How to Install Foam Board Insulation on Interior Walls

What is Foam Board Insulation?

Foam board insulation is a product made from polystyrene and polyurethane used to insulate interiors from exterior temperatures. Generally speaking, foam board is most often used to insulate areas of a structure unfriendly to batts or blown insulation. An example might be an unfinished basement where the walls are made from cement blocks. Foam board insulation is anywhere from 1”-4” thick, which often provides an r-value of approximately 2 to 8. Some versions can also serve as a vapor barrier, which is vitally important where moisture is prevalent.

Where Would I Use Foam Board Insulation?

How Do I Install Foam Board Insulation?

Foam board insulation can be installed in a number of ways, depending on where it is going. For our purposes today, we will assume that the surface receiving the foam board is accessible and free of any fasteners. Next we will discuss the typical methods the pros use to install foam board insulation, and the tools they use to perform the project safely and successfully.

What Tools Do I Need to Work with Foam Board Insulation?

Working with foam board insulation requires only a few hand tools. These include (but are not limited to:

• Sharp Utility Knife
• Measuring Tape
• Straightedge
• Hammer
• Framing Square
• Marker
• Caulk gun (if using construction adhesive)

Wood Framed Interior Walls (Surface Mount)

Step 1 Measure and Cut

The first step in installing foam board insulation is to decide how much you’ll need for the job. To do this, you’ll need a tape measure and a little math. Most foam board insulation comes in sheets (also known as boards) 4’ wide by 8’ long, which equates to 32 square feet, although some versions may have 40 square feet. If the wall is 8’ high and the wall studs are on 16” centers, a sheet can be installed without additional trimming. If the walls are more (or less) that 8’ tall, at least a few boards will need to be adjusted.

Generally speaking, the boards will only have to be trimmed lengthwise, because wall studs are typically on either 12”, 16”, or 24” centers. Since each of these spacings is evenly divisible into four feet, the edge of each sheet should always fall on a stud. However, if the wall is out of plumb or irregularly shaped, each sheet can be trimmed with a sharp utility knife.

Step 2 Hang the Foam Boards

As mentioned previously, hanging the foam boards can be done in a number of ways. In this case, since the material is being installed on stud walls any of the common methods can be used, but button cap nails and/or construction adhesive are the most common. In this example, we’ll use button cap nails. Starting in one corner, the board is set vertically in place and one button cap nail is driven into either top corner. The board is then pivoted as needed until the board is plumb (as measured with a level) with the wall stud it will be attached to. 

If the spacing is correct, the leading edge of the foam board should fall in the center of the stud, allowing two adjacent sheets to be connected to the same stud. By attaching two boards to the same stud, the boards are tied together, adding lateral strength to the wall. It is important to install the boards vertically, because in most cases the drywall will be hung horizontally. If the insulation and the drywall were hung in the same configuration, eventually the joints would intersect and likely cause a crack later on.

Step 3 Seal the Joints

Sealing of the joints is considered optional in an interior installation, but it is still a good idea. A special joint tape designed for use with foam boards is used for this purpose. After the button cap nails are fully seated, the tape (which is usually about 2” wide) is applied over the nails and joint.

Wood Framed Interior Walls (Inside the Wall)

Step 1 Measure and Cut

Installing foam board insulation inside the wall is usually done with 2” x 6” walls, which are thicker than the typical 2” x 4” wall. In most cases, this is because foam board does not have the same insulative value by volume as expanding closed cell foam. As such, 2” x 4” walls are generally not deep enough to allow for the r-value required by most building codes. 

The first step is to measure the spacing between the studs. Although most 2” x 6” walls are on 24” centers, some will be on 16”, or even 12” centers. The important part of this step is to measure inside to inside the studs, not outside to outside. For example, the space between studs on 24” centers is actually 21”, so this becomes the width of the foam board. In most situations, up to five layers of foam board are then glued together with construction adhesive and the entire block is placed between the stud cavity and glued into place.

Masonry/Cement Block Walls

Step 1 Decide On a Method

When installing foam board insulation on a block wall, the basic measuring and calculating steps are the same as with a stud wall. The real contrast is in how the foam board insulation is attached. Foam board is usually attached to furring strips, which themselves have been attached to the block wall. It is generally not recommended to glue the foam board insulation directly to the block. In most applications, this is because the insulation will be more effective when an air gap exists between the wall and the board. 

Step 2 Mount the Furring Strips

The pros will usually install 1” x 3”x ¾” furring strips with a pneumatic nail gun, using masonry nails. These furring strips are placed on consistent centers just like wall studs, and run from ceiling to floor. The foam board can then be attached to the furring strip using an appropriate nail, or construction adhesive. Using this method provides for a ¾” gap between the block foundation wall and the foam board, which prevents moisture from the block coming into contact with the foam. Air trapped in the gap also adds insulative value, making the insulation that much more effective.