Musings of an Energy Nerd

Every now and then, a GBAGreenBuildingAdvisor.com reader posts the question, “How do you install windows in a wall with exterior rigid foam?”

The answer to the question is surprisingly complicated. The best method will depend on several factors, including the answers to these questions:

• Are the windows innies or outies?
• What type of water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB) does the wall have: Zip sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , housewrap, or rigid foam?
• How thick is the rigid foam? (For more information on this question, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

GBAGreenBuildingAdvisor.com Prime subscribers have access to many articles that aren't accessible to non-subscribers, including Martin Holladay's weekly blog series, “Musings of an Energy Nerd.” To whet the appetite of non-subscribers, we offer a “GBA Prime Sneak Peek.” This GBA Prime blog was originally published on July 3, 2015.

A recent paper on the cost-effectiveness of weatherization work has received much more attention in the popular press than have similar studies in the past. The researchers concluded that weatherization measures performed at five nonprofit community action agencies in Michigan weren’t cost-effective. Newspaper headline writers have had a field day, trumpeting generalizations that aren’t supported by the limited data collected by researchers.

Water causes all kinds of problems for buildings. When rain leaks into walls through a poorly flashed window sill, or when the humidity in summer air contacts a cold water pipe and condenses, mold or rot can easily develop.

One possible way to handle localized leaks or intermittent humidity spikes is to build with hygroscopic materials that provide hygric buffering and hygric redistribution. To say the same thing in simpler terms: installing building materials that can absorb and store water may help handle moisture events.

Green builders usually specify high-performance windows and above-code levels of insulation, while striving to reduce air leaks in their homes. As a result of these efforts, most green homes have relatively low heating and cooling loads.

Building scientists talk about several different moisture transport mechanisms. Most of these mechanisms — for example, water entry due to a roof leak — are easy to understand. Other transport mechanisms, like vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. , aren't quite as intuitive.

First, some basic definitions. Water vapor is water in a gaseous state — that is, water that has evaporated. It is invisible.

Water vapor diffusion is the movement of water vapor through vapor-permeable materials. Vapor diffusion happens through a solid material even when the material has no holes.

In most climate zones, achieving the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard requires more expensive windows, higher insulation levels, and greater attention to air-sealing details than less stringent construction practices. It should come as no surprise that homes with these features cost more to build than homes complying with easier-to-meet standards.

A few weeks ago, my wife and I went on a short hike with our college-age son. As the three of us drove to the trailhead in Norwich, Vermont, we passed a construction site. “Looks like a zero-energy house,” I observed. The sign out front read, “Prudent Living Homes.” I decided to get more information on the house and return later to try to talk with the builder.

I called up Prudent Living Homes, and the owner of the company, Paul Biebel, agreed to meet me at the site. When I showed up a few days later, two carpenters, Gary Castellini and Maynard White, were working on exterior details.

Most green builders who need a layer of horizontal insulation under a concrete slab specify expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.), an affordable product that performs well in this application. If a builder specifies high-density EPS rated for below-grade use, the product is very durable.

That said, many green builders don’t like EPS. Some object to the fact that polystyrene is made from petroleum, while others worry about possible health problems associated with the brominated flame retardants that polystyrene manufacturers add to EPS.

Fully aware that I am engaging in gross oversimplification, I’m going to offer a cartoon version of the History of Vapor Barriers. (I’m not a cartoonist, though, so someone else will have to make the drawings.) Here goes:

Panel 1: In the late 1940s, residential building codes in the U.S. began requiring the installation of vapor barriers on the interior side of walls and ceilings. These requirements had complicated historical origins but were not based on credible building science.