Trained Eye


Insulating Exhaust Fan Duct


I am having a problem with moisture in my attic. My kitchen stove fan vents thru the roof. I get quite a bit of frost around the piping in the attic. I wrapped insulation around the piping and taped it with duct tape. I also put a turbine vent above this same area. I still get some moisture coming thru the ceiling above the stove when it warms up. I just read one of your articles where you said the vapour barrier is important, but air circulation is more so. My soffit vents are spaced about six or seven feet apart. Could this be the main problem? I would very much appreciate your opinion on my little problem and any recommendations you might have.


The source of the moisture in the attic in question undoubtedly comes from the warm moist air produced in the kitchen, from cooking, washing and normal daily life. The fact that he is exhausting some of this outside is a major step in reducing the high level of humidity in the house. It is not uncommon for a duct/vent in the attic to sweat and drip back down to cause stains on the ceiling below. Insulating the duct will help but may not totally eliminate the problem, as is the case here.

The first thing to consider is the level of insulation on the duct and in the attic itself. If either of these is minimal, then a significant amount of warm air can still escape the ceiling through convection. If the vent is not well insulated and sealed, it will act like a magnet for that warm air and condensation will form inside or outside the duct. If the duct does not have any vapour barrier, then the moisture can still penetrate the insulation and condense. Insulating the duct is good, but it should also include a polyethylene or other vapour barrier. Many supply houses sell pipe wrap insulation which has an integral vapour barrier included.

The next thing to check is the area around the duct where it penetrates the ceiling. If there are gaps in this area, air will leak out. These gaps, if present, may be filled with caulking or spray foam insulation for easy sealing. There should be a vapour barrier on top of the ceiling sheathing, under the insulation, and it may have been cut when the duct was installed. If one is present it should be sealed directly to the new vapour barrier installed around the duct. This is critical to prevent any small area from leaking air, as a small hole will concentrate the escaping moisture and will produce bad stains in this one spot. If an air/vapour barrier is not present in the attic, one should be installed. If this is too difficult for the entire attic space, adding one over the kitchen will improve the situation, but will not be totally effective.

The final thing to check is the exhaust fan and ducting itself. The vent cap on the roof as well as the exhaust fan should have dampers installed. These will minimize the cold air entering the duct when the fan is not in use. If the roof vent cap damper is damaged, it may not be opening fully, and may allow warm air to remain in the duct too long. If one is not present, cold air can enter the duct unabated, when the fan is not operating. Either of these situations could cause excess condensation that will run back down the vent and cause the damage in question. Look at these on a cold day when the fan is in operation. There should be a sufficient quantity of warm air and steam escaping the roof vent cap. If there is little air being removed, the kitchen exhaust fan and ducting may have to be replaced with a system that has a higher CFM (cubic foot per minute) rating. The flashing on the roof vent cap should also be properly sealed and may be allowing rain or snow runoff to enter the duct.

The venting system installed in the attic sounds like it should be sufficient in this case. Soffit vents spaced 6 to 7 feet apart are fine if they are not too small. Inspect the vents to make sure they are not covered with insulation or painted over on the soffits. If they are open and the roof vent is clear, then the problem is likely air leakage and can be reduced or eliminated by improving the insulation and vapour barrier as described.