Image Credit: Fine Homebuilding Older homes often lack insulation under the basement slab. Even if you decide to leave your basement floor uninsulated, it's a good idea to insulate your basement walls. Rigid foam insulation is a good choice against the concrete; many experts recommend leaving the stud bays of the 2x4 basement wall uninsulated.
Image Credit: Fine Homebuilding Thermomass walls sandwich a core of XPS insulation between two layers of concrete.
Image Credit: Thermomass This drawing illustrates why exterior basement wall insulation is incompatible with brick veneer. According to building scientist Joe Lstiburek, "There is no known practical cost-effective solution to the thermal bridging brick veneer problem when exterior basement insulation is used in residential basements." [Credit for illustration: Building Science Corporation]
Image Credit: Building Science Corporation If you decide to install 2x4 studs on the interior side of your basement wall, don't forget the fire blocking. This detail comes from "Fire-Blocking Basics" (Journal of Light Construction, February 2009).
Image Credit: Journal of Light Construction
Here at GBA, we regularly receive questions from readers about the best way to insulate a basement wall. Since these questions pop up frequently, it’s time to pull together as much information as possible on this topic.
In this article, I’ll try to explain everything you always wanted to know about insulating basement walls.
Is it worth insulating a basement wall?
If you live in Climate Zone 3 or anywhere colder, it’s cost-effective and wise to install basement wall insulation. This advice applies to those who live in most of New Mexico and most of Alabama, as well as all of Oklahoma, Arkansas, and South Carolina, and anywhere colder than these states. (Click here to see a climate zone map.)
Canadian researchers who studied basement insulation methods and costs in five Canadian locations (Toronto, Ottawa, Halifax, Edmonton and Victoria) concluded that “for all types and sizes of basements assessed in this study, the lowest total life-cycle cost was associated with basements insulated internally, full-height to a nominal level of R-20.”
How much money will basement insulation save you annually? According to a report published by the U.S. Department of Energy, the annual savings attributable to R-20 basement insulation in a 1,500-square-foot home ranged from $280 per year in Washington, DC to $390 per year in Buffalo, New York, assuming that natural gas costs $0.72/therm. (However, energy consultant Michael Blasnik cites two Minnesota studies that show lower levels of savings. See his 6/29/12 comments posted below.)
What do building codes require?
The 2012 International Residential Code requires basement insulation in Climate Zones 3 and higher. Here are the minimum code requirements for basement wall insulation — assuming that you are insulating with foam, not fiberglass batts:
Should I insulate the wall on the inside or the outside?
I used to believe that the best location for basement wall insulation was on the exterior. In recent years, however, I’ve decided that interior basement insulation makes a lot of sense.
However, there are valid reasons for both approaches, and either way can work fine. So if you prefer one approach, don’t hesitate to use it.
Here are the advantages of exterior basement insulation:
- Exterior insulation keeps the concrete within the home’s thermal envelope; this increases the amount of interior thermal mass and reduces the likelihood of temperature swings if heating and cooling equipment stops working. (However, it’s worth pointing out that the advantages of interior thermal mass are often exaggerated.)
- Exterior insulation protects the dampproofing or waterproofing layer from damage during backfilling.
- Insulating on the exterior allows a builder to install an uninterrupted layer of rigid foam from the foundation footing to the rafters. While this approach isn’t required — you don’t have to insulate above-grade walls with exterior foam if you don’t want to — many builders like it.
- Exterior insulation provides more interior space in your basement than interior insulation.
- It’s easier to insulate and air-seal the rim joist area with exterior insulation than with interior insulation.
- If you insulate on the exterior, you avoid the expense of interior studs and drywall.
- Exterior insulation leaves the interior of the concrete wall exposed (assuming the basement is unfinished) so that the concrete can be inspected at any time for cracks.
Here are the advantages of interior basement insulation:
- The insulation work integrates more smoothly with the construction schedule, since it happens after the building is dried in rather than when the excavation contractor is eager to backfill the foundation.
- It’s easier to provide an uninterrupted connection between the below-slab insulation and the wall insulation when the insulation is on the interior. If you install the wall insulation on the exterior, the footing will usually interrupt insulation continuity. (For more information on this issue, see Foam Under Footings.)
- If you insulate on the interior, you avoid the hassle of figuring out how to protect the above-grade portion of the exterior basement insulation.
- If you plan to install brick veneer on your above-grade walls, interior basement insulation makes more sense than exterior insulation. (For more information on the incompatibility of brick veneer with exterior basement insulation, see Image #4, below.)
Briefly, how are basement walls insulated on the exterior?
After the basement wall has been protected with a dampproofing or a waterproofing system, insulation is installed from the top of the footing to somewhere near the top of the rim joist. Acceptable insulation materials include extruded polystyrene (XPS), expanded polystyrene (EPS), closed-cell spray polyurethane foam, or mineral wool. Polyisocyanurate insulation should not be used because it can absorb water.
Below-grade insulation does not need to be attached to the concrete; it is held in place by the backfill. The best backfill material is a fast-draining granular material like gravel or crushed stone with a thin cap of soil or clay.
Above-grade insulation may or may not need to be attached to the concrete — fastening methods include foam-compatible adhesive, TapCons with washers, and specialty fasteners like Hilti IDP fasteners or Rodenhouse Plasti-Grip PMF fasteners — depending on the height of the exposed foam and the method used to protect it.
Some builders cantilever their 2×6 perimeter walls so that the basement insulation isn’t proud of the siding. If the basement insulation ends up proud of the siding, you’ll have to protect the top of the basement insulation with metal flashing. The top of the flashing needs to include a vertical leg that extends upward and is lapped by the housewrap; the flashing should be sloped, and the bottom of the flashing needs to terminate in a drip leg that extends beyond the insulation and the insulation protection materials.
If I insulate on the outside, how should I protect the above-grade foam?
The above-grade portions of all types of exterior insulation must be protected from physical abuse and sunlight. Among the products than can be used for this purpose are the following:
- A cementitious coating or cementitious stucco (for example, Styro Industries Brush On ST), with or without metal lath
- A cementitious coating that includes chopped fiberglass (for example, Quikrete #1219 foam coating or surface-bonding cement)
- An acrylic coating like Styro Industries FlexCoat or Styro Industries Tuff II
- EIFS (synthetic stucco)
- Cement backerboard, with or without a layer of stucco
- Pressure-treated plywood
- Metal flashing
- A fiberglass panel like Ground Breaker from Nudo Products
- Styro Industries FP Ultra Lite panels (XPS coated with mineral granules adhered to one side)
- Protecto Wrap Protecto Bond (a flexible peel-and-stick membrane with a textured, gritty coating)
- ProGuard Cement Faced Insulated Sheathing.
- Boral TruExterior brand composite siding and trim products, which are rated by the manufacturer for ground contact.
For more information on this topic, see How to Finish Exterior Foundation Insulation.
Briefly, how are basement walls insulated on the interior?
The best way to insulate a basement wall on the interior is with foam insulation that is adhered or attached directly to the concrete. Any of the following insulation materials are acceptable for this purpose: closed-cell spray polyurethane foam, XPS, EPS, or polyisocyanurate.
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 using Hilti IDP fasteners to attach rigid foam to a basement wall, see Marc Rosenbaum’s article, Basement Insulation — Part 2. For more information on Rodenhouse Plasti-Grip PMF fasteners, see New Green Building Products — June 2013. For more information on methods of fastening rigid foam or furring strips to a concrete wall, see Fasteners for Concrete and Brick.)
To prevent interior air from reaching the cold concrete, make sure to seal the perimeter of each piece of rigid foam with adhesive, caulk, a high-quality European tape, or canned spray foam.
Building codes require most types of foam insulation to be protected by a layer of gypsum drywall. Many builders put up a 2×4 wall on the interior of the foam insulation; the studs provide a convenient wiring chase and make drywall installation simple. (If you frame up a 2×4 wall, don’t forget to install fire blocking at the top of the wall. For more information on fire blocking, see Fire-Blocking Basics.)
One brand of rigid foam, Dow Thermax polyisocyanurate, meets code requirements for a thermal barrier and can therefore be left exposed in a crawl space (and in some jurisdictions, in a basement) without the need for a layer of gypsum drywall. If your basement doesn’t need wiring, studs, and drywall, then Thermax is probably the brand of insulation to use. (However, be sure to check with your local building official before going this route.)
If you plan to insulate your basement walls with spray foam, the best approach is to frame your 2×4 walls before the foam is sprayed, leaving a gap of 1 to 2 inches between the back of the studs and the concrete wall. The gap will later be filled with spray foam.
For information on insulating rim joists, see Insulating Rim Joists.
If you live in an area where termites are a problem, your local building code may require that you leave a 3-inch-high “inspection strip” of bare concrete near the top of your basement wall. To find out what details are required in your area, talk to your local building official.
What if the basement has an interior French drain?
Some basements have an interior French drain at the base of the basement walls. This type of French drain may be installed on just one or two walls, or it may be installed around the entire perimeter of the basement. Usually, the French drain (a shallow trench) includes perforated drain pipe that leads to a sump; the drain pipe is usually surrounded by crushed stone.
Before insulating a basement wall with an interior French drain, you’ll probably want to cover the wall with a dimple mat. The dimple mat allows any water that seeps through the wall to find its way to the French drain at the base of the wall. (For more information, see “Using a Dimple Mat to Keep a Basement Wall Dry.”)
Can I insulate on the interior with fiberglass batts, mineral wool batts, or cellulose?
No. Fiberglass batts, mineral wool batts, and cellulose are air-permeable. When this type of insulation is installed in contact with concrete, the moisture in the interior air condenses against the cold concrete surface, leading to mold and rot. That’s why I advise builders that fiberglass batts, mineral wool insulation, and cellulose should never be installed against a basement wall.
The risk of moisture problems is reduced if the concrete is first covered with a continuous layer of rigid foam or closed-cell spray foam. If that is done, some builders then install a 2×4 wall on the interior side of the foam insulation and fill the stud bays with fiberglass batts. This approach is less risky than installing fiberglass directly against the concrete. However, I don’t think that fiberglass batts belong in a basement. My advice: if you want a higher R-value, just install thicker rigid foam, and leave the stud bays empty.
If you decide to combine rigid foam installed on the interior side of the basement wall with fluffy insulation between 2×4 studs installed on the interior side of the rigid foam, you may be wondering, “How thick should the rigid foam be to keep the wall free of moisture problems?” A conservative approach for below-grade walls is to follow the guidelines for above-grade walls detailed in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Does interior basement insulation need to be vapor-permeable?
No. The idea that a damp concrete wall should be able to dry towards the interior — in other words, that any insulation on the interior of a basement wall should be vapor-permeable — is mistaken. In fact, you don’t want to encourage any moisture to enter your home. Your concrete wall can stay damp for a century; that dampness won’t hurt the concrete.
For more information on this topic, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.
Should I include a polyethylene vapor barrier?
No. Basement wall systems should never include any polyethylene. You don’t want poly between the concrete and the insulation; nor do you want poly between gypsum drywall and the insulation. You don’t want poly anywhere.
Paul Ellringer, an energy and mold consultant in Saint Paul, Minnesota, has a collection of slides showing moldy basement insulation. In most cases, these basement walls were insulated with fiberglass batts, and included two layers of polyethylene — one on each side of the studs. Ellringer calls this a “diaper wall,” and reports that most of them are a mess. “Fibrous insulation and poly are inherently problematic, and should not be used in below-grade walls,” says Ellringer. “Sometimes when you open it up, the fiberglass is soaking wet. If the house is two to four years old, the studs are often beginning to rot.”
What about ICFs or the ThermoMass system?
If you build a new basement with insulated concrete forms (ICFs) or the ThermoMass system, your wall already includes insulation, so you don’t need to add any more.
Both approaches work. The main disadvantage of these systems is their high cost compared to conventional poured concrete walls.
ICFs have a core of concrete sandwiched between two layers of rigid foam. ThermoMass walls have a core of rigid foam sandwiched between two layers of concrete. It seems to me that the ThermoMass sandwich makes much more sense than the ICF sandwich: since foam is more fragile than concrete, it makes more sense to protect the fragile layer with concrete than to put the fragile material on the outside of the sandwich.
If you decide to use either ICFs or the ThermoMass system, pay close attention to the wall’s R-value. Many ICF and ThermoMass walls have relatively low R-values. If you’re going to buy such an expensive wall system, be sure to specify thick foam.
What do I need to know if I am installing insulation on an existing house?
If you want to insulate an existing basement, you’ll probably be working from the interior. Before installing a layer of foam insulation on an existing wall, the first step is to verify that the basement wall doesn’t have a water-entry problem.
Diagnosing and fixing water-entry problems in existing basements is a big topic in its own right, and is beyond the scope of this article. Suffice it to say that if your basement walls get wet every spring or every time you get a heavy rain, the walls should not be insulated until the water-entry problem is solved.
Among the possible solutions to this problem:
- Adjusting the grade around your house so that the soil slopes away from the building on all four sides;
- Installing roof gutters connected to conductor pipes that convey the roof water away from the foundation;
- Excavating the exterior of your foundation and installing new footing drains leading to daylight;
- Installing an interior French drain around the perimeter of your basement and connecting the drain to a sump equipped with a sump pump; and
- Installing a layer of dimple mat against the basement walls before insulating.
For more information on these issues, see Fixing a Wet Basement.
If your basement has stone-and-mortar walls, you can’t insulate the walls with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam.
If your basement has poured concrete or concrete-block walls, you can proceed with the same methods used for new construction — as long as you’re sure that the walls don’t have a water-entry problem.
What about crawl space walls?
Crawl space walls should be insulated with the same methods used for basement walls. For more information on insulating a sealed crawl space, see Building an Unvented Crawl Space.
Basement wall insulation is a cost-effective measure
Remember: if you live in Climate Zone 3 or anywhere colder, installing basement wall insulation is almost always cost-effective. Performing this work will lower your energy bills, and will also provide an important side benefit: insulated walls are less susceptible to condensation and mold.
That means that insulated basements stay dryer and smell better than uninsulated basements.
Last week’s blog: “Understanding Energy Units.”