What’s the proper way to add exterior insulation to my home?
One of the common ways to retrofit an older house is to add exterior insulation to the walls. It can make a big difference in heating costs since it cuts down on heat loss through the framing and through air leakage, but it can also trap moisture in the walls if not done properly, leading to health and durability concerns.
Most framed homes in Alaska contain plastic sheeting behind the drywall, which is designed to keep the water vapor generated indoors from condensing inside the walls and ceilings during winter. The plastic sheeting is rarely perfect, particularly in older homes, and usually allows some degree of moisture to collect inside walls during winter. This isn’t the end of the world, as walls can often dry out in the summer as moisture exits to the outside. However, if you add foam board to the exterior, it makes it difficult for the wall to dry to the outside.
The best approach is to add enough exterior insulation so that the existing wall never cools to the point that condensation can form. Determining the proper amount to add depends on local winter temperatures and how the original wall is built. In the Fairbanks area, for example, conventional wisdom says you should add at least two-thirds the amount of total wall R-value to the exterior. That means if you have a 2×6 wall filled with R-21 fiberglass, you should add at least R-40 to the outside (equivalent to 8 inches of EPS foam board). Or a 2×4 wall with R-11 should have at least R-20 on the outside (equivalent to at least 5 inches of EPS foam board). This ensures all the wood components of the wall stay warm and dry. Unfortunately, this much exterior insulation is often prohibitively expensive.
If not enough exterior insulation is added to keep the existing wall warm enough, controlling indoor humidity through mechanical ventilation becomes extremely important to reduce the potential for moisture problems inside the wall. In Interior Alaska’s dry climate, a properly sized and installed ventilation system can readily reduce indoor humidity levels to 20 percent. Window condensation is a good indicator that you have too much humidity in the home, however an inexpensive indoor humidity monitor would be more accurate (which you can buy at a local supply houses or online).
Exterior insulation retrofits can result in big energy savings, however, each home needs to be assessed on an individual basis. Local climate, variations in existing wall construction, occupant awareness, mechanical ventilation, and the materials and methods used in the retrofit all need to be taken into consideration to maximize performance and minimize risk.