On the main floor the ceiling is full of penetrations. Wherever a light fixture, fan, smoke detector, crack in the drywall or a seams in a T&G wood ceiling is located, a chase is created for moisture laden air to enter the attic space. These areas are best sealed from the attic side. The insulation would be temporarily swept away and the penetration sealed with a caulking or spray foam product. This option is not a cost effective means of dealing with the problem.
In every home the natural movement of air from the lowest level to the highest is referred to as the “stack effect” of the home. Essentially, your house is working like a chimney as the air warms it rises. When this air rises from the lowest level it creates a negative pressure in the basement (cellar or crawlspace) this is why it is important to have no opening in the basement walls or foundation that would allow the movement of uncontrolled, unconditioned outside air to enter the lower levels of the home. Should these openings exist, as they often do in the form of venting, they should be terminated.
As the air within the house rises, it comes into contact with the interior side of the ceiling. At this point, we started to create a positive pressure where the conditioned air is trying to migrate into the attic space. Should this occur, the warm moist air will condense on the underside of the cooler roof sheathing material.
During the winter months this causes the snow on the roof to melt about an eighth of an inch of the snow that is in direct contact with the roof sheathing. The rest of the snow above works like an insulating blanket and results in water running down the roof (in winter) until it reaches the overhanging portion of the roof or what is known as the soffit. Due to this, now being beyond the heated portion of the roof, the melted water now freezes and creates what we refer to as an “ice dam”.
Also important to understand is that the attic space should always be atmospheric pressure, that is the same pressure that is outside is inside (the attic).
Should an excessive amount of roof vents be installed, these vents now have the capacity to vent more air out of the attic than what the soffits can bring in. The end result of this is that instead of the attic being atmospheric pressure, it is now a negative pressure (a vacuum). This induces greater air leakage into the attic space.
Given this scenario, we now have a positive pressure on the interior ceiling side and a negative pressure on the attic side, thus, inducing significant air leakage into the attic. Once again contributing to a greater potential for further ice build up, and condensation problems.
Remediation:
The objective is to always reduce the movement of conditioned air into an unconditioned space, such as attics and crawl spaces.
Unfortunately sometimes even the best laid plans can be flawed. Approximately 5% of ice damming in a “typical” winter is caused by external sources, which we have no control of. It is this 5% that would justify the use of a trace heater cable. The “Heat Line” would be installed in a “W” fashion, the upper points of the pattern would project up the eaves edge of the roof to approximately 16” beyond the exterior wall. This pattern would ensure that the melt water would have a continuous unobstructed avenue that would allow the water to enter the eavestrough. It is important to note that the eaves and downspouts will also have to have a continuous run of heat trace to keep the gutters and downs free of ice.
Mike