ASK A BUILDER
By CCHRC Staff
The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.
Q: I want to put insulation on the outside of my home before winter. I heard it has to be applied in the right ratio. What does that mean?
At CCHRC, we receive many questions from property owners who are interested in retrofitting homes with additional insulation. This can make a big difference in heating costs since it cuts down on heat loss through the framing and also due to air leakage. Adding exterior insulation will influence how the home performs in other ways too, as a home with less air leakage often accumulates more humidity indoors during the winter.
Most framed homes in Alaska contain plastic sheeting behind the drywall. This sheeting 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 some moisture may collect inside walls during winter. Usually the walls will dry out in the summer, as moisture can still exit through the outside of the wall. If rigid foam board is added to the exterior, this can make it difficult for the wall to dry out.
The best approach is to add enough exterior insulation so that the existing wall never cools to the point that condensation forms. Determining the proper amount of exterior insulation required to eliminate moisture problems inside the wall depends on local winter temperatures and the original amount of insulation in the wall. Generally accepted formulas for the Fairbanks area say that a house with 2-x-6-inch walls should have between 6 and 11 inches of exterior foam board (depending on the type) to compensate for the insulative effect of the existing insulation. Unfortunately, this much exterior insulation is often 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. Windows can provide a visual indicator of moisture issues when they attract condensation, however an inexpensive indoor humidity monitor would be more accurate. Monitors can be obtained through local supply houses or on line.
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. CCHRC is conducting research on exterior insulation retrofits under best and worst case scenarios for indoor humidity. These results will be published at the conclusion of the experiments.
Alaska HomeWise articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, e-mail us at akhomewise@cchrc.org.You can also call the CCHRC at (907) 457-3454.