5 Important Things to Know When Insulating Basement Walls

One of the most important steps to finishing a basement is installing basement wall insulation and basement ceiling insulation. Insulating basement walls keeps heat inside during colder months and prevents heat from entering during warmer months.

Before insulating basement walls, take these five important considerations into account so that your project goes off without a hitch.

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1. Any and all moisture issues should be dealt with before insulating basement walls.

Moisture in the basement is a major problem that needs to be handled before installing any type of insulation. Otherwise, moisture can seep into the insulation, wood, drywall, and other materials, causing irreparable damage. For this reason, conduct a basic dampness test by taping a small piece of plastic sheeting to a bare concrete wall and leaving the plastic patch in place for about 48 hours. After this period of time, check the interior and exterior of the plastic sheet for condensation to determine if water is seeping through the basement walls.

If you find that water is leaking into the basement, you will need to conduct a more thorough investigation to determine the cause. In many cases, the issue is that the gutter system is draining too close to the home’s foundation. Other reasons could include cracks in the foundation; gaps around pipes, conduit, or vents; or flooding in the area. Assess the situation and take proper steps to rectify the issue, and then make sure everything is dry before installing any basement wall insulation.

2. Rigid foam insulation is often the best option for DIYers.

There are several types of insulation materials to choose from when insulating basement walls. Fiberglass insulation is common for other projects, but fiberglass is not the best type of insulation for basement walls. It can hold onto moisture, leading to potential mold, mildew, rot, and water damage.

Spray foam insulation can be an effective option for basements, especially for filling narrow crevices, gaps, and cavities like around light switches. It doesn’t retain moisture and has good sound absorption properties, though spray foam tends to be more costly than other insulation options.

Often, the best insulation for basement walls in terms of moisture resistance, cost, quality, and ease of installation is rigid foam board insulation. It comes in sheets that are easy to measure, cut, and install. The material is also water-resistant, so it’s less likely to retain moisture. However, rigid foam boards are a difficult option for narrow openings. Consider pairing a small amount of spray foam with foam board insulation for a combined approach.

3. Installation methods include furring strips or a standard 2×4 wall system. 

Typically, installing basement wall insulation is done using one of two methods. The first method uses furring strips to secure the insulation to the wall, while the second method involves framing out the wall over the insulation. Be sure to check your local building code to determine if a vapor barrier is also required with either method.

Furring Strips

Furring strips are thin pieces of wood used to mount insulation to walls. First secure the insulation to the concrete wall using foam-compatible adhesive. If the foam board insulation has a built-in pocket to accept a 2-inch by 3-inch furring strip, use this pocket to mount the furring strips directly to the concrete with concrete screws, masonry nails, or spring spikes. However, this will not leave any space for electrical wires to run through the walls.

If you need space for electrical service, build up the furring strips in a grid on the walls. Start with horizontal strips, then lay out vertical furring strips on top of the horizontal strips to create a gap for the wiring. Position furring strips over the insulation and secure them directly to the concrete. Drywall can then be installed over the insulation, using the furring strips to form a positive attachment.

2×4 Framing

2×4 framing is the more common method of installing insulation and finishing basement walls. This method leaves space for electrical and plumbing connections and creates a moisture-free space where extra insulation can be added to increase walls’ R-value.

Start by attaching the insulation to the concrete using foam-compatible adhesive; then measure and cut a piece of pressure-treated lumber to serve as the bottom plate of the frame. Fasten the bottom plate to the concrete with concrete screws, masonry nails, or spring spikes. Once in position, measure, cut, and secure the vertical pieces of the frame, ensuring that insulation sits flat against the wall. Add the top plate and secure it to the ceiling joists. Finish the wall by measuring, cutting, and mounting drywall to the frame.

4. To prevent condensation, all gaps in the insulation should be sealed with tape, caulk, or spray foam.

Small cracks, gaps, or holes in the insulation can create cold spots along the basement wall. When these cold spots come in contact with warm air, condensation can form. While a little condensation one or two times isn’t a huge problem, ongoing or severe cases of condensation can lead to mold growth, rot, and extensive water damage.

Sealing and insulating the basement walls helps to reduce humidity and thermal loss, while preventing moisture from seeping into the home. To avoid condensation formation in the basement, it’s recommended to use sealing tape, caulk, or spray foam insulation to fill any crevices, gaps, or narrow openings that are too small for rigid foam board insulation.

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5. Most building codes require a thermal barrier when insulating basement walls from the interior.

According to the 2021 International Residential Code, unless allowed by sections R316.5 or R316.6, foam plastic insulation (including polyisocyanurate, polystyrene, and polyurethane) must be separated from the building interiors with an approved thermal barrier. A thermal barrier is typically a ½-inch or greater piece of gypsum wallboard, drywall, or some type of equivalent.

While insulating outside of the basement can also help to reduce the flow of heat and sound through the walls, it isn’t cost-effective to dig up your foundation just to install exterior insulation around a subterranean basement. This is especially true given that the effect on energy consumption is about the same, regardless of whether the insulation is installed inside or outside.