant structure matched with standard windows and doors offers the best available package for passive solar. Some may, in the interest of increased passive gains, suggest lower U-factor windows only on the south side in order to improve SHGC coefficients. However, the mismatched elements logic does not support that choice on the insulation side, as that would proportionally weaken the value and investment in all stronger elements, including premium expense on all non-south-facing windows, but especially on any wall system upgrades.
In addition to southern glass and thermal sinks, passive solar designs often call for a robust thermal envelope to retain the heat accepted from the sun. A common misconception about overall performance is the significant compromise of passive solar heat gain when windows are placed in thick walls. Another reason for a code-compliant structure with passive solar design is that thinner walls allow more direct sunlight to cast onto indoor surfaces and possibly thermal sinks. The window extensions across thick walls, such as with double-stud or ICF structures, are not likely–or advised to be–made from thermal mass material and instead they block more of the sun’s direct rays during all period of the day except the few moments when they are perpendicular to the glass.
Another misconception about passive solar is the value of precision in orienting southern collection and control zones. Even though active PV systems are not significantly compromised within ten degrees of true south, passive solar systems are. This is the result of designed shading, which cannot compensate for imprecise orientation. A skew just a few degrees west of south provides too much/lengthy shade of the sun during its eastern arc throughout the winter, and too much/lengthy heat gain from the western sun in swing seasons. We do not recommend passive solar design unless southern exposure can be oriented to true south, or within just one or two degrees. Once again, the case project presents a relevant example.
The case home was constructed on a hillside with the fall line oriented ESE-WNW. Squaring the building to the fall line (and property lines) would have oriented the south face approximately 15 degrees west of perfect south. In excavation and setting the foundation, we were able to turn the house slightly to achieve a southern orientation of 188 degrees, just 8 degrees west of due south. This precise azimuth was surprisingly optimal for PV solar production, due to local weather conditions, but there was minimal fall-off within 10 degrees, meaning that 180 degree orientation would have meant little sacrifice in active solar production. Unfortunately, our team did not give enough attention to passive solar at the constructed orientation at 188 degrees. The full passive design included masonry trombe walls (thermal sinks; see image below) just inside large south-facing windows on two levels. This is common packaging for passive heat gain; design for the sun’s incident solar radiation to pass through the glass during heating seasons to warm a thermal mass, which captures the energy during the day and radiates the heat through the night.
The technique for passive solar in homes with both heating and cooling seasons is to design eaves to shade southern windows from direct sunlight during cooling and swing seasons. The eaves on the case house were designed and sized for its precise latitude, which is critical for sun angles, but it assumed perfect south orientation, and that feature was not adjusted when the final footprint was set at 188 degrees. With the house just 8 degrees west of due south, the actual result is some sacrifice of desired solar heat gain during morning hours of heating months, and too much afternoon solar heat gain during swing seasons. To be more precise, the afternoon sun begins casting onto trombe walls in mid August, when there are still 6-8 weeks of cooling conditions. In the spring, the southern windows are not fully shaded until mid April, several weeks beyond the optimal swing. There are increasingly undesirable trade-offs for passive design with each degree of orientation away from perfect south. Illustrating with the case home, lengthening southern eaves to extend afternoon shading of southern glass during swing seasons would further diminish passive solar gain during morning hours when desired.
The logical conclusion is that unless the house can be oriented with a southern face within 1-2 degrees of perfect south, it is not worth any additional design or expense to include passive solar elements. It is still better for homes with heating seasons to have windows on the south side than on other flanks, but unless the home can be oriented to true south, other features of passive solar are likely to incur negative returns. Furthermore, triple-paned windows on the case house, selected for best insulating value, meant compromising solar heat gain through southern glazing. This reduced passive solar effectiveness, and return on investment for the additional cost of the trombe walls.
The case home also provides some graphic and numeric examples of the problem of weak links and mismatched elements in the thermal envelope. Thermal imaging highlights these concerns with color and temperature readings. The images captured below link a photograph (left side) with its paired thermal image (right side). The crosshairs on the thermal image indicate the spot temperature (in degrees fahrenheit, upper left), and the scale on the right shows the range of temperatures in the image frame. All these images were taken with 32°F outdoor air temperature, and 67°F indoor thermostat setting.
These first images show the outbound side of the ERV, with the upper inlet pipe and lower exhaust. In the first two pairs, the ERV has not been operating, but the compromise around the collars is evident in the cold readings of 56.5°F and 50.9°F, respectively, even while some surfaces in this utility room are as high as 71°F. Sealing out air infiltration around penetrations through the thermal envelope is exceedingly difficult, especially across mixed materials and when any have high thermal bridging properties, as metal does. While the images do not capture the entire sacrifice of the uninsulated “hole” through the thermal envelope (ICF in this case), the colder temperatures on the surface of the pipe indicate ambient outdoor air finding its way through the thermal envelope; this is a constant incursion. The third pairing shows these same two pipes, but with the ERV in operation. Predictably, the coldest temperatures around the pipe collar, where outdoor air first enters the home, drops to 32.4°F. Furthermore, look at the cold temperatures (by color) on the entire section of metal pipe between the entry through the thermal envelope and the inlet side of the ERV; this is effectively a cooling coil through interior conditioned space. The warmer exhaust-side pipe indicates the heat that is lost out of the building for the benefit of mechanically ventilating for fresh air.
These few images depict several of the compromising weak links that diminish the value of stronger elements of the thermal envelope (e.g., wall and ceiling structure and insulation). Two six-inch holes have been punched through the wall, thereby eliminating insulation across 57 square inches (almost half a square foot), with no damper at or outside the thermal envelope. The connection collars are leaking air, even with careful attention and robust sealers. The highly-conductive metal material through the wall wicks (transfers) cold air into the home and indoor conditioned air to the outside, and the open six-inch pipe allows ambient outdoor air into the thermal envelope at all times, even when the ERV is not in operation. And when the ERV operates, the inlet pipe fills with cold air, which then radiates through the highly-conductive metal pipe wall like a cooling coil.
The ERV penetrations provide just one example of thermal envelope weak links. Many more are documented with images; please reference the full chapter for that data.
Overall the case house was a hard lesson that we needed to learn by doing, measuring, testing, and living in the home. We followed the conventional wisdom in the industry, and we did not yet have the benefit of our own research and findings. The case house has an ICF wall system, which we now know to be significantly diminished in its purported insulating value due to the many weak links and mismatched elements. The added cost for this premium thermal envelope was well in excess of $100,000, yet it required more energy to operate than the code-minimum house that the homeowners moved from. Fortunately, solar PV was installed from the beginning, which cleanly produces enough energy for household and transportation needs, but this all could have been accomplished for much less cost, and reduced environmental impact.
ConclusionsMarginal analysis is critical to find optimal thresholds in bolstering the thermal envelope, and that informs the diminishing returns to scale of insulation. Performance profiles for different insulation products and thicknesses demonstrate that building codes across the U.S. already require insulation values in excess of a cost-benefit optimum. Our findings rebut the conventional wisdom that thicker walls and insulation is better; we show how it is worse from both a financial return perspective and ecological impact. Too little thought and research goes into the impact of utility penetrations through the thermal envelope, and how that further diminishes the intended value of the most robust thermal systems. Weak links and mismatched elements abound in American homes, which is not a critique of those compromises as much as it enlightens the folly of thermal envelope upgrades. Our findings of energy use by wall type and living space was both surprising and disappointing at first. We expected to apply the tools of environmental economics to show attractive return on investment for thermal envelope upgrades when accounting for the ecological externality. That was impossible when whole-house, lived-experience cases seemed to show no benefit at all from premium upgrades. Though initially mystifying, a careful cataloging of weak links and mismatched elements provided the logic we now see in the data. Our team had fallen into the industry pattern of trying to analyze component parts in isolation, but our data suggested a broader, whole-systems review. Adding the tools of finance, and especially opportunity cost of funds and energy inflation, drove the nail still deeper into the coffin of thermal envelope upgrades.
As long as we live with utilities in the home, and operable windows and doors, the weak links will always overwhelm theoretical benefits of nearly all thermal envelope upgrades. Our findings point strongly toward an optimal thermal envelope simply built to code standards. While this was unexpected and disappointing at first, we soon realized that placing this knowledge in the context of whole-life sustainability makes this recommendation very good news indeed. It means that the most sustainable choice is also the most affordable. Homeowners building or buying a code-minimum house are more likely to have it appraise and financed at actual cost, and they are more likely to see an acceptable return on investment at resale. If homeowners resist expensive upgrades to the thermal envelope, they are more likely to add solar PV, which on reasonably good installations, accrues attractive financial returns. Eschewing expensive thermal envelope upgrades would preserve household resources for the transition to electric vehicles. These are the opportunity costs of thermal envelope upgrades foregone.
Many people believe that premium thermal envelope homes are tight enough to require mechanical ventilation. Our findings suggest that every thermal envelope type can be constructed and maintained tight enough to create debilitating concentrations of CO2. Options for fresh air ventilation are addressed in the next chapter; however, here we can debunk the theory that the threat occurs only in certain construction systems and methods.
Our study on passive solar uncovers further misconceptions. Windows have improved dramatically over the past few decades in insulating value, and that is helpful to narrow the gap between mismatched elements in the thermal envelope. However, the sacrifice of stronger U-values is less solar heat gain through windows, and this alters the calculus for passive solar design. In cold climates, windows are always better placed in south-facing walls, but unless the home can be oriented to perfect south, extra effort, costs and provisions for a full passive solar package is likely to offer a poor cost-benefit return.
Understanding diminishing returns for building envelope upgrades beyond code-minimum will lead to the most sustainable outcome, with an attractive financial return on investment, that also would cut 50% of climate emissions in the U.S. Code-minimum houses, built with quality, are the least expensive among a wide variety of options. Adding solar PV to meet household and transportation energy actually provides an attractive and stable rate of return. The industry has homeowners thinking building envelope first and solar second; however, with building envelope upgrades off the table, solar should be planned as a priority from the beginning. Finally, with a least-costly building envelope, and a strong financial investment in solar PV, homeowners will have more resources to transition to emissions-free transportation, charging their electric vehicle with the clean energy of their home solar system.
- Learn enough about diminishing returns to apply it intentionally in all areas of life.
- Learn enough about opportunity costs to apply the concept intentionally in all areas of life.
- Apply the tools of finance to understand return on investment for envelope upgrades.
- Orient structure to 180° (true south), if possible, for optimal solar heat gain and control.
- Prefer window placement on south side, for cold-climate homes, as optimal for passive.
- Build thermal envelope to local code compliance (varies by region), with quality control.
- Employ verifiable craftsmanship to minimize weak links in the thermal envelope.
- Give consideration to mismatched elements when selecting windows and doors.
- Plan for a system of ventilation to keep CO2 within healthy levels in all rooms
- Given a reorientation about the value of thermal envelope upgrades, plan for solar PV as a first priority in any new home construction project; design for it!
- Place choices about home construction in the context of broader life issues and impact.
- Don’t assume that thermal envelope upgrades will reduce HVAC energy use.
- Don’t spend money on thermal envelope upgrades beyond what is code-compliant.
- Don’t design for passive solar unless true south orientation range between 178°-182°.
- Don’t assume that any house would not exhibit unhealthy levels of CO2
- Don’t run an ERV (or other mechanical ventilation, if installed) more than necessary.
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