The Moisture Issue
Controlling moisture is one of the greatest challenges for homes in cold climates. That’s because we produce a lot more moisture inside our homes than most people realize through everyday activities like cooking, bathing, and having plants and pets. If you don’t deal with this moisture in a controlled way (through ventilation, for example), it can get into the building envelope and cause problems. While warmer climates may be more forgiving, the extreme cold temperatures in Alaska can lead to mold, rot, and structural issues. That’s why you need to control moisture levels inside the home and build in such a way that allows the house to “breathe.” This article takes a closer look at the issue.
Understanding vapor drive
Moisture inside the home mainly takes the form of water vapor, and primarily enters the building shell through “vapor drive”–as warm, moist air inside the house seeks to escape to the outside where it is colder and drier (based on the law of physics that water naturally seeks a less concentrated state). This process is exaggerated by the extreme winters in Alaska; because the air outside is so cold and dry, the effect of vapor drive will be stronger.
The key to preventing vapor from getting inside the walls is to minimize the number of holes in your building envelope. That’s why builders in cold climates typically install a vapor retarder and air barrier in the wall. Yet, as the saying goes, there is no such thing as a perfect vapor barrier. Penetrations caused by electrical outlets, fasteners, or vents create holes in the membrane that inevitably allow moisture to get into the wall.
Moisture can also originate from outside the wall. Alaska’s coastal climates experience a phenomenal amount of precipitation, often accompanied by winds that can drive moisture into the building shell. When a weather resistive barrier is placed immediately behind the siding without an air gap to promote drying, it may be susceptible to moisture damage over time. Even if the framing is protected from liquid water, moisture absorbed in siding can diffuse inward from solar heating. This can create vapor pressure differentials across the building envelope even greater than those produced by dry, arctic conditions where the vapor drive occurs predominantly from the interior.
Balancing moisture and efficiency
One of the quandaries of cold climate housing is attaining a humidity level that is healthy for both people AND buildings. While humans need 40-60% indoor humidity for optimal health and comfort, this range is too high for most buildings. That forces many people to compromise on one or both fronts. If you follow the proper building science and incorporate good moisture control, however, you should be able to maintain healthy humidity levels inside the home without creating moisture problems in the structure.