Adding exterior foam to walls works well for new construction. As long as you meet local wind and earthquake codes, it’s usually possible to build a foam-sheathed wall without structural oriented strand board (OSB) or plywood sheathing. Foam-sheathed walls are braced using one of four methods:
• Diagonal 1×4 let-in braces.
• Diagonal T-profile steel strapping, such as Simpson TWB.
• Inset shear panels.
• A few strategically placed pieces of OSB (they are usually installed at corners).
Of course, you should check with an engineer and your local building official before finalizing your wall-bracing plan.
Exterior foam sheathing is often installed on existing homes as an energy-saving
retrofit when new roofing or siding is needed. You can install rigid foam on the inside of a wall as well, but adding exterior foam increases the R-value of a wall or roof without eating up interior living space.
Installed on the roof, exterior foam makes the living space below more comfortable and reduces the likelihood of ice dams.
The illustration on the facing page shows two layers of OSB or plywood roof sheathing: a lower layer conventionally nailed to the rafters and an upper layer installed as a nailing surface for the roofing material on top of the rigid foam. The type of fasteners used and the way they need to be spaced for the top layer of sheathing depend on the pitch of the roof and roof loads, particularly wind and snow loads. It’s fairly easy to find screws with a pullout-resistance rating exceeding 400 lb., even when they are secured just to plywood or OSB sheathing. The fastener rating increases if the screws are driven into the rafters. (Fastener sources include Wind-lock® and FastenMaster®, which manufactures HeadLok® and OlyLog® screws.)
Most foam-sheathed walls include a rain — screen gap between the foam and the siding. After the foam is tacked in place temporarily with a few nails, it is secured in place with vertical 1×3 or 1×4 strapping that is screwed through the foam to the underlying studs. Some siding types, including cedar shingles, may require a drainage mat, kerfed horizontal furring, or an OSB or plywood nailer over the foam.
Does Exterior Foam Create a Wrong-Side Vapor Barrier?
Some builders worry that exterior foam sheathing is a "wrong-side vapor barrier" that can trap moisture in walls. In new construction without interior polyethylene vapor barriers, the worry is baseless. As long as the foam sheathing is thick enough, it will reduce the chance that moisture will accumulate in a wall. By warming the wall cavity, exterior foam eliminates cold surfaces where moisture can condense. Here are the minimum R-values for exterior foam for 2×6 walls:
• R-15 in climate zones 7 and 8
• R-11.25 in climate zone 6
• R-7.5 in climate zone 5
• R-3.75 in climate zone 4
When exterior rigid foam is used as an energy-saving detail in warmer climates, condensation is not a concern, so no minimum R-values apply.
Since exterior foam reduces a wall’s ability to dry to the exterior, foam-sheathed walls should be able to dry to the interior. That means that foam-sheathed walls should never include interior polyethylene or vinyl wallpaper. Painted drywall has a high enough permeance to allow any incidental moisture that enters a wall cavity in the winter to evaporate through the drywall during the summer.
If you are considering installing exterior foam on a house with interior 6-mil polyethylene, proceed with caution. If water ever enters a wall with foam sheathing and interior poly, the wall has a very limited ability to dry. This raises the stakes, and water — management details must be impeccable.
After all the siding has been removed, inspect the existing wall sheathing for stains or moisture damage. If you find either, you’ll need to diagnose the cause and implement remedies. If the existing sheathing is clean, dry, and sound, it’s safe to install exterior wall foam, as long as the new siding is installed with a rain-screen gap and meticulous flashing.
Martin Holladay is a contributing editor to Fine Homebuilding.
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