One significant way to improve energy efficiency in your historic house is to reduce air infiltration through the walls. Air infiltration allows heat to escape. You can prevent air infiltration in your historic house by:

• Making smart insulation decisions
• Sealing leaky areas on the exterior of your house
• Applying special paint to your interior walls

Consider Your Insulation Options for Original Plaster Walls

Wall insulation might seem like the obvious solution to heat loss in your historic house, but this is usually not a good practice. If your historic house has original plaster walls, you should not blow insulation into the exterior walls. Loose-fill cellulose or fiberglass insulation that has been blown into the walls is one of the main causes of exterior paint failure, termites, mold and structural damage in old houses. Old houses with plaster walls have no vapor barrier under the plaster to stop moist air from saturating the insulation. Instead, plaster walls are designed to allow the free exchange of air and moisture.

Warm, moist air is generated in your home by cooking, taking showers, running the dishwasher, having plants—even breathing. That warm, moist vapor enters the wall through hairline wall cracks, outlets, switches and window trim. When loose, blown-in insulation gets wet, it becomes a moist mass at the bottom of the wall cavity. This mass creates an inviting place for termites, carpenter ants, mold and dry rot. The moisture can enter the exterior sheathing and wood siding, often causing permanent exterior paint failure.

Once these conditions begin inside a wall, the exterior paint often begins to peel from excess moisture. Many homeowners decide to solve the problem by having vinyl siding installed. However, replacement vinyl siding will make the problem worse. Vinyl siding is installed with backer board (insulation board), and the combination of these two layers creates a vapor barrier on the outside of the wall. This barrier stops the free exchange of air and traps more moisture. In fact, the Federal Trade Commission (FTC) issued a ruling (PDF, 4 MB) indicating that the installation of replacement siding over original siding adds little to no insulation benefit.

There is one type of insulation you can install to prevent heat loss in your historic house. Heat loss in a house happens primarily in an upward movement, like a chimney. Therefore, you should seal and insulate your attic space to a minimum of an R-49 with eave ventilation. It's also a good idea to seal and insulate the box sills in your basement — the area where the beams or floor joists rest on top of the foundation.

One other factor in your insulation decision is payback. You might spend $8,000 to have your house walls insulated and save about $400 per year on heating and air conditioning costs. In this case, it would take 20 years for you to recoup the money you spent on the insulation. That is not a good payback in any situation, and it does not take into account the costs of addressing termites, dry rot, mold or paint failure.

Dense-pack cellulose insulation is also not a good product for old houses. Dense-pack cellulose is installed at higher pressures than loose, blown-in cellulose and can cause "pillowing." Pillowing happens when the insulation is packed into the wall so tightly that it bows the wood lath holding the interior plaster to the wall. The bowed lath loosens the lath nails and often causes catastrophic plaster failure.

Closed-cell and open-cell foam insulation products have the same effect on original plaster. Wet foam insulation is installed into walls blindly, which means the installer cannot adequately control the expansion of the foam.

TIP: If your historic house already has insulation blown into the walls, you can remove it. Remove several courses (rows) of siding and sheathing from the bottom of each side of your house and above the windows and doors. Pull out the wet insulation, allow the wall to dry out and then reinstall the siding and sheathing. You should also remove the 1-inch-diameter plastic plugs from your exterior siding. These plugs were placed in the holes that were drilled in your exterior walls to blow in the insulation. Replace these plugs for three to six months with screened and louvered 1-inch-diameter vent plugs. You can buy these plugs at lumberyards. These plugs will allow the wall cavity to dry out after you’ve removed the wet insulation.

Treat Your Gutted Walls Like New Construction

If your historic house has plaster that is not repairable, you can remove the plaster and insulate your gutted walls from the inside like new construction. Building code regulations for new construction in Wisconsin require that a plastic vapor barrier be installed under the drywall. The vapor barrier prevents warm, moisture-laden air from turning into wet condensation in the insulation.

When you have gutted your walls, install fiberglass batting, foam insulation or dense-pack cellulose insulation from the inside. Place a 4-mil plastic sheeting vapor barrier over all three of these types of insulations. Be aware that the plastic sheeting will shrink as the formaldehyde off-gasses. Also, be aware that your wall studs contract and expand. This action will break the seal between foam or dense-pack cellulose insulation and the stud, which will allow air infiltration.

Seal Exterior Leaky Areas

You can prevent some air infiltration in your historic house by tightening up leaky exterior areas by doing the following:

• Add quality storm windows to your house
• Caulk every pipe and exterior penetration on your house
• Insulate your attic and basement box sills
• Maintain the paint on your exterior walls
• Repair loose mortar on your house
• Weatherstrip your windows and doors

On the interior plaster side of your walls, you can install rubber gaskets around electrical outlets and switches to prevent air infiltration.

CAUTION: One place you should not caulk is under each clapboard of your clapboard siding. Caulking the clapboards will seal your house too tightly.

Create a Vapor Barrier with Paint

You can create a vapor barrier on your plaster walls by applying vapor-barrier grade interior paints. The effectiveness of these special paints is limited, so you should still caulk all your windows, outlets and switches.