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FYI if you live in a cold climate and venting through your roof

13 years ago

Summary:

If you live in a cold climate area and are venting your bathroom (or kitchen) through your roof, you may want to consider installing a double-wall flue vent pipe and cap instead of a standard roof vent cap to avoid the risk of an ice dam and associated water damage.

Full story:

Last summer I installed an in-line vent fan in a bathroom. Per the installation manual, I used insulated flex-duct and terminated it through the roof with a roof vent cap (Broan 644). The fan works great; it draws the air out well and is very quiet. The vent cap is located about 12 feet up from the edge of the roof and was installed by a professional roofer. The attic is well insulated (R-49), the roof plane is northern facing, and I live in a suburb of Columbus, OH.

This winter the warm air exiting the vent cap melted the snow on the roof below the cap. This happened repeatedly as we had a prolonged period of sub-freezing weather during which there were numerous snowstorms. The water ran down under the snow cover on the roof and froze, creating an ice dam. Water backed up underneath the shingles and through the roof sheathing. In the attic, the water flowed down the sheathing to a rafter and then straight down a stud inside the exterior wall.

Last week, when I was outside on our back deck, I noticed a vertical line of moisture coming through the stucco siding. This line is directly below where the vent cap was installed and the moisture appeared to pool just above the second level floor. Last weekend I climbed up into the attic and found the wet sheathing from the ice dam. Next, I went to the interior of the wall where I saw the moisture outside and cut through the drywall. The fiberglass insulation was wet and the exterior wall sheathing was saturated and has mold growing on it.

I called the fan manufacturer and described the situation. The technician I spoke with said this was not the first time he'd heard of a problem with roof caps creating ice dams. He said that in cold climates, double-walled flue vent pipe and a cap (like that used for venting a wood stove or furnace) should be installed instead of a surface-mounted vent cap. Unfortunately, there is no mention of this in their installation manual and all of the accompanying graphics show a surface mounted roof cap.

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Comment (1)

MongoCT
13 years ago
Good advice. I never install the surface-mounted types of vent hoods that were used in your installation for just that reason.

Ice/water membrane will normally help protect against ice dam melt water intrusion, but with your hood being that high up it's probably above the membrane.

Somewhat unrelated, yet related...a house I built two years ago, the owners called me up in late January with "a huge, huge, problem". They still have a blanket of snow on their roof, it's been there since the first snowfall in December. Meanwhile, all the neighbor's houses have had snow come, then melt off the roof, then more snow, then again another melt off to expose a clean roof.

I drove them around to a few other houses in town that I've built, all my roofs are holding snow.

Took them a while to comprehend that it means that the heat they're paying for is staying inside the thermal envelope of their house and not going, literally, "through the roof".

The good news is you've identified the problem and it'll be easy to rectify.

Good on you for identifying the problem.
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