attic insulation and vents

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Old 01-29-02, 01:58 AM
tfsawmill
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attic insulation and vents

I just bought a 1945 home in northern Minnesota and I have frost buildup in the attic. It has 2 vents on each side of the house and cellulose blown insulation. What can I do to prevent frost buildup? Any advice is appreciated.
 
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Old 01-30-02, 07:49 AM
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To prevent frost in the attic you require free venting. The rule with free venting is 1 : 300 with a vapor barrier and 1 : 150 without one. Most blown in insulation is installed without a vapor barrier. The way this rule applies is, let's say the attic floor is 30' x 30' = 900 sq. ft./150 = 6 sq. ft. of required free venting. The way free venting is applied is 3 sq. ft. of high vents, such as gable or ridge vents and 3 sq. ft. of low vents, such as eave or soffit vents.

Where your roof meets the attic floor is usually where the eave of soffit vents are located. There is a good chance the cellulose is covering them. On the other hand, in 1945 they might not have installed them. In either case, it's not difficult to correct this problem. Any roofer or carpenter can resolve this problem.

You may even want to seal up any attic by-passes, like attic entrances and around plumbing stacks, etc. Heat from the house going through these areas will produce the same problem you're experiencing now.
 
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Old 02-01-02, 12:26 AM
tfsawmill
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Resercon , thanks for the advice! one more question. If I don't have a vapor barrier (plastic) already, would it be a good idea to add one?
 
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Old 02-01-02, 04:57 AM
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No. The reason for that is the vapor barrier should go between the insulation and the warm side of the house. That would mean lifting up the insulation and putting the plastic and then putting back the insulation. That's way too much trouble.

The other way would to paint the ceiling with a vapor barrier paint. Again, I don't like to paint and on top of that the vapor barrier paint is expensive.

You have to add the vent area regardless if you use a vapor barrier and it would be a lot easier on you just to add the venting. I would strongly recommend paying attention to the by-passes like the attic entrance and vent stacks and sealing and insulating those areas well.
 
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