Warning: This post is more technical than most others.
When I first set out to design our house, I didn’t think very much about construction technology beyond trying to assure myself that what I designed was feasible to construct without extreme costs. (Some of my early designs would have failed that test.) I set out to learn enough about house construction to assure myself that I wasn’t specifying something stupid. I discovered the very interesting field of building science. At websites like Green Building Advisor and Building Science Corporation, I found smart people debating how to go beyond stupidity avoidance to make houses more comfortable, more durable, and more energy efficient. (Stupidity avoidance continues to be an important aspect of building science.) Although builders in my region have adopted standard methods that are used on the vast majority of houses, there are alternatives to these standard methods. Should I be using these alternatives? After considerable research, I have decided to depart from standard practice for my region in a couple ways. One such departure is adding a layer of exterior rigid foam.
The primary functions of the exterior walls are separating the interior from the exterior and holding up the roof and higher floors. Ensuring that the walls will continue to perform these functions well for many years requires a structure that avoids rot, by keeping any wood parts from being warm and wet simultaneously. A second function of the exterior walls is to limit heating and cooling loads. This function requires avoiding the flow of air between inside and outside and resisting heat conduction. The traditional approach to resisting heat conduction is to fill the areas between the studs with insulation. Once this is done, the studs themselves become a “thermal bridge” providing a path for heat to flow around the insulation.
A traditional residential wall in Southeast Michigan has wood studs that carry the vertical loads, OSB sheathing to provide rigidity, housewrap to keep out bulk water, and some form of cladding like siding or brick veneer. There is typically cellulose or fiberglass insulation between the studs and drywall on the interior. Some air from inside the house typically gets past the drywall into the wall cavity. In Winter, water vapor in that air is adsorbed by the OSB sheathing which is cold. Hopefully, wetness in the sheathing dries out by vapor diffusion in Spring before the wall gets warm enough for rot to form. To reduce heat conduction, 2x4 studs have given way to 2x6 studs, increasing the effective R-value of the wall from about 9 to about 14. Although the nominal R-value of the insulation is greater than that, the effective R-value of the wall is reduced by thermal bridging through the studs.
A number of alternative wall stuctures are in use. One approach to reducing the thermal bridging of the studs is to increase the space between the studs from 16 inches to 24 inches. Another approach is a double stud wall which has an inner set of studs and an outer set of studs separated by a few inches. Usually only one set of studs supports the weight of the roof and upper floors. Taping the seams between OSB panels reduces air leakage through the wall. Some builders use a layer of spray foam between studs to reduce air leakage. Structural Insulated Panels (SIPs) rely on OSB layers bonded to each side of a rigid foam layer to support the structure, eliminating most of the studs (except where SIPs are joined to one another). Insulated Concrete Forms (ICFS) use continuous rigid foam layers on the outside and inside to provide R-value and use steel reinforced concrete in the middle to provide the structure and the air barrier. By eliminating wood, ICFs avoid wood rot. Each of these alternatives have advantages and disadvantages.
After studying alternatives, I decided on the wall structure illustrated above. Two inches of continuous rigid foam insulation is attached to the outside of the OSB sheathing. I selecting Platinum Insulfoam EPS which provides R-5 per inch. Since this is not reduced by thermal bridging, the effective R-value of the wall increased from about 14 to about 24. One drawback of rigid foam is that it dramatically limits drying to the outside in Spring. Therefore, it is important to use a thick enough layer of rigid foam to keep the sheathing warmer than the dew point during the winter such that the sheathing does not adsorb water. In climate zone 5, where this house is located, R-7.5 is enough to accomplish that.
One of the most problematic air leakage areas is the rim joist. To address this, peel and stick flashing is added on the outside and spray foam is applied to the interior of the rim joist. As shown in the picture below, spray foam is also used at the top of the walls to join the OSB sheathing, which is the air barrier for the walls, to the drywall which forms the air barrier for the ceiling.
On the outside of the rigid foam, 3/4 inch furring strips create an air gap called a rain screen. The air gap allows the cladding to dry from both sides and prevents vapor that may be driven out of the cladding by sunshine from going into the wall structure. Screws through the furring strips into the studs secure the rigid foam to the wall. Some areas of the house will have brick veneer cladding while other areas will have Hardi-Board fiber-cement siding.
In the basement, two inches of foam separate the poured concrete foundation wall from a framed wall, keeping all of the wood above the dew point. I looked into alternatives to poured concrete basement walls, such as Superior Walls. The Superior Wall system uses wall sections pre-manufactured in a factory and assembled on site. The wall sections include insulation, which would have allowed elimination of the interior framed wall. Although there are some advantages to the Superior Walls system, I concluded that those advantages were not sufficient to depart from the system my builder is familiar with.