Frost in attic


Old 12-26-01, 06:00 PM
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Frost in attic

I'm having problems with frost forming on the roof sheathing in my unheated attic. I have an old home (built in 1922) and I reinsulated the attic this fall. I sealed all the attic bypasses I could find and stapled down plastic sheets between the joists before blowing in loosefill cellulose. Now I'm disappointed to find frost forming on the roof sheathing. The attic is much colder than it was last year -- before there was only 3 inches of old rock wool insulation, and I could feel warm air rushing up from below. This year it's much colder, but last year I don't recall any frost forming in the attic.

Any suggestions on how to handle this problem, both short-term (how do I get rid of the frost before it melts onto the insulation) & long-term?


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Old 12-26-01, 06:45 PM
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Frost in the attic

Frost in the attic is usually due to high humidity in the home and/or poor roof ventilation. Moist warm air in the home rises to the attic where it condenses and freezes and forms frost. Proper ventilation would allow adequate air exchange and there would be no frost problems. A combination of ridge vents and soffit vents is reportedly more effective than gable end vents. Concerns about frost and condensation in the attic are damage to insulation, wood, and plaster. Sounds like you need to get some fans in the attic to circulate air and improve roof and attic ventilation next summer.

Last edited by twelvepole; 12-27-01 at 04:27 AM.
Old 12-27-01, 01:42 AM
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What free venting does for you in the winter is to prohibit what's occurring in your attic. Insulation does not stop heat, it resists it's flow. Since all heat has moisture in it, it is that moisture that is condensing on your roof sheathing because heat usually condenses against cooler surfaces. And your roof sheathing is the coolest surface in your attic. Cold air usually contains less humidity than warm air. The attic vents allow colder air to enter your attic. Since that air has less humidity in it, it possesses that ability to absorb the higher humidity in the heat that manages to get pass the insulation. Thereby reducing the ability of the warm air condensing on your roof sheathing. This is referred to as equilibrium relative humidity. Vapor barriers basically do the same thing, by reducing the humidity in warm air, it prohibits it's ability to condense.

Even with a vapor barrier, you should have adequate free venting to prohibit the problem you're experiencing. If you want to know more about free venting and the rules that apply, click on the little house icon with www next to it at the bottom of this message and read topic "Ventilation".
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