UPDATED on July 20, 2015
What’s the best way to install foam insulation on the outside of a wall?
Although GBA has published many articles and videos on the topic, we continue to receive frequent questions from readers asking how to install rigid foam sheathing on exterior walls — so it’s time to provide a primer on the topic.
There are three major types of rigid foam: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate. All brands of EPS and XPS sold in the U.S. include a brominated flame retardant — hexabromocyclododecane (HBCD) — that many environmentalists find worrisome.
Moreover, most green builders avoid using XPS because it is manufactured with a blowing agent with a very high global warming potential.
That leaves polyiso, which enjoys a reputation as the most environmentally friendly type of rigid foam insulation. However, polyiso doesn’t perform very well at cold temperatures. (For more information on this issue, see In Cold Climates, R-5 Foam Beats R-6.) The bottom line: green builders in hot climates tend to prefer polyiso, while green builders in cold climates tend to prefer EPS.
If you prefer not to use rigid foam, you can use mineral wool panels instead; for more information on this option, see Installing Mineral Wool Insulation Over Exterior Wall Sheathing.
If you are building a new home, there are two basic ways to install rigid foam on the exterior of a wall: The foam can either be attached directly to the studs, or the walls can be conventionally sheathed with OSB or plywood before the foam is installed.
If you decide to omit some or all of the OSB or plywood sheathing, you’ll need to come up with a plan to brace your walls. There are at least four ways to brace a foam-sheathed wall: