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The best way: exterior   - Although more costly (in an existing building), the insulation of foundation walls from the outside is the best way to provide thermal efficiency and protection from moisture problems, mold, and mildew. Proper sealing and drainage are important in any exterior foundation insulation project.

Another exterior option   - An alternative to insulating and sealing all the way to the footing on the exterior foundation wall is to do it part way (sometimes called the "apron" method). Dig a trench about two feet wide and about 18 inches deep and insulate the wall with a vertical piece of rigid foam. Then place a horizontal piece of rigid foam slanted away from the wall at the foot of the vertical piece. This approach will provide some insulation for the basement wall and (if done properly) can aid in reducing the intrusion of water through the basement wall. Attention to detail regarding flashing, surface preparation, and joint sealing will reduce the likelihood of basement water problems.  

Head for the basement

Most Minnesota homes have basements with either concrete block or  poured-concrete walls. While these make sturdy foundations, they’re  poor insulators and have a very low R-value. Therefore, an uninsulated  basement can account for a significant amount of a home’s energy loss.

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From the inside  - Because of the risk of mold and other moisture problems, the insulation of interior basement walls is  very  challenging to do properly. Unless basements have a low risk of moisture potential, it is better to leave the walls unfinished to allow drying and reduce mold and mildew issues.  Fiberglass batts are no longer recommended!  The steps outlined to left are guidelines; attention to details is essential in order to reduce potential problems and provide complete thermal insulation.

Check the top of the basement foundation

The wooden rim (or band) joist area—where the house’s wooden structure rests on the concrete foundation—is the best place to begin. It’s the simplest and least expensive basement area to insulate, and it will bring you the highest return on your investment. And, because it is usually above ground, there is little risk of moisture migration from the exterior.

Insulate with cut-to-fit pieces of rigid, extruded polystyrene or polyisocyanurate foam, caulked or foamed into place, to avoid air leaks. Fiberglass is not recommended because it does not provide an air-tight seal. Moisture from the basement can migrate through the fiberglass where it condenses on the cooler rim joist, leading to potential problems with mold or wood rot.

Insulating the foundation walls 

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Rim joist insulation - Install rigid foam pieces, carefully sealing edges and around pipes, wires, and vents to prevent moisture from damaging wood framing. Alternatively, the application of a spray foam (usually done by a contractor) can provide good insulation and air sealing. The material must be fire-rated to the interior.

Concrete (whether block or poured) is extremely porous, and unless sealed from the outside it is a likely source of moisture into a basement. Because migrating and condensing moisture can be trapped behind wall insulation—leading to mold and mildew growth—insulating basement walls is challenging. Even historically dry basements can be subjected to water migration or seepage during severe storms or flooding. (Don’t be misled by claims that products that are painted or applied to interior basement surfaces will prevent moisture from seeping through walls or floors—they can’t.)

Attaching wooden studs to the foundation wall and installing fiberglass insulation (a method used in previous years) is no longer recommended. The wood and insulation can become a good medium for growing mold and mildew should any moisture find its way between the foundation wall and the insulated wall.

Interior basement walls—that have a very low risk of moisture potential—can sometimes be insulated successfully with rigid foam panels; proper assessment and installation by a professional contractor is strongly suggested, however. Attention to details—such as proper sealing—can reduce the chances of damaging mold and mildew problems. It is important to note again, however, that foundation walls that are not sealed from the outside will potentially allow moisture to migrate into the basement.

Unfortunately, the recommended approach to basement insulating can also be the most costly: removing the soil outside the house all the way to the footing, sealing the foundation wall, providing for proper drainage, then insulating with rigid foam on the exterior of the wall. A project of this magnitude requires hiring an experienced contractor.

Insulate floors over unheated spaces 

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Cantilever insulation - Any projection over an exterior wall should be insulated and sealed to prevent heat loss and moisture intrusion. If fiberglass is used to insulate, sealant (caulking or spray foam) should be applied at all framing joints and cracks. Rigid foam boards should be taped at corners and joints.

Floors over a basement that have a heat source such as a furnace, boiler, or wood stove don’t need to be insulated. However, floors over an unheated area such as a garage, a porch, or open ground can be a source of considerable heat loss. Window projections (such as bay or bow windows) also need to be insulated and sealed where they protrude from the surrounding wall.

  • A cantilevered floor over an exterior wall needs insulation and air-sealing just as much as any other exterior surface. Depending on how the floor is built, there are several ways to make cantilevered floors more efficient. One is to hire a professional to spray in polyurethane, polyicynene, or blown-in cellulose insulation through small holes on either the interior or exterior. Air-sealing (with rigid foam insulation and other products) is critical to prevent moisture from damaging insulation or building components.
  • Use rigid board insulation or spray-in foam to insulate the “floor” and the “roof” of a bay or bow window projection.
  • If your home was constructed slab-on-grade, the cold slab can damage wood and carpets if water and ice condense on the floor. During construction, rigid board insulation should have been installed underneath and around the entire perimeter of the slab. If not, a professional can insulate the slab using rigid insulation board with plywood flooring on top.

If you have a crawl space

Any part of the house that is heated must be separated from unheated space by insulation and air-sealing, including crawl spaces. If your crawl space has a dirt floor, cover it with polyethylene sheeting before insulating and extend the plastic several inches up the walls. As with a basement foundation wall, the preferred method for insulating is from the outside. Other options include:

  • Insulate crawl space foundation walls and the floor to a value of R-10 or higher with a rigid foam insulation, appropriately sealed.
  • Insulate between the floor joists with sprayed foam or rigid foam to a minimum of R-10. (Because of the difficulty of a proper air-sealed installation, fiberglass batts between the joists is not usually recommended.)

Regardless of the method used, crawl space ventilation is not recommended, due to moisture and condensation concerns.