Rigid foam insulation is common for insulating interior and exterior concrete basement walls. On interior walls, contractors apply it directly to the concrete before framing for drywall and electrical. If your basement has framing but no insulation, you can add rigid foam board insulation between studs.
Why Install Rigid Foam Between Studs
Rigid foam board works on exterior below-grade applications because of its moisture resistance and on interior walls since 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.
Before a contractor adds framing to the basement wall, they’ll apply rigid foam insulation. Rigid foam is better than fiberglass insulation for a basement since fiberglass can absorb moisture and lose its insulating properties.
If your walls are already framed, you’re left with limited insulation options. You can have a contractor spray foam the walls, which is expensive and often messy, or you can install rigid foam insulation between the studs. Installing rigid foam between studs is time-consuming but an inexpensive DIY project.
You can also use rigid foam between the studs to insulate the above-grade walls of the house, but it’s seldom used for this purpose. 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. You can also install rigid boards between studs of interior walls for soundproofing bathrooms, bedrooms, and home theaters.
Types of Rigid Foam Insulation
There are three types of rigid foam board insulation for insulating between the studs. All three provide good R-value and are easy to install. You can glue them to the wall or friction-fit them between studs.
- Expanded Polystyrene (EPS). R-3.6 – R-3.8 per inch. Usually white in color and 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 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 waterproof, moisture-resistant, 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.
- More affordable than spray foam insulation.
- Moisture resistant.
- Mold and mildew resistant.
- Better R-value per inch than fiberglass.
- 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.
Step 1: Measure the Area
Measure the entire area you’ll be insulating. Since there will be a waste, don’t be too exact with your measurements. 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.
Step 2: Measure the Cavities
A well-framed wall should have consistent-sized stud cavities. Measure more than a couple to check. If they vary, measure and keep a list of each individually. Measure the top, middle, and bottom. Some of the studs may have warped after installation, and rigid foam does not compress or stretch.
Step 3: Cut the Foam
Cut the rigid foam boards 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. If it’s too tight, the edges may break as you force them 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.
Step 4: Install the Pieces
Install the cut board into each cavity. If they are too loose, recut 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 or the board may break.
Seal any gaps between the boards and studs with acoustic caulking, which doesn’t 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.
Why not just install the foam over top of the studs?
It’s acceptable to install rigid foam board over the studs, but you will lose floor space.
What can I do if the walls are already finished, and I don’t want to remove drywall?
You can add rigid foam insulation over the drywall. Then add more drywall. Again, keep in mind that you are reducing floor space.