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Question DetailsAsked on 12/16/2016

Should attic vents be capped in winter

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1 Answer


NO - while they remove heat in the summer as you know, in the winter you still get moisture coming in from the house to even the best insulated and sealed attics, plus if there is any leak in the roofing the sheathing can get wet- but if ventilated the moisture can be vented away with the attic air movement from eave to vents. Also, during cold weather, as the temperature drops on the roof, you commonly get (in fully ventilated attics of course) condensation and/or frost on the underside of the sheathing, and sometimes on the framing and on top of the insulation as well - just from the cold roof being colder than the warmer house-heated air in the attic. As the warmer attic air cools, it cannot hold the amount of moisture it could before, so that moisture drops out as condensation and frost - both becfause the air is cooling, and because it comes into contact with cold framing and sheathing, forming hoarfrost (the frost you see on trees on frosty mornings after a snap cold night).

If there is attic ventilation the outside air moving in through vents moves that more humid air out, hopefully before it can form significant condensation or frosting - if the air is trapped in there, you can get moisture buildup. In attics that have not been fully sealed (meaning almost all, including new construction), so there are passages to the house itself via holes for pipes or wiring, unsealed or vented ceiling light fixtures, gaps in the vapor barrier (orno ceiling vapor barrier at all) - all these contribute to movement of warm moist air into the attic in the winter which needs to be removed from any open-to-the outside attic. About the only exception are of course fully heated attics (hence warm roof, and would have no attic vents), and attics in building without any heat or moisture source like sheds and barns.

Most extreme cases - I have seen ceiling failures in very cold areas due to 2-3 FEET of frost buildup in the attic. I worked on one failure investigation at an 8-plex I think it was military housing complex in Alaska where there was structural ceiling/roof collapse due to such frost buildup - granted, in an area that experiences month-long period of 40 below temps.

Keep your ventilation open and functioning year-around.

Answered 3 years ago by LCD

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