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Community and Q&A

Insulating an attic floor and ceiling

Ryan Lewis - Zone 4A | Posted in General Questions on

Ok. We all know this is a no no, because of the risk of the attic temperature going below the dew point. 

I’m curious what happens if you apply say R-5 of foam to the underside of a roof, while leaving your attic floor at R-50. 

wood itself has some R value, so clearly there is some R at which there is too much insulation.

saving this, if you just air sealed the ceiling without insulating say with this:
https://foursevenfive.com/visconn/

it would still make the thermal envelope more airtight.. but would it have any negative moisture effects in the attic? 

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Replies

  1. Expert Member
    Zephyr7 | | #1

    You add some potential moisture risk doing this, you don't gain much insulating benefit -- none at all if you have a vented attic -- and you don't help in terms of code since code doesn't let you add R values together when the insulation isn't in the same place.

    If you want to gain something with a quick install under the sheathing, this is one place where a radiant barrier makes sense. Get basic radiant barrier material (not the bubble stuff), staple it under your rafters. This will help you with heat gain in the summer, with no real risk as long as you leave the top and bottom open for ventilation. You're not dealing with "real" R value here, either, so there are no issues moving condensing surfaces around.

    Bill

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