High-Performance Walls: Page 6 of 6

Foam ICFs. Several kinds of foam are used to make most concrete forms, including EPS (the most popular), XPS, and expanded polyurethane. A typical EPS form consists of two parallel sheets of foam separated by plastic or metal webbing. Blocks are stacked together to form walls. Builders add steel rebar and then pour concrete into the forms. Foam ICFs are available in fully assembled blocks or as ready-to-assemble components that are shipped flat.

The R-value of the blocks depends on foam type and foam thickness. EPS, for example, is about R-4 per inch, while XPS is rated about R-5 per inch. Polyurethane foam used in ICFs is about R-6 per inch. The concrete inside an ICF wall contributes virtually nothing to its insulating qualities. Forms are available in a variety of widths to produce concrete cores from 4 inches to 24 inches or more. Thicker walls are stronger because of the increased concrete content, but are not necessarily better thermal insulators.

ICF construction has some advantages—high strength and low rates of air infiltration are two important ones—but overall wall R-values aren’t as high as with some of the alternatives. In fact, the material used to make some types of ICFs—polystyrene beads and cement or wood chips and cement—have lower per-inch R-values than rigid foam, sprayed-in foam, or even dense-pack cellulose. That makes it difficult to get the R-values that high-performance buildings often specify—R-40 in the walls and R-60 or more in the ceiling. As a result, builders might consider ICFs for the foundation but choose another option for above-grade walls, such as wood-frame construction with both cavity insulation and a layer of rigid insulation over the sheathing. ICFs made with EPS foam are relatively effective thermal insulators, but it would still take 10 inches of foam— 5 inches on each side of the concrete core—to equal R-40.  

Insulation
On the Outside

One of the most radical high-performance wall systems is the “pressure equalized rain screen insulated structure technique” (PERSIST), developed by the National Research Council of Canada in the 1960s. Walls are framed with 2-by-4s without any roof overhangs, sheathed with plywood or OSB, and covered with a peel-and-stick membrane, such as Grace Ice & Water Shield (a rubberized asphalt adhesive backed by a layer of high-density cross-laminated polyethylene). Then the building is wrapped in one or more layers of rigid foam insulation, which in very cold regions could be as much as 8 inches thick. Using XPS foam at that thickness would mean walls of R-40. The membrane provides an effective air and vapor barrier and, because it’s installed on the warm side of the insulation, does not foster condensation inside the walls.

Builders apply strapping over the foam on the walls and attach the siding to the strapping. This creates what’s called a “rain screen.” Any water that is forced through the siding (in a driving rain, for example) drains away, and the back of the siding dries more effectively, which extends siding life. On the roof, sleepers—2-by-4s on the flat—are installed over the rafters, followed by a second layer of sheathing. Rakes and eaves are applied.

Houses built this way are extremely resistant to air leaks, with greatly reduced thermal bridging, and the insulation-free stud cavities are excellent places to run wiring or plumbing without interrupting the air barrier. On the down side, all that membrane and the layers of rigid foam are expensive—and there’s extra time involved in detailing the doors, windows, and roof overhangs.

The PERSIST is effective in all climate zones, not just the far north. A similar approach called residential exterior membrane outside-insulation technique (REMOTE) uses almost the same wall system but a more conventional unconditioned attic. As explained by the Cold Climate Housing Research Center (CCHRC) in Fairbanks, Alaska, REMOTE eliminates the need to build a second roof. It also calls for the addition of cavity insulation in the framed wall, with one-third of total wall insulation on the warm side of the membrane and two-thirds on the outside. (Note that this ratio of inside to outside insulation would be dictated by climate to keep the dew point to the exterior of the vapor barrier).

Given the design, air leakage rates can be very low. A blower-door test of a REMOTE house by the CCHRC showed air tightness at 0.4 ACH50 (air changes per hour at 50 pascals of pressure, a standard industry measure). This is an extremely low number, and about one-fifth the leakage of a conventionally built comparison house. In the CCHRC house, building costs were about $0.85 more per square foot of heated space than a conventionally built house, including labor. Keep in mind that cost differences depend on a variety of factors, and that this side-by-side comparison took place more than 10 years ago, so costs are out of date.

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Scott Gibson co-authored Green from the Ground Up and Toward a Zero Energy Home (The Taunton Press). He is a contributing writer at Fine Homebuilding magazine, and at GreenBuildingAdvisor.com. Scott and his wife live in southern Maine.