how to install foam board insulation on interior walls

Musings of an Energy Nerd

Walls With Interior Rigid Foam

When remodeling an older home, it sometimes makes sense to install rigid foam on the interior side of exterior walls

**Some remodelers install rigid foam on the interior side of exterior walls.** While the best place to install rigid foam is usually on the exterior side of the wall, that approach often can't be used on an existing house.
Image Credit: Image #1: Iso R Plus

There are two main ways of reducing thermal bridging through studs: you can build a double-stud wall, or you can install a continuous layer of rigid insulation on one side of the wall.

Most builders who install a continuous layer of rigid insulation use rigid foam (polyisocyanurate, expanded polystyrene, or extruded polystyrene); a small minority of builders use semi-rigid panels of mineral wool.

Builders who install rigid foam on the walls of a new building usually install the foam on the exterior side of the wall. There are several reasons for this:

The exterior side of the wall has fewer interruptions (for example, electrical boxes and partition intersections), so it's much easier to install an uninterrupted layer of insulation on the exterior than the interior.
Exterior rigid foam is a good way to insulate rim joists.
Exterior rigid foam helps keep wall sheathing warm and dry.

Unlike new home builders, remodelers can't start with a blank slate. If a remodeler wants to reduce thermal bridging through studs, the exterior foam option is usually prohibitively expensive — especially if the siding is in good shape. So many remodelers ask, "Why can't I put the rigid foam on the interior side of the wall?"

The answer is, you can.

Back to the 1980s…

In the 1980s, most cold-climate builders were taught that walls needed an interior vapor retarder, and needed to be able to dry to the exterior. When a few pioneers started installing exterior rigid foam, some people thought the practice was risky.

"Rigid foam is a vapor retarder, so it belongs on the interior," some old-time builders opined. "Exterior rigid foam is risky, unless you live in Florida. It's a wrong-side vapor barrier."

Eventually, researchers proved that these builders were wrong. (To learn more about the history…

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