Exterior Wall Insulation

Many older homes have little or no exterior wall insulation, resulting in unnecessarily high heating and cooling energy costs. While retrofitting exterior wall insulation can be difficult, it can yield significant energy savings as well as enhanced comfort.

The first step is to try to determine whether, and to what extent, your exterior walls are insulated. You can do this by removing the cover from an electric outlet (be sure to turn off the power first!) and shining a flashlight around the edges of the outlet box, or probing with a plastic straw or crochet hook. You should check multiple outlets, as insulation in one wall doesn't necessarily mean all walls are insulated.

Retrofit Options

If there's no insulation present, you can have professional insulation contractor blow insulation into the wall cavities through small holes drilled through the exterior. After the insulation is installed, the holes are filled and can be painted to match the exterior. This method is not recommended when there is currently some insulation in the wall.

If you're considering new siding, an alternative is to have a layer of rigid foam board insulation installed under the new siding. The thicker the layer, the greater the energy savings, but the thickness is usually limited by the depth of the window and door frames and other factors.

Where to Insulate

In addition to the exterior walls, you should also insulate any walls between the living space and unconditioned areas like garages or attic storage spaces. There is no need to insulate between areas that are both conditioned.

If you're adding a room or finishing a previously unfinished area, be sure to insulate the exterior walls. If you're remodeling and your project involves removing interior drywall or other finish material, be sure to insulate the walls before installing the new interior surface.


For newly added walls or renovations where the wall cavities are exposed, the simplest material to use is fiberglass batts. Be sure to use the correct thickness - compressing a six-inch batt to fit a two by four wall cavity will actually result in a lower R-value than a standard three-and-one-half inch batt. Be sure the coverage is complete - cut the batts carefully to fit around pipes or wires and narrow gaps around windows and doors. You should also install a vapor retarder directly behind the drywall or interior finish material to prevent any moisture migration into the wall cavity (see "Moisture Control" below).

Blown-in insulation is typically used when the walls are fully enclosed. Materials include loose fiberglass, cellulose and several types of foam-in-place insulation. Some contractors use a "dense-pack" installation (usually with cellulose) that can provide better thermal performance and reduce the likelihood of the insulation settling and leaving voids. The foam-in-place method often results in the highest R-value and also seals many of the small gaps that can increase infiltration or drafts.

Moisture Control

During cold weather, the air inside your home contains more moisture than the outside air. This moisture will try to migrate toward the outside through any small cracks or openings, and can even permeate through materials like drywall. If the moisture is allowed to contact a cold surface, condensation will occur.

This movement of moisture can be reduced by making the wall as air tight as possible (through caulking and other air-sealing measures) and by installing a vapor retarder. Batt insulation typically comes with a treated paper or foil facing that acts as a vapor retarder. Be sure the vapor retarder faces the "warm in winter" side (directly behind the drywall). With blown-in insulation, consider a low-permeability paint or wall covering for the interior of the wall.


The primary benefit of exterior wall insulation is obviously reduced energy costs. Insulating your walls can also improve comfort by keeping the interior wall surfaces closer to room temperature. During cold weather, an uninsulated wall literally sucks heat from your body, causing you to feel chilled (and often prompting you to raise your heating thermostat setting). Some methods of insulating walls can also reduce air leakage or drafts, saving even more energy and further enhancing comfort.

Other benefits include reduced noise from outdoors (insulation acts like sound-proofing) and potentially enhanced resale value.

Cost vs. Savings

While retrofitting exterior wall insulation can be difficult and costly, it can often pay for itself in reduced energy bills. The more you spend on annual space heating and cooling energy costs, the more you'll save and the faster you'll recover the cost of installing the insulation.

If you've already completed the Home Energy Profile, you can compare the estimated savings for wall insulation with the contractors' bids to get an idea of the simple payback. Simply divide the installed cost by the estimated annual savings - the result will be the number of years it takes for the insulation to pay for itself.

Next Steps

Once you've decided to have your exterior walls insulated, contact several reputable insulation contractors with experience in retrofitting wall insulation. Be sure to ask about their experience and training - don't just take the lowest bid. Ask about the materials and methods they use, and question any energy savings claims that sound unrealistic. Be sure to ask for and check references. If possible, check ones that go back several years - some problems caused by poor installation can take time to become noticeable.

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