Published on May 6th, 2018 | by Henri


Ice dams appear after insulation upgrade

Q. About 6 or 7 years ago I added another new layer of insulation in my attic. The good news is that my oil usage went down by several hundred gallons a year. The bad news is I’ve been getting ice dams ever since and just discovered that some of the trim boards along the edge of the roof are rotting.

I’m assuming that the reason for the rot is due to not enough, but some heat still escaping through the attic resulting in the ice and that it must be melting into the boards and rotting them out. The edges of the roof, which was replaced about 10 years ago, have edge vents. To ultimately solve this problem what do you suggest? One thought I have had is to pull the insulation back from the edges of the attic leaving a bigger gap at the edges. This would allow more heat to escape and melt what is above. Thank you for any help you can provide.

A. It’s surprising that you didn’t experience ice dams before with the lower level of insulation.

Ice dams form when the snow cover on a roof melts because the attic’s temperature remains above freezing. When the melting snow reaches cold areas of the roof, such as its eaves, it freezes and ice builds up to where it can cause further melting snow to back up under the shingles and leak inside the building.

I assume the trim boards you are referring to are fascia boards fastened to the bottom of the rafters and onto which gutters are installed. The fascia boards are more likely to be rotting from another cause: Roof water following surface tension along the metal drip edge and getting into the fascia boards joints (boards that were most likely installed “raw”, – i.e unprimed) over a long period of time as is too often the case. They can be replaced with one of the PVC boards that are now the favorite.

Since you are getting ice dams, it means that even though you added insulation, there is still enough heat in the attic to melt the snow cover. If the ice dams are evenly spread across the entire roof, there may still be warm, moist air convecting into the attic from the conditioned space.

If the ice dams are more locally found at the eaves below skylights and chimneys, there isn’t much you can do about the skylights, but the accessible parts of chimneys in the attic may be insulated.

You mention vents at the edge of the roof; I am assuming that you mean that you have soffit vents in the roof overhangs, and I hope that they are continuous across the entire soffits.

But do you also have a ridge vent at the peak of the roof? If so, you must make sure that the soffit vents are not blocked by the added insulation in order to ensure that there is an uninterrupted air flow between the soffit and the ridge vents. This air flow is helpful in keeping the roof sheathing cooler and in dissipating any moisture in the air that may convect from the living spaces.

To accomplish this, there should be baffles at the bottom of each rafter space where the rafters meet the walls, and it is best if these baffles have a vertical leg that protects the end of the fiberglass insulation from any air currents that would reduce its effectiveness. The baffles should be such that they span the entire width of each rafter space and leave a space of at least 1-1/2-inches for air circulation.

The eventual solution to the elimination of ice dams is an energy audit to determine the source(s) of the heat loss into the attic. The most likely cause would be convection resulting from recessed light fixtures, bath fans, etc.

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