Rigid Foam Insulation Between Studs

Installing rigid foam board insulation between studs is not usually a typical type of insulating application. But occasionally it is the best option available.

Why Install Rigid Foam Between Studs

Rigid foam insulation is often used to insulate concrete basement walls. On the outside below grade because of its moisture-resistant properties. It is used on the inside because it provides a high R-value per inch and takes up less floor space. Rigid foam insulation also acts as a vapor barrier if it is two inches thick.

Rigid Foam Insulation Between Studs

When applying rigid foam directly to concrete, framing is added after to accommodate wiring and drywall. Most people do not want to tear out framing if it is already in place. Installing fiberglass batt insulation between the stud and against concrete almost always leads to moisture being absorbed by the fiberglass. Wet fiberglass has no insulation value and will not dry out in an enclosed stud cavity.

You are left with limited options when insulating. One is to have a contractor spray foam the walls. This is an expensive and potentially messy operation. Or you can install rigid foam insulation between the studs. This is a relatively inexpensive DIY project when compared to spray foam. It can be time-consuming when installed properly.

Rigid foam board can also be used between the studs to insulate the above-grade walls of the house. It is seldom used for this purpose. Many other less expensive options are available including fiberglass batts, mineral wool batts, and cellulose loose-fill.

Rigid insulation between rafters is a popular option when insulating vaulted ceilings. Rigid board can also be installed between studs of interior walls for soundproofing. For bathrooms, bedrooms, and home theaters.

Types of Rigid Foam Insulation

There are three types of rigid foam board insulation commonly used to insulate between the studs. All three provide good R-value and are easy to install. They can all be glued to the wall or friction fit between studs.

Expanded Polystyrene (EPS). R-3.6 – R-3.8 per inch. Usually white in color. Often called beadboard.
Extruded Polystyrene (XPS). R-5.0 per inch. Blue, green, or pink in color. Commonly called Styrofoam which is a Dow Chemical trade name for their blue product.
Polyisocyanurate (ISO). R-6.2 – R-6.5 per inch. Faced with reflective foil or kraft paper. Reflective foil-type insulation can add an extra R-1.0 to the R-value.

All three products are available in many thicknesses–allowing for the exact thickness if you want to fill the entire stud cavity. For instance: if the framing is 2 x 3 adding a layer of ½” foam to 2” foam will bring the board flush to the inside of the studs.

Rigid Foam Insulation and Water

All rigid foam boards are considered waterproof and moisture-resistant. They are also mold and mildew-resistant. Mold and mildew can grow on the surface in the right conditions but will not enter the foam. These qualities make foam a better choice than fiberglass.

Pros and Cons of Rigid Foam Insulation

As with most products, there are good qualities and some not-so-good qualities. Rigid foam boards between studs are no different.

Pros:

More affordable than spray foam insulation.
Moisture resistant.
Mold and mildew resistant.
Better R-value per inch than fiberglass.

Cons:

Installation requires more precision.
Can be difficult to seal gaps.
More expensive than fiberglass.

How to Install Rigid Foam Boards Between Studs

Before starting, decide on the R-value you want to achieve and the type of product that will accomplish your needs. Foam boards are available in multiple sizes including 2 x 8 and 4 x 8 among others. Choose the size that will produce the most efficient coverage and fewest joins.

Measure the Area

Measure the entire area to be insulated. Do not try to be too exact by removing stud widths from the calculations. There will be waste. For example: walls framed at 24” on center have cavities of 22 ½”. You will get two pieces out of a 48” wide board–leaving a 3” strip that probably will not be used.

Measure the Cavities

A well-framed wall should have fairly consistent-sized stud cavities. Measure more than a couple to check. If they vary, you might have to measure and keep a list of each individually. Measure the top, middle, and bottom. Some of the studs may have warped after being installed and rigid foam does not compress or stretch.

Cut the Foam

Rigid foam boards can be cut with a circular saw, a table saw, an ordinary hand saw, or a utility knife. Using a drywall square or straight edge draw a line with a small point permanent marker. A chalk line also works well and is quicker.

The individual pieces should fit snugly but not tight. Too tight and the edges may break as they are being forced into place. One way to avoid this problem is to cut on the “wrong” side of the line. This will make them up to ⅛” narrower overall–giving you a little wiggle room.

Install the Pieces

Install the cut board into each cavity. If they are too loose you may want to recut some as required. Or apply a few lines of foam board adhesive to the backs. (Certain types of adhesives will melt the foam and not work.)

If a piece is close to fitting but a little too snug, use a wooden block and hammer to force it in. Only tap along the edges–not in the center. The board may break.

Seal any gaps between the boards and studs. Acoustic caulking does not dry and crack. It will remain adhered to both the foam and studs as they expand and contract. Low-expanding spray foam is another option but if a stud warps too much, the foam will crack and leave a gap.