How to cope with outside insulation


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: What are some of the risks with adding insulation to the outside of my home?

It is a complex issue, and what works in one part of the state may not necessarily work in another. The key to successful exterior insulation is to keep moisture from entering the wall from the inside and from the outside. If you cannot ensure this will not happen, then you are insulating at your own risk. If too much moisture moves into your walls or ceiling, then you could end up with mold and rot.

For years, people in Fairbanks and other parts of the state have been applying various amounts of foam insulation to the exterior of their houses. However, this method can also contribute to moisture problems if not done correctly. These problems can originate from inside or outside the house.

In local building code, cold climate construction requires a vapor retarder, placed near the interior wall surface. Most times this consists of a well-sealed layer of polyethylene under the sheetrock.

This barrier membrane is designed to prevent indoor moisture from getting inside walls where it can condense. When you have heat and humidity inside, and a leaky vapor barrier, moisture may get inside a wall. If there are not enough outside layers of foam insulation, this moisture will cool, condense and begin causing problems. Having the proper ratio of insulation on the outside of the wall to insulation on the inside of the wall will help solve this problem.

Also, if you have a good existing interior vapor retarder and add exterior foam, you are creating, what many call, a “double vapor barrier,” as the foam is also relatively impermeable. Should enough moisture find its way into a wall under these conditions, it may have a difficult time drying out.

In some environments, there is the possibility that rain water can work its way into the wall, especially in combination with wind. To minimize these problems, your home may benefit from large overhangs, gutters and proper drainage. Another option is a good draining type of house wrap installed in combination with adhesive or metal flashings, which are applied before putting on the foam. These steps will ensure that if any water gets behind the foam, it will drain down into the ground rather than soak into your framing.

If you put exterior insulation on your house, it will become much tighter. As a result, your indoor humidity levels may increase and the house may no longer receive enough fresh air, making it necessary to better ventilate your home.

An early indicator of elevated indoor humidity levels are your windows. If you are seeing a lot of condensation, especially on a good quality doublepane or triple-pane, that is a good reason to look for a cause.

Q: Are electric stoves more energy efficient than propane ranges?

Strictly, electric energy is very efficient in a range, but it depends on what the cost of propane is, and what the cost of power is in relation. You would have to figure out how much energy, in BTUs, you are getting out of your propane and what you are paying for your kilowatt hours, and compare the two.

Alaska HomeWise articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, e-mail us at You can also call the CCHRC at (907) 457-3454.