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Question DetailsAsked on 10/28/2017

Original insulation in attic looks to have a vapor barrier on both sides can I add insulation on top of it?

House built in 1950-60. Original insulation looks has paper on one side facing the inside of the house the other side has a tar like paper on it.

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Answered 1 year ago by LCD

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Not a good idea to do it like that - I would check if the tar paper is actually part of the insulation or not - may just be laid over it so easy to remove - was commonly so done before about 1960.

Even if it is double-faced, usually is not tough to pull it off the batting (the asphalt paper layer) because it is just lightly glued or asphalt adhered - though of course, if reinsulating you do need to consider whether all-new insulation with full insulation value is a good idea or not, while doing the job anyway.

You can find a lot of previous similar questions about attic insulation, and the moisture entrapment problems you can get into by adding insulation if you do not first seal all the attic penetrations and seams (which means pulling the existing insulation up to do so), in the Home > Insulation link under Browse Projects, at lower left.

One of the ironic things about attic insulation is, in poorly sealed attics, less rather than more insulation may waste energy but can actually avoid insulation entrapment problems. Especially in winter when under existing conditions the moisture might dissipate adequately into the attic air and be dissipates, but put more insulation in without sealing the warm humid air from the house and you can induce a situation where, because of the added insulation, the freezing front moves from the top surface of the insulation to a point within the insulation. I have seen cases in cold climates where the moisutre buildup was so severe it caused ceiling failures and in one case a total attic framing collapse from several FEET of ice accumulation in a newly applied insulation layer.

When adding insulating the type of insulation makes a difference too - fiberglass tends to be a lot more tolerant of moist air coming from below than cellulose, which tends to act as a vapor retarder - and also soaks up free moisture.


Here are a couple of related links which might be of interest too:


http://answers.angieslist.com/Why-tar...


http://answers.angieslist.com/Cellulo...


http://answers.angieslist.com/Whats-b...


http://answers.angieslist.com/Insulat...


http://answers.angieslist.com/house-b...


http://answers.angieslist.com/E-Sheil...

Answered 1 year ago by LCD




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