Building envelope systems

These trends set up an uncomfortable juxtaposition. It is still assumed that being environmentally responsible means building a house with a premium envelope. But the high cost of that premium makes this option accessible only to the wealthy and, even for them, it can be a poor financial investment due to financing and resale markets. Those who cannot qualify for a larger mortgage that would be needed to finance a premium envelope, or who cannot afford down payments in excess of 20% because appraisals do not fully value envelope upgrades, or who simply cannot allocate a higher percentage of a limited and squeezed income, have no choice but to seek more affordable housing, and that usually means a more basic thermal envelope. With these realities, it is no surprise that so few houses in the U.S. are built with premium building envelopes–the presumed sustainable home–and conventional wisdom suggests that most homebuyers, then, do not build or buy sustainable homes. However, due to declining costs of solar PV, and the advantages now of onsite renewable energy generation, we reject this juxtaposition as a false choice. This is where our three part typology is relevant, and how onsite renewable energy generation is linked with building envelope decisions.

Implications for SNAIL, SORTA, and SOAR houses

Where solar PV or other clean energy generation is not available on a residence, the only sustainable choice is to make building envelope decisions to reduce heat loss and operational energy, the same prescription as proffered by the green building industry. Even while recognizing that most envelope upgrades have higher embodied energy, the purported continuous operational energy saved over the life of the building would be expected to surpass the additional embodied energy and more damaging environmental impact of the more robust envelope. A far better outcome, however, is the SOAR home, where onsite solar PV generates all operational energy needed (annually); ironically, this points to a more basic and less costly building envelope–if built with quality and integrity–that is also less environmentally damaging from embodied energy. The SORTA home, where solar PV is available, but limited in production at levels below operational energy demand from a basic envelope, strategic upgrades to the envelope might be selected to bring operational energy needs down to available (limited) solar production. To recap, SOAR homes are optimal from a whole house, whole life, sustainability view, and fortunately they are also the least costly to build, most affordable to own, easiest to sell, and most likely to return the highest investment value on resale. SORTA homes are second best among these three options, and SNAIL homes are the most damaging to the environment in both embodied and operational energy, most costly to construct, least affordable, and most difficult to sell at prices that would recoup investment value.

The comparison of building envelope upgrades (generally) vs. solar PV on both economic and environmental costs and benefits is not even close. Increasing the relative size of solar PV has far less embodied energy implications than most building envelope upgrades, and far superior return on investment from the stream of operational benefits. To put this into perspective, an entire solar PV system providing 100% of annual residential energy needs often has less material weight than many of the individual upgrade systems to the building envelope, each of which may reduce energy use/loss by a mere fraction of the whole, if any at all. Of course the materials are different, and not all equal in environmental impact, but the reality of this scale and magnitude helps keep perspective when comparing these two systems, both of which attempt to either save or provide energy and reduce environmental harms.

What Building Envelope Upgrades to Make

This leads to an interesting question: are there any building envelope upgrades beyond code-minimum standards that offer reasonable and recommended financial and environmental tradeoffs? In other words, what upgrades are advisable because lower energy use and cost in operations offset higher embodied energy and costs in construction? Air infiltration, insulation, and thermal bridging are the three primary concerns of the building envelope from an operational energy perspective, and moisture and condensation/vapor control is an issue determined by the interrelated performance of all three. Insulation and air infiltration are commonly understood and descriptive by name. Thermal bridging may be new to some; in construction it refers to the heat transfer properties of materials. As an example, steel is highly conductive and transfers heat through the material; a thermal bridge in wall construction would be metal studs in an exterior wall that conduct outside heat into the building in the summer, and cold into the building in the winter, regardless of insulation within the stud cavities. A building envelope is most effective against operational energy losses when thermal bridging and air infiltration are relatively low, while insulation value is relatively high. Furthermore, these three elements need to be considered as interrelated and complementary components of the whole envelope. For example, thick and expensive insulation matters little if gaps are left for easy air infiltration, or if thermal bridging conducts heat through the envelope.

Of the three principles targeted to reduce heat transfer through the building envelope, reducing air infiltration has the largest impact on performance and is also one of the least expensive areas to address. Relatively small investments–in both dollar cost and embodied energy–to seal the envelope of air infiltration will pay large dividends over the long life of the building. We strongly recommend a blower door test just before insulation is installed, to identify and seal air leaks in the structural envelope. Low cost (dollar and environmental) sealants such as caulk, spray foam, and tape can be applied to air gaps in the outer shell before they are covered by insulation and inaccessible. Note that this practice applies to any envelope system, and the investment of a few hundred dollars will pay off in a few years with both environmental and economic benefits. Furthermore, the embodied energy of this task and material pales in comparison to commensurate reduction in operational energy over the life of the structure. With very few resources and materials used for a pre-insulation blower door test and seal, this is more of a process inclusion than a materials or systems upgrade to the building envelope, most of which have much higher costs (in both dollars and embodied energy) and diminishing returns. Let’s now apply these principles to each of the building envelope elements; foundations, roofs, and walls.

Foundations

Three predominant foundation methods are used in the United States. The National Association of Home Builders (NAHB) reports that 54% of houses are built on concrete slabs, 30% on full or partial basements, and 15% over crawl spaces; the small remaining balance (about 1%) are built on stilts, pilings, or use other methods (Siniavskaya, 2014). The optimal choice is based on many factors, but primarily weather conditions, local geology, and cost; see the graph below for the U.S. breakdown by region.

Colder climates benefit from envelope contact with the earth, since soil is an effective insulating buffer in extreme cold, and it also blocks air infiltration. The regions with the coldest winters are the five in Figure 5.1 using mostly basements. Ground temperature at the surface varies (though lags) with air temperature changes, but that variation modifies below grade, and progressively with distance from the surface. In many areas of the U.S., ground temperature below the frost line hovers near 50-55° Fahrenheit year round; this is fantastic insulation in the winter when air temperatures drop well below 50° in many parts of the country, and this is one reason basements are common in colder climates. Standard construction practice for concrete floors on soil include four to eight inches of gravel, then plastic sheeting as a vapor barrier, before pouring a four-inch concrete slab. Concrete has relatively high thermal bridging properties, which draws the coolness of the earth onto the surface of the slab; many homeowners lament the cool/cold surface of concrete basement floors, which also cools surrounding air and increases the indoor heating load. One upgrade we advise is installing two inches of foam (either spray or rigid) between the gravel and plastic to both insulate and break the thermal bridge from the soil to the slab; this adds minimal cost in both dollars (about $1.30 per square foot) and materials, makes the concrete more comfortable to live on, and will reduce heating costs and energy use in the winter. Rigid foam also works well for slab edge insulation, in this case to break horizontal thermal bridging from adjacent earth to the slab.

Crawl spaces work best when they are within the conditioned envelope, but that requires energy to heat and cool unusable space. Unconditioned crawl spaces are very tricky to get right, since they need a measure of air exchange to control moisture and prevent mold growth, but this further weakens the insulating value of the envelope. Since conditioned areas over a crawl space are not rammed against the earth, substantial insulation is recommended in the floor system, and great care should be taken to seal any gaps or penetrations through the floor to minimize air infiltration. Crawl spaces often have an advantage over slab floors in thermal bridging; floor joists are less conductive (if wooden) than concrete. Builders should consider breaking the thermal bridge below floor joists, but only in consultation with an integrated strategy to control condensation and moisture. Since the complexities of crawl spaces are many, we strongly advise consulting history, best practices, and engineered specifications outside of this resource.

Roofs

Most residential roof structures in the U.S. are wood-framed. Wood is a better material for reduced thermal bridging than most other structural options; by contrast, it is very difficult to isolate metal roof members from the thermal bridging problem. Additionally, metal expands and contracts more with temperature variation, and that has the potential for stressing building materials and opening cracks over time that allow air infiltration. If metal must be used for structural purposes, insulation should be placed between the metal and outside elements to reduce temperature variation of the metal and thermal bridging problems. While not as critical as with metal, insulating between structural members and outside elements is good practice for any structural material. This can be accomplished with a design that places the envelope ceiling below an upper structure for roofing, and then adding insulation on top of structural ceiling members. Figure 5.3 below shows loose cellulose being blown into truss cavities, then on top of ceiling joists; this forms an integrative solution to the top plane of the building envelope that reduces air infiltration and thermal bridging while providing substantial insulation from recycled material.

The upper plane of the building envelope encounters the most extreme temperature differences and therefore benefits from more insulation than walls; for example, R-38 is recommended or required in U.S. Region-4. Concentrated heat in the summer, either by direct sunlight on the roof or heat buildup in an attic space, encounter temperatures well above ambient outdoor air, and an insulation shield helps keep that unwanted heat out of conditioned spaces below. During colder weather, well-insulated ceilings helps reduce heat losses and energy use from within, since heat naturally convects to the highest elevations. For these reasons, building codes in the U.S. require higher R-value in ceilings than in walls and, for reasons detailed in the next chapter, we think adding more insulation than code requires is a poor use of resources and offers a poor set of tradeoffs. Due to the substantial value of insulation in the upper plane of the envelope, penetrations through ceilings and roofs should be minimized and, where necessary, sealed with spray foam, caulk, tape, or other sealing agent. Large housings, such as for recessed lights, should not be placed in the envelope structure; these become dramatic weak links in the overall system and prime areas for condensation to occur, which can lead to mold growth and/or structural damage. For similar reasons, skylights and solar tubes (natural light tubes) introduce weak points in all three compromising areas (air infiltration, insulation, and thermal bridging) and are best avoided.

Insulation

Before moving to the wall systems of the building envelope, a better understanding of the properties and benefits of insulation is instructive. Insulation is a critical component of the building envelope, but how much is needed and what insulation types and systems offer the best tradeoffs among cost, performance, and environmental impact? Performance of building insulation, of all types, offers progressively diminished returns (benefits) with each additional unit after the first; meanwhile, the costs of increasing insulation–on both environmental and dollar measures–are closer to linear. For example, installing insulation where none is present adds tremendous value in insulating performance, but each additional unit beyond the first–say, each additional inch–provides a declining additional performance benefit (see chart below). Meanwhile, on the cost side, doubling the thickness of insulation comes close to doubling the installed dollar cost and the embodied energy in the material used.

Common insulating materials in building construction are fiberglass (blown and batts), mineral (rock) wool, cellulose, and foams, among others. As an example of the diminishing returns of additional insulation, the graph to the right illustrates the reduction in heat loss (as a percentage) for each inch of polyurethane foam. Just one inch of spray-in-place urethane reduces heat loss by 70% compared to no insulation. Adding a second inch improves the insulating benefit, but by a smaller margin, combining with the first inch for 90% reduction in heat loss. Each additional inch will surely improve insulation performance, but at diminishing rates; in the case of polyurethane, the difference between six and seven inches is hardly discernible in performance value, both on the graph and in heat loss and energy bills.

This illustration on the performance rates of urethane foam shows that there is very little–and progressively less–gained in insulating value by installing more than just a few inches. Furthermore, the environmental damage of using more of any one type of insulation is largely linear, along with the dollar cost; this raises relevant questions about how much (how many inches of) insulation provides the best balance of insulating value vs. cost of product vs. embodied energy of installed material. This analysis has been attempted for a typical house and installation in the U.S., though many related factors influence whole house performance, such as the number and size of windows and doors, and of varying quality. Still, some sense of expected return on insulation upgrades is helpful to gain a sense of magnitude; the chart below offers such an analysis.

Returns calculated for the chart above utilize a simple payback formula, dividing the investment value of each premium by its respective predicted annual savings in energy costs. That analysis is quick and simple, but it leaves out important financial realities that vastly change payback periods. To provide a more realistic return on investment, we include an expected inflation rate of energy and the cost of funds, or the opportunity cost of money spent on each upgrade. Since most homeowners finance their residence with a mortgage, we think a reasonable lower bound on cost of funds is 4.5%; those paying cash might consider a higher opportunity cost, such as against equity or fixed income investments. Overall inflation in the U.S. has averaged 3% in the modern U.S. economy (since WWII), and while energy inflation has been higher, on average, we believe that 3% is a conservative rate for this kind of analysis. Using the same underlying data as the chart above, but correcting for energy inflation at 3% and variable rates (base of 4.5%) for cost-of-funds, the chart below shows realistic payback periods.

Even at a relatively low rate for cost-of-funds (4.5% represents competitive mortgage rates for top-tier borrowers in 2018), it is hard to make a financial argument for upgrading insulation beyond R-8. However, if onsite renewable energy generation is not available, then increasing the R-value of insulation in the building envelope may be the responsible environmental choice (up to a point) to minimize energy use and the household carbon footprint from operations. This is where our three scenario classification is helpful again. SNAIL homes lack the ability to generate clean energy onsite, and given that operational energy far exceeds embodied energy over the lifetime of a building, the best environmental choice where the energy is derived from fossil fuels is to decrease energy use and losses, even though it nets a financial loss. Reducing air infiltration and thermal bridging should be priority before increasing insulation beyond what building codes require in cavity spaces between structural members. Even a SNAIL home will encounter insulation (and R-value) thresholds where the environmental impact of adding insulation in embodied energy exceeds the marginal performance and environmental impact of reducing energy use in operations; these will be discussed later in this chapter and the next. On the other end of our typology, SOAR homes are assured of generating sufficient and clean onsite energy, so they can use fewer resources, less insulation, and lower R-values (and embodied energy), and still achieve net zero energy. SORTA homeowners will add building envelope premiums only to the point of reducing energy use to within limits placed on solar PV or other onsite renewable energy generation, and reducing air infiltration and thermal bridging should be prioritized before any increases in insulation.

Most envelope structures already have insulation cavities deeper than necessary to achieve an R-value of 8, and most building codes require R-values higher than eight in each of the three envelope elements. Additionally, breaking thermal bridging and addressing air infiltration most effectively utilizes applications outside of the structure, and outside of the insulation cavity, which add further insulation and R-value. That aspect will be addressed further in the section on walls, but what is clear from a straight economic cost-benefit analysis is that building code requirements and structural cavity spaces already provision insulating values well beyond the economic-performance optimum. This gap between optimal and achieved R-values widens still further when thermal bridging and air infiltration are addressed. Walls are further complicated by weak(er) links in whole-wall sections from doors, windows, utilities within walls (e.g., electrical boxes and piping), and utility penetrations (e.g., bath fans, dryer vents). These weak(er) links render higher R-values in walls even less effective, both in terms of energy use and loss, and in economic return; discussion of these mismatched elements are taken up in the next chapter.

Before assessing wall sections of the envelope, a few more broad understandings of building insulation are needed. Figure 5.4 showed the insulation profile for polyurethane foam; this pattern is similar for other insulation materials, though each one has its unique performance profile. Keep in mind that these ratings are static for a cross-section of insulation material and does not account for quality of installation, integration with other systems, or the wall as a whole which will have lower ratings across structural members (e.g., studs) and other weaker links (e.g., windows, doors, utilities). Installation quality can significantly compromise effectiveness of insulation; for example, if fiberglass or rock wool batts are not cut with precision to fill an entire wall cavity, or if they are not notched carefully around impingements like electrical boxes, they leave insulation gaps, which quickly overwhelm the R-value rating of the insulation. Similarly, if cellulose is not blown to the correct density, it will settle over time and leave an uninsulated gap at the top of each cavity; again, this breach overwhelms much of the benefit of the insulation material. With those caveats in mind, the following table (Figure 5.6) provides maximum R-value ratings, by inch thickness, for many of the common insulation materials used in most residential construction in the U.S.; these relative values may be helpful in comparing different insulating materials, and in helping to inform wall thickness and wall system decisions.

Walls

Wall sections of the building envelope have more variables than foundations and roofs. Some walls may be below grade, and above-grade sections have windows and doors. People enter and exit through walls, which exchanges air, as do vents from bath and exhaust fans and dryers. Walls tend to have more utility incursions, especially electrical boxes, in addition to piping and wiring. All of these complicate options and selections of wall systems and insulation. Let’s start with below-grade wall systems where structural strength and moisture control are preeminent, along with thermal bridging and insulation. Most below-grade wall systems are engineered for greater strength because they bear the weight of the whole house above and lateral pressure from bermed earth. Moisture control is critical because these wall sections either abut, or are in close proximity to, soils that absorb and hold water that would dam against the wall system if drainage is not intentionally and carefully provided.

Wood structures are commonly used in residential construction above grade, but they are not optimal below grade for the potential of water penetration and structural compromise. Wood could also be more accessible to termites or other bugs in some regions if used below grade. Masonry block was common in basement walls in many regions of the U.S. That system relied on effective and durable waterproofing as a thin membrane on the exterior surface to keep moisture at bay; unfortunately, too many of these systems failed, leaving basements and crawl spaces damp and musty, and potentially unhealthy from mold colonies. Fortunately, there are better and improving products on the market today, both for wall systems and waterproofing. Insulated concrete forms (ICF), Superior Walls, and poured concrete walls all add greater structural integrity than masonry block and more comprehensive moisture control, but all still require effective waterproofing applications on the exterior surface up to grade.

Above-Grade Walls

Above-grade wall systems really get complicated. Building science provides us with tested performance data on insulation and R-value, as in many of the charts and graphs in this chapter, yet those metrics are for a static cross-section of the insulation material. Whole wall R-values are far lower than the insulation metrics for a variety of reasons, including thermal bridging across structural members, air infiltration, and the presence of windows and doors that have very low R-values relative to wall insulation. While infinite in possible permutations, some basic principles can be drawn from this complexity. The diminishing returns of better insulation, either because of material selected or thickness of the application, is further diminished by any breaches or weaker links in the wall system, or building envelope. Breaches from air infiltration or thermal bridging, and weak links from doors, windows, vents and utility penetrations, all compromise the building envelope regardless of R-value in the walls (or floors and ceilings, for that matter). This principle suggests that better insulation (material or thickness) improves heat loss performance, but by progressively smaller amounts than the static R-value ratings might suggest; this further diminishes the marginal effectiveness of heavier insulated wall systems. Even the best energy modeling software cannot predict overall performance with precision because there are so many elements beyond the control of the systems, such as poor quality of construction, a window that doesn’t latch well, or a door that loses its seal.

The basic above-grade wall system used across much of the U.S. is 2×4 wood structure, with studs at 16-inch centers, half-inch OSB sheathing and wrap on the exterior, and fiberglass batt insulation in the stud cavities. Using this wall system as a base, and considering a number of upgrades using cost of funds rate of 4.5% and energy inflation of 3%, we found very few upgrades that offered reasonable returns, both on economic grounds exclusively, and even when adding ecological impact. However, adding insulation on the exterior of the base wall system is one upgrade worth making because it both adds R-value to the whole wall (except where there are windows and doors) and it breaks the thermal bridging through the solid wood members (studs and plates) of the wall structure. Adding exterior insulation, however, can complicate the trapping and drying of moisture in the wall, potentially causing structural damage and/or mold growth. Let us look at this specific issue of moisture in and through walls and then return to exterior insulation.

Note that the thicker wall (2×6, in this case) requires higher R-value insulation on the exterior; this is because thicker wall cavity insulation keeps the inside surface of sheathing colder from interior warmth. Note also that R-value requirements increase for colder zones, and there are no requirements in the warmer Zones 1-4 (areas of Zone 4 not in the Marine region). The map below shows the climate zones across the U.S.

Moisture Control

Sheathing on the exterior of walls will get wet, or moist. Moisture can encroach from the exterior through any defects or breaks in the water-resistive barrier (WRB); poor workmanship, or quality problems in installation, can allow this breach from the outset, but gaps will also open over time as building materials cure, shift, and degrade. Moisture can also arrive from forces inside the house. In the winter, warm inside air can leak into the wall via cracks or gaps, such as through wall switches and outlets, poorly fitted insulation, and under the lower edge of the drywall and the lower wall plate. When this warm moist air meets the sheathing, which is cold from low outdoor temperatures, the vapor in the warm air is condensed and absorbed by the sheathing; a process called ‘sorption’ (Holladay, 2010). This was less problematic in the past with basic wall systems, which allowed enough warmth from livable space to infiltrate the wall and warm the inner side of sheathing, but thicker and more insulated walls allow the sheathing to become colder, thereby exacerbating the sorption condition on the inside surface. As this condition led to mold growth and rotted sheathing, building codes began adding requirements for exterior insulation in cold climates to protect the sheathing from getting too cold. The chart below indicates minimum R-values of exterior insulation, with examples if using expanded polystyrene (EPS).

It is not advisable to expect that water/moisture will never be present inside a walls. The presence of moisture is not problematic if the wall system is designed to facilitate drying and, ideally, a wall should be able to dry both to the interior and exterior. Drying will occur to the inside when outdoor temperatures moderate and inside air is dehumidified. Drying to the outside will take place naturally when the ambient air is dryer, but the sun’s energy driving against the wall is the most effective drying agent. In the northern hemisphere, north-facing walls dry more slowly; another reason to buffer the north side of a residence with the garage or other semi-conditioned or unconditioned space. To permit drying, the other materials in the wall system must be vapor permeable. For example, closed-cell foam insulation is a vapor barrier, whereas open-cell foam is not; both are effective air barriers.

Returning again to exterior insulation, which we suggest is a reasonable upgrade to the base 2 x 4 wall, though it’s not critical if not required by local building code. Even a thin cover of insulation that breaks the thermal bridging of the structural members of the wall (studs, top and bottom plates, window and door frames, etc.) will return measurable benefits in reduced heat loss and energy bills. We like rockwool rigid board for this application, due to its properties of being a natural mineral, flame retardant, and advantages dealing with moisture and drying. Board as thin as half an inch would be ideal, but it more commonly sells in thicknesses of one inch or more. Rockwool in standard 1.25-inch thick rigid board adds an R-value of five to the whole wall, except over windows and doors, vents, or other penetrations. Thickness (and R-value) of continuous exterior insulation should be guided by requirements for specific climate zone (see Figure 5.x above). Rockwool breathes, which is important for outbound drying of the sheathing; it also sheds (or drains) water effectively, and its structural integrity is not compromised by moisture. Exterior insulation that is not too thick allows fixing of siding to the structure without the additional step of adding furring strips, and rockwool in a thin rigid board package provides greater support than softer materials for exterior fixing through the material. The relatively thin depth also requires less labor and material for window and door extensions than thicker applications.

Recommendations for SOAR homes

Homeowners with SOAR conditions, and who are able to install enough solar PV to meet their annual energy needs, will find that the most environmentally-friendly building envelope structure is simply complying minimally with local building codes. It bears repeating here, as at several other key places in this text, that quality and integrity in construction of the building envelope is paramount to maximizing sustainability objectives through longevity of use and life of construction materials. Building codes in the U.S. already require R-values well in excess of maximum cost-benefit gains from insulation, so there is no need to add more for economic reasons, and since SOAR homeowners are generating all of their (net) energy cleanly onsite, there is no need to add more insulation for environmental, or sustainability reasons. There may be other reasons a homeowners may wish to add wall thickness and more insulation, such as for greater indoor living comfort, but in our view that would be sacrificing environmental objectives for other personal concerns. Safety and security in areas at high risk to natural disasters provide a notable exception. Homes built in coastal regions susceptible to hurricane winds and possible storm surge will follow location-specific recommendations, and these most often require more extensive connecting hardware, and in some cases a more robust structure. The difference in these regions is that structural upgrades are required or recommended for personal safety and structural longevity rather than for energy use and loss.

The chart below offers the recommended wall system from our team for SOAR homes that utilize solar PV energy generation, for the first seven climate zones. There is some variability in local building codes, notably in natural disaster-prone regions, and those will legally supercede these broad recommendations by climate zone.

Note that the recommendation of exterior insulation is to meet minimum requirements of the International Residential Code (2015), and this is primarily for effective long-term management of moisture in wall systems rather than for energy use and loss. It is interesting that moisture management in some climate zones drives insulation requirements beyond what would be optimal from an economic and environmental cost-benefit basis for energy use and loss.

Recommendations for SORTA and SNAIL homes

SORTA homeowners are working to reduce operational energy needs just to the limits of restrictions on solar PV, and we recommend addressing the weakest links in wall systems, but not the walls themselves, as we will explain in this chapter and the next. SNAIL homeowners may also wish to reduce operational energy needs, and we again recommend a priority of addressing the weak links of windows, doors, and utility incursions; more on that later. There are too many possibilities, in an evolving and dynamic industry to make specific recommendations, but these principles can be applied with the counsel of an architect, builder, and energy modeler.

To gain as much value as possible from the insulation cavity, we recommend minimizing incursions of utilities in exterior walls. This starts at the design stage with careful thought given to where plumbing fixtures are located, preferably adjacent to interior walls where piping can run without compromising the insulation envelope. Even where plumbing fixtures need to be placed against an exterior wall, as is often the case with a kitchen sink, the supply and drain pipes can be routed through conditioned floor and cabinet spaces rather than stubbed out from exterior walls. Design work should include locating light switches on interior walls, where practical, and we advocate for surface-mount electrical boxes where placement is necessary on exterior walls, either by owner preference or by code (see image to the right). Surface-mount wall outlets can be integrated with the baseboard to minimize intrusion into room space (see image to the right). While there are surface-mount electrical boxes and fittings on the market, we would like to see more attractive and integrated options; outlets and switches integrated with trim as factory-finished products. Electrical wiring may need to route through exterior walls in order to meet code requirements. Those lines do not compromise the insulation cavities in a significant way, but if batt insulation is used in the cavities, it is very difficult to notch perfectly around cables, and that typically results in gaps and compromise. This is one reason we advocate cellulose over batt-type wall insulation.

Insulation challenges and solutions at corner sections of wood-framed walls.

At both the design and construction phases, it is worth paying special attention to insulation and thermal bridging at the connecting points between building envelope elements. Where walls join and where walls meet ceiling and roof structures are areas often neglected from an insulation and heat loss perspective. The wall-roof connection needs to be considered first in the design stage, with sufficient space to retain the R-value of the wall through the corner and into the roof/ceiling insulation plane. Too many designs either pinch the corners, allowing less depth for insulation to match the R-value of the walls, or the design makes it a challenge for installers to reach and effectively fit the requisite insulation.

The insulation challenge where walls intersect is more of a concern for the builder than designer. The figures below show typical installations on the left, where a section of the wall is left without insulation, whereas the image on the right side demonstrates an alternative framing design that avoids insulation gaps and provides more space for stud cavity insulation.

Windows and Doors

“A chain is only as strong as its weakest link” is a helpful metaphor to think about the whole building envelope. Weak links in the thermal envelope can appear in unintended ways such as from air infiltration or thermal bridging, as noted above, but they also come from intended design elements, such as windows and doors which can have insulating values significantly lower than the walls they are mounted in. The insulating value of windows and doors is represented as a U-factor, or U-value, and commonly ranges between 0.07 and 1.20. U-factor is the inverse of R-value, meaning that dividing U-factor into 1.0 provides an equivalent R-value to compare against other insulation profiles. Using this formula, some of the best windows on the market have an equivalent R-value of about 14, and some of the worst examples have an R-value below ONE! Lower U-factor (higher R-value) windows and doors are clearly preferable; however, they sell at a very high price premium, which requires a long-term cost-benefit analysis.

The best performing windows have triple-pane glazing, fiberglass framing, and casement style, whereas typical standard windows today are often double-pane glazing, vinyl framing, and double-hung style; there are also many other variations. One of the most reliable studies on window performance was conducted by the Department of Energy’s Pacific Northwest National Laboratory (PNNL), which looked at the specific difference between double and triple-pane glazing. While the PNNL study showed a whole-house energy savings of 12.2% from triple-pane versus double-pane glass, the price premium for the better insulated windows provided very long payback periods of between 23 and 55 years (DWM Magazine, 2013). The Family Handyman estimates payback in “a few decades” (see sidebar). Furthermore, if cost of funds and energy inflation are added to the analysis, which again is more realistic, triple-pane windows likely never return a financial payback. Still, SNAIL homeowners wanting to make the most sustainable choices for overall ecological impact may wish to upgrade on the weak links of windows and doors. However, rather than jumping to triple pane glazing, we recommend starting with type (casement over double-hung), then to frame material (wood over vinyl), and then inspect meticulously for proper installation and air-gap sealing.

The best window installed poorly will perform worse than the worst window installed well. Since SNAIL homes cannot generate onsite renewable energy, and if they receive energy directly or indirectly from fossil fuels, then the sustainability goal is to reduce operational energy demand in the home, and this needs to begin with the weakest links. Even for SNAIL homes, we do not recommend bulking up wall systems with structure and insulation beyond code minimum because the performance benefits are slight, yet the ecological impact from materials use is significant; we explain this over the next two chapters. Furthermore, when premium walls are coupled with windows, which are always weaker in resisting heat flow, the heat loss breach through weak-link windows overwhelms the benefit of increasingly robust wall systems, further jeopardizing the benefits and investment of a premium structural envelope. The weaker the weakest link, the weaker the chain, and the less value derived from the strongest elements. This leads to a principle of matching and integrating various elements of the thermal envelope, a topic explored in the next chapter.

Fortunately, there is helpful independent data available when researching and selecting windows and doors. The National Fenestration Rating Council (NFRC) provides independent energy ratings as a consumer service. Manufacturers display performance ratings in product specifications and on a window label. Air leakage would be the worst compromise for a window or door, but there is little variance in that metric across brands and models. U-Factor, then, becomes the most salient performance and comparative data point. Solar Heat Gain Coefficient and Visible Transmittance will be addressed in the chapter on energy. NFRC also does this testing on doors; see www.nfrc.org for more information. However, the same principles outlined above for windows apply to exterior doors.

As with wall insulation, the U-factor rating on windows refers only to the glass; it does not include heat loss performance of window frames or assemblies, nor does it provide qualification about air seals and infiltration from installation quality. Just as full-wall R-values are lower than the R-value performance of just the insulation in the wall, whole-window R-values are effectively lower than just the glass component rated in U-factor, and unfortunately whole-window R-value ratings are not available. Fixed windows are both less expensive, and they perform better on heat loss than operable windows. In rooms that have more than one window, consider making only one operable for egress and ventilation, and the rest can be fixed glass.

Need for Ventilation

One question often asked about a robust building envelope is whether it can be built too tight. Before the days of advanced building science, precision tools, and durable sealants, it was impossible to build a house too tight. However, as methods and materials improved, tighter building envelopes reduced heat loss and energy use, but they also trapped more moisture and stale air, both of which may be unhealthy for inhabitants. Opening windows is a low-tech solution to this problem, but it’s not recommended when outdoor temperatures or humidity could cause more harm, or more energy to run HVAC equipment. In those conditions, mechanical ventilation is recommended to exhaust stale air and replace it with fresh outdoor air, though through a mechanism to scrub humidity and temperature differences. The most common solutions today are the energy recovery ventilator (ERV) or the heat recovery ventilator (HRV); both of these are effective in exchanging air, and they offer about 90% efficiency in heat loss, but they also require energy to operate, and the outdoor air they bring in often requires more energy in HVAC operations. These trade-offs are discussed more in the energy chapter. Any house built with integrity, and detailed attention to air infiltration, should plan for mechanical ventilation. Even a standard code-minimum house can be just as tight as a home with a premium building envelope, and any tight house will suffer unhealthy levels of CO2, among other risks. Most code-minimum homes in the U.S. today would benefit from an ERV, but they are not required by code and are still quite rare. Our research showed that new and recently-built homes in the U.S. have an undiagnosed CO2 problem if there is no mechanical ventilation; we’ll address this concern over the next two chapters. Most custom homes built with a robust envelope do include mechanical ventilation; however, the operational energy needed to power this equipment works against the objective of minimizing operational energy use and loss from a premium envelope.

Opportunity Costs

Another factor to consider in building envelope upgrades is the opportunity costs; that is, what could be done with the same funds if not for the upgrades. Bolstering structural elements of the thermal envelope and upgrading windows and doors can quickly add tens of thousands of dollars to construction costs, and even into six figures; that is well more than would fund a solar PV system that could provide 100% of the home’s energy needs. For an average size American home, the cost of a premium envelope, with premium windows and doors, could purchase two new (long range) electric vehicles plus a solar PV system large enough to power the home and transportation. Shifting housing and transportation to clean renewable energy would cut the average American’s greenhouse gas emissions by at least 50%. If the homeowner laments the social and global injustices of climate change, a premium envelope forgone would fund enormous and perpetual relief to keep many from starving, or help thousands of poor people adapt to changing climate conditions. The possibilities for opportunity costs are endless, with just a few mentioned here. Stepping back from the homeowner’s tree (their home) to view the forest of global implication and consequence can help keep these choices in perspective.

Build it with Quality

One final note before our case study to briefly address durability and longevity of the structure. Our data and analysis favors a lighter building footprint, especially when onsite renewable energy generation is available, as in a SOAR home. However, too many houses built to code minimum standards are also built with poor craftsmanship. The industry, as it works in practice, generally incentivizes short-term outcomes for the initial sale and warranty period, but it does not incentivize long-term durability and structural longevity. We strongly recommend that homeowners employ an independent, third party inspector to maintain quality control. This could be an architect or a trusted and independent builder. Our research found that homes built to U.S. code minimum standards can last indefinitely with careful use and effective maintenance, but that requires initial integrity and quality in every aspect of the construction process.