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Installing fiberglass batts in attic

I have recently finished up air-sealing my unfinished attic, and am going to be installing R30 faced fiberglass batts in between the joists. My question is in regards to how aggressively should I be pressing down on the batts when I install them? Do I need to really push down on them to make sure that the vapor barrier on the batt is pressed closely to the attic "floor" made up of drywall? Or should I be focusing on keeping the batt as uncompressed as possible so that its R-value does not drop, even if this means that the vapor barrier on the batt isn't pressed directly up against the attic "floor".

Thanks!

Asked by DrJimesTooperMD
Posted Jun 13, 2018 8:46 PM ET

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Dr. Jim,
Ideally, the batts should (a) be trimmed if necessary to fit perfectly (or a little snugly) in each rafter bay, and (b) installed in such a way as leave no voids. You don't want to try to force a 15.5 inch batt into a rafter bay that is only 13 inches wide, or a 23.5 inch batt into a rafter bay that is only 21 inches wide.

Pressing the batts to the drywall is important, but the reason has nothing to do with the vapor barrier or vapor retarder. You want to press down the batt so that you don't end up with voids at the bottom of the bay or in the corners.

If necessary, you can re-fluff the batt by pulling it back up after you press it down -- somewhat of a learned skill.

For more information on this issue, see Installing Fiberglass Right.

Answered by Martin Holladay
Posted Jun 14, 2018 5:14 AM ET

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