What’s the difference between a “hot” and “cold” roof?
The big challenge for Alaska roofs is to retain heat effectively and control moisture trying to escape from inside. The colder the weather and the longer the winter, the more pronounced the issues can become. Deficiencies and poor building practices that may pass in a more forgiving climate become very apparent in Alaska. A basic understanding of your roof system and the challenges it faces can help identify the sources of problems.
Roofs fall into two categories: “cold” and “hot.” They can suffer from the same ailments.
A cold roof maintains a continuous air space between the underside of the roof and the insulation, designed to do two things. To some degree, it allows an exit path for moist air that has leaked from holes in the ceiling vapor barrier into the insulation cavity. It also creates a thermal break that helps prevent escaping interior heat from conducting directly to the roof’s underside, where it can cause the snow above to melt.
A hot or unvented roof relies on high levels of insulation to slow down heat transfer to the exterior. For this type of roof, it’s critical that the ceiling vapor barrier be as close to perfect as possible to prevent moisture-laden air from entering the roof cavity, where it can become trapped.
Roofs in this climate are prone to several problems, including ice damming. A relatively common sight in Fairbanks mid-winter, the classic symptoms are large icicles hanging off of eaves and exposed spots on the roof where snow has melted away. Ice damming is caused by three main factors: air leaking from the living space into the roof, insufficient insulation, or not enough ventilation.
Roof problems become more pronounced in colder temperatures due to increased “stack effect” creating a pressure imbalance in the building. As warm inside air rises, it tries to exit the building through leaks at the ceiling level. To replace it, dense, cold outside air is drawn through cracks in the bottom of the house, like a chimney. The greater the temperature difference between inside and outside, the stronger the stack effect, amplifying the heat loss.
Vapor follows similar laws. During winter there is a huge imbalance between moist, heated indoor air and extremely dry, cold outdoor air. Because water vapor molecules by nature try to reach equilibrium, they will move through any vulnerable areas (including solid wood) to balance the moisture levels–called vapor drive. The greater the temperature difference, the more intense the vapor drive. When a house has high indoor humidity, the combination of stack effect and vapor drive can cause severe moisture problems inside the roof if it is poorly sealed. Gone unnoticed, this can lead to structural damage as well as mold and its accompanying health-related issues.
Regardless of the roof system, good indoor moisture control, adequate insulation, and good sealing are three key elements to keep potential problems at bay.