Your basement needs wall insulation. Eight inches of uninsulated concrete has an R-value of 1.35ea, meaning a tremendous amount of heat is lost through uninsulated concrete walls.
Any basement used as a living space–or at least more than a large storage room–should be well insulated for comfort and energy savings. Many local jurisdictions have building codes that require basement insulation of various thicknesses.
Insulating your basement walls is an interesting DIY project. Some insulation requires special equipment and training. But all types of basement wall insulation will add more comfort to your home and lead to heat savings.
Benefits of Insulation Basement Walls
All basement walls should be insulated–for one or more reasons.
- Living Space. You are paying for the entire house. You may as well live in the whole house.
- Comfort. Your family will enjoy the basement entertainment center more without wearing coats and boots.
- Savings. You will save on heating and air conditioning costs.
- Resale Value. Even if your basement is not finished, an insulated basement will add value to your home.
Basement Wall Insulation Is Required By the IRC
According to the International Residential Code (IRC), all finished basements in climate zones #3 and higher require basement insulation. (Table 1102.1.3)
- Zone 3: R-5
- Zone 4 (not including Marine Zone 4): R-10
- Zones 5, 6, 7, 8 (and Marine Zone 4): R-15
Note: Section 1102.2.8 provides the exception to the code. If your basement is unfinished and isolated with insulation from the main floor (including things like the underside of stairs), the walls can remain bare.
Some local building codes require all basement walls–finished or unfinished–to be insulated. For example, I live in an area where all new construction basement walls must be insulated to an R-12 minimum.
Foam – The Best Basement Wall Insulation
The best concrete wall insulation involves foam–either foam board or spray foam. At R-5 to the inch, closed cell foam provides the best return on your investment. Both types of foam act as a vapor barrier provided that they are a minimum of 2” thick and all seams and holes are filled and sealed.
Foam is most efficient when applied directly onto the concrete wall. The International Residential Code (IRC) Section R316 requires foam to be separated from building interiors by a thermal barrier–such as ½” drywall minimum. The drywall is necessary because of the foam flame spread. The code has some exceptions–sections R316.5 and R316.6–that don’t apply to walls.
Before installing any insulation on your basement walls, find and seal leaks. Water leaking behind the wall and insulation may go undiscovered for a while–leading to an expensive, time-consuming repair.
Rigid Foam Insulation
Rigid foam board insulation is very versatile. You can glue it on the concrete, fit it between the studs of a framed wall, nail it on the face of framing, or even nail it on top of the drywall. Three of the most popular and effective rigid foam boards are:
- EPS. (Expanded Polystyrene) High density. R4.2 per inch.
- XPS. (Extruded Polystyrene) High density. R5.2 per inch.
- ISO. (Polyisocyanurate) Foil-faced high density. R6.8 per inch. It may degrade to R5.5 within 5 – 10 years because of off-gassing.
Overall, XPS is the best choice–because of its consistent R values and reasonable pricing.
Installing Rigid Foam Board Over Concrete
Applying foam to concrete walls is quick and easy. Some of the products you’ll need for a successful installation include Loctite PL300 Foam Board Adhesive, Quad Window and Door Spray Foam, and Foam Joint Tape.
Here’s how to install rigid foam board over concrete walls:
- Clean the Wall. Use a broom to remove dust and lumps. Do not wet the walls.
- Install Vertically. Most basement walls are less than 8’ high, so install sheets of foam board vertically. Apply glue in ¼” beads to the back of the boards and then press them into place.
- Foam Joints and Penetrations. Seal all joints and penetrations with window and door spray foam.
- Tape Joints. Seal all joints and penetrations with Foam Joint Tape.
Note: PL300 is low VOC (Volatile Organic Compound) glue but it will off-gas a certain amount. Although closed-cell foam is a benign product, you may want to use safety equipment such as an N95 mask or respirator, gloves, and a hazmat-type coverall.
Consider using some mechanical fasteners to ensure the foam stays where you put it while the glue dries. These are the best options:
- Self-Impaling Pins. You can glue these pins to the wall and press foam panels into them. Then a large washer slides onto the nail, and the excess is cut off.
- Hilti Powder-Actuated Gun. Available with special foam-holding pins.
- Self-Tapping Concrete Screws. Need to predrill holes to use these.
After you’ve installed the foam, construct a wall to hang drywall on. You can use 2 x 4s, which allows you to add fiberglass batts for more insulation. Plus, 2 x 4s will stay straight. A 1 x 2 may be too flimsy, and some 2 x 2s tend to warp.
If you use 2 x 4 framing, you can add even more insulation by installing fiberglass batts before drywalling.
Note: Any wood in contact with concrete must be pressure treated.
Installing Rigid Foam Board Over Existing Framing
Your basement may have existing framed walls. To improve the insulations value, you can:
- Remove and replace the insulation. Remove the entire structure and insulate the wall as described above.
- Remove the drywall. Remove the existing drywall, fit rigid foam against the concrete between the studs, slide foam pieces behind the studs, and seal all gaps. Add batt insulation if desired, and then install new drywall.
- Cover it. Covering the walls is the easiest option. You can nail rigid foam boards over the existing drywall and then add another layer of drywall on top of the foam.
Spray Foam Insulation
When done properly, spray foam insulation provides a seamless blanket on any surface. It covers imperfections on the wall and pipes and other services installed against or next to the wall.
Although you can buy the product and rent equipment (masterpkg.com), foaming an entire basement is best left to the experts. Improper mixing and application can cause off-gassing of odors and toxins–making everyone in the house uncomfortable and possibly very ill.
Benefits of spray foam insulation include:
- Elimination of Air Leaks. Spray foam on basement walls can eliminate air leakage by 15% – 20%.
- Reduced Condensation. Foaming basement walls change the dew point where warm air meets the cool wall–reducing condensation more than other insulation materials.
- Seal. Complete seal resists conductive heat transfer from the warm to the cold side of walls.
- Mold Resistant. The chemical composition of spray foam is not a friendly environment for mold growth.
The only downside to insulating basement walls with foam insulation is the loss of floor space. When done correctly, foam insulation will be approximately 6” thick–including foam, framing, and drywall. So in a 1000-square-foot basement (nominally 25’ x 40’), you will lose close to 65 square feet–6.5% of the total floor area.