Rigid Foam Board Insulation

There are three common rigid foams and they are not interchangeable.

EPS (expanded polystyrene) is the white beadboard. It has the lowest R-value per inch of the three, around R-4, but it is the least expensive, it does not lose R-value over time, and it handles ground contact and moisture well when you buy a dense enough grade. It is our default for under-slab and for foundation walls.

XPS (extruded polystyrene) is the pink, blue, or green board. It runs around R-5 per inch, is stronger in compression, and absorbs very little water. It is the traditional choice for below-grade and slab edges. Newer XPS formulations are a bit lower in R-value than the old ones as the blowing agents have changed, so we check the current label rather than assuming.

Polyiso (polyisocyanurate) is the foil-faced board. It has the highest R-value per inch at room temperature, often R-6 or better, and is common on commercial roofs and above-grade walls. The catch for Williston: polyiso loses R-value in deep cold. The blowing agent inside it condenses when the board gets well below freezing, and its performance drops right when you need it most. That makes it a poor fit for the outer layer of a wall here and a bad idea below grade, where it also takes on water. We use EPS or XPS below grade and at the cold face of a wall, and reserve polyiso for spots where it stays warmer, like the inner layer of a thick exterior stack or an interior basement wall.

For a new house or a re-side, we fasten foam board over the sheathing before the housewrap or, depending on the system, over the housewrap with the seams taped as the drainage plane. One inch of XPS gets you the R-5ci the code asks for. Going to 1.5 or 2 inches adds margin and keeps the sheathing warmer through a Williston cold snap, which matters for moisture because the inside face of the sheathing is where condensation forms when warm indoor air leaks into the wall.

Thicker foam changes the details. Window and door openings need extension jambs or bucks, siding fasteners need to reach through the foam into framing, and the bottom of the foam needs a drip edge and insect screen. We work those out with the builder or siding contractor ahead of time rather than finding out at the trim stage. Post-frame and metal buildings have their own version of this, where board goes between the girts and the steel skin or on the inside face of the girts.

Below grade, rigid foam is usually the best answer. The Climate Zone 7 minimum is R-15 for a basement wall and R-10 for slab edges and crawl space walls. On the outside of a foundation, we set XPS or dense EPS against the concrete from the footing to grade, protect the above-grade portion with a coating or cover board, and let the backfill hold it. On the inside, board goes against the concrete, seams get taped, and a framed wall or furring and drywall goes over it as the required thermal barrier.

The slab edge is the detail that gets skipped most in this region and it is one of the worst heat leaks in a slab-on-grade house or a heated shop. Concrete at the perimeter sits right against frozen ground and outdoor air. Two inches of XPS or EPS at the edge, run down along the stem wall or turned under the slab, keeps the floor near the walls from being cold and keeps frost from working under the footing.

Under a heated slab, especially one with in-floor radiant, we lay EPS across the whole area before the vapor barrier and the pour. Without it, a good share of the boiler's heat goes into the ground.

Foam board only works as an air barrier when the joints are sealed. We stagger the seams, butt the boards tight, and tape every joint with a tape made for foam, pressed down with a roller so it bonds. Where two layers go on, we offset the seams between layers. Around windows, penetrations, and at the top and bottom of the wall, the foam gets foamed or flashed to the adjacent material so there is no path for air.

Below grade, taped seams matter less for air and more for keeping water from getting behind the board. We keep the board tight to the concrete and finish the top edge so water running down the wall goes over the foam, not behind it. Small details, but they are what separate a foam layer that performs from one that is just expensive sheathing.

Board type and thickness set the material price, with polyiso and XPS above EPS per sheet and every added inch adding cost. Square footage is the multiplier. Where the foam goes is the next factor. Under-slab work on open ground is fast. Exterior walls take fastening, taping, and coordination with the siding. Foundation exteriors need excavation access and protection above grade.

Details add time: number of windows and doors to buck out, penetrations to seal, corners, and whether we are doing two layers with staggered seams. If we are also furring or framing over the foam inside a basement, that is its own line. We lay it all out on the quote so the board, the labor, and the detailing are visible separately.