Three Ways to Insulate a Basement Wall
For interior foundation insulation, the safest choices include rigid foam board and spray polyurethane foam.
Basement wall insulation can be placed on the exterior side of the wall, on the interior side of the wall, or on both sides of the wall. In this article, I’ll discuss the three most common ways to insulate a basement wall from the interior. (For a comprehensive discussion of basement wall insulation, including exterior basement insulation, see my 2012 article, “How to Insulate a Basement Wall.”)
General principles
Make sure that your basement is dry. Before installing any interior wall insulation, verify that your basement doesn’t have a water-entry problem. For more information on this topic, see “Fixing a Wet Basement.”
R-value minimums. In Climate Zones 3 and higher, basement insulation is required by the 2012, 2015, and 2018 International Residential Codes as follows: at least R-5 in Climate Zone 3, R-10 in Zone 4 (except Marine Zone 4), and R-15 in Marine Zone 4 and Zones 5, 6, 7, and 8.
That said, local codes may differ from these general guidelines, so it’s worth asking your local building department about minimum R-value requirements in your community.
Note that the IRC lists two different R-value requirements for basement walls: a lower number (for example, R-15 in Zone 5) for continuous foam, and a higher number (for example, R-19 in Zone 5) for “cavity insulation”—usually interpreted as fluffy insulation like fiberglass installed between studs. Since it is inadvisable to insulate a basement wall with fluffy insulation like fiberglass unless the wall has first be insulated with a layer of continuous rigid foam or spray foam, it’s generally best to focus on an approach that uses continuous insulation, and to ignore the “cavity insulation” approach.
Insulation choices. On the interior side of a basement wall, all three common types of rigid foam insulation—polyisocyanurate, expanded polystyrene (EPS), or extruded polystyrene (XPS)—perform well. That said, green builders usually avoid the use of XPS, since most brands are manufactured with a blowing agent that has a high global warming potential. (For more information on this issue, see “Choosing Rigid Foam.”) Problematic blowing agents are also used in most brands of closed-cell spray foam, so if you plan to use closed-cell spray foam, seek out a brand of insulation that uses one of the new, more environmentally friendly blowing agents—for example, Demilec’s Heatlok HFO spray foam.
Rim joists. If you are installing interior basement wall insulation, don’t forget to insulate the rim joists. For more information, see “Insulating Rim Joists.”
Avoid the use of polyethylene. Basement wall systems should never include polyethylene sheeting—neither between the concrete and the foam insulation, nor between the gypsum drywall and the insulation. In these locations, polyethylene can trap moisture, leading to mold or rot.
Termites. If you live in an area where termites are a problem, your local building code may require that you leave a 3-in.-high termite-inspection strip of bare concrete near the top of your basement wall. These requirements vary widely from jurisdiction to jurisdiction, so it’s wise to seek local advice on this issue.
Airtightness matters. During the winter, indoor air tends to be warm and humid, while concrete foundation walls tend to be cold, setting up ideal conditions for potential condensation. You can limit the chance of condensation or mold by preventing any interior air from contacting cold concrete. If you are installing interior rigid foam, all of the foam seams need to be sealed with caulk, high-quality tape, or canned spray foam. If you are hiring a spray-foam contractor to insulate your wall, make sure that there are no gaps or shrinkage cracks in the foam that could allow indoor air to contact the concrete.
Thermal barriers. When rigid foam or spray foam is installed on the interior side of a basement wall, the foam must be separated from living spaces by a so-called thermal barrier—that is, a layer of 1/2-inch drywall or a material that has been approved as equivalent in fire resistance to 1/2-inch drywall. (For more information on thermal barriers, see “Thermal Barriers and Ignition Barriers for Spray Foam.”) If you don’t want to install any drywall, you can use Thermax, a brand of rigid foam insulation that can be left exposed (because it has passed tests for thermal resistance), or you can use mineral wool insulation as a thermal barrier.
Assembly #1: A continuous layer of interior rigid insulation
One simple way to insulate the interior of a basement wall is with a continuous layer of rigid foam that is thick enough to meet the minimum R-value for your climate zone. This approach is shown in the illustration at the top of this page. If you can’t reach your R-value target with one layer of rigid foam, it’s perfectly acceptable to install two layers of rigid foam. (If you are installing two layers, make sure to stagger the foam seams.)
Rigid foam can be adhered to concrete with foam-compatible adhesive or can be attached with special fasteners like Hilti IDP fasteners or Rodenhouse Plasti-Grip PMF fasteners. (For more information on methods of fastening rigid foam or furring strips to a concrete wall, see “Fasteners for Concrete and Brick.”)
Once the rigid foam is installed, you can either install a 2×4 stud wall on the interior side of the rigid foam, or you can install 1×4 strapping, 16 inches on center, to facilitate installation of the drywall. If you frame a 2×4 wall, don’t forget to install fire blocking at the top of the wall. For more information, see “Seven Common Fireblocking Locations.”
Assembly #2: Continuous interior rigid foam with adjacent stud wall filled with fiberglass or mineral wool batts
If you decide to install a 2×4 wall on the interior side of your rigid foam, the question arises: should you fill the stud cavities with fibrous insulation like fiberglass?
Many builders prefer to leave the stud bays uninsulated, as shown in Assembly #1, because it’s common for basements to experience occasional flooding, and fibrous insulation becomes a soggy mess if it ever gets wet.
On the other hand, you may prefer to boost the assembly’s R-value by insulating between the studs. If so, keep these principles in mind:
- Your rigid foam layer needs to be thick enough to prevent condensation problems. A conservative approach calls for at least R-2.5 of rigid foam in Climate Zone 3, at least R-5 of rigid foam in Zones 4 or 5, at least R-7.5 of rigid foam in Zone 6, and at least R-10 of rigid foam in Zone 7 or 8.
- Mineral wool batts generally perform better in damp environments than fiberglass batts.
Assembly #3: Interior closed-cell spray foam
If you plan to insulate your basement walls with spray foam, you’ll want to frame your 2×4 walls before the foam is sprayed, leaving a gap of 1 1/2 inch to 2 inches between the back of the studs and the concrete wall. The gap will be filled later with closed-cell spray foam. (Note that open-cell spray foam is too vapor-permeable to be suitable for basement walls.)
If your basement has stone-and-mortar walls, you can’t insulate them with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray foam.
Although it’s possible to buy do-it-yourself two-component spray foam kits for this type of job, it’s generally less expensive to hire a spray foam contractor for big jobs like basement walls.
Final thoughts
In most U.S. locations, basement wall insulation is required by code. Properly installed, basement insulation will save energy, improve basement comfort, and reduce the likelihood that your walls will be damp. With a lower chance of dampness, there will be fewer opportunities for mold growth—so your insulated basement will probably smell better than it used to.
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Originally published on GreenBuildingAdvisor.com.
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