Can I insulate the underside of an addition that has a 2×8 framed floor that is elevated above dirt?
Homehelper
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The room was previously insulated with fiberglass Kraft faced with the paper facing up,towards the living space. It was held in place with chicken wire. The rodents loved it. I was thinking of putting u faced fiberglass back and then 2″ tongue and groove polystyrene . The frame is about 2 feet above grade and is open on 3 sides. The fourth side being the original foundation. It is basically framed as you would frame a deck. Any suggestions would be appreciated.
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Replies
Sal,
Your plan will work. After installing new fiberglass batts and a layer of rigid foam with taped seams on the underside of your floor joists, the rigid foam should be protected by a layer of OSB or plywood.
Thank you. Do you agree that unfaced Batts is the way to go? My thought was the 2" polystyrene would be the vapor barrier. And the fiberglass has to be installed from below so it would be impossible to properly install the Kraft face up towards living space . Thanks again Martin
Sal,
Most building codes do not require an interior vapor barrier; however, some building codes require an interior vapor retarder (a less stringent layer than a vapor barrier).
In your case, the plywood or OSB subflooring is your vapor retarder. For more information on this issue, see Do I Need a Vapor Retarder?
To know whether or not you'll have moisture issue with that stackup we need to know:
What is your climate zone/location?
What is the R-value of the batts?
What is the R-value of the polystyrene?
What is the vapor permeance of the polystyrene? (2" of 1.5-2" XPS runs 0.5-0.7 perms, 2" of 1lb EPS runs 2.25-2.5 perms, 2" of Type-II EPS runs 1.25-1.5 perms).
The vapor permeance of dry half inch plywood is about 1 perm but becomes more vapor open as it's moisture content rises- it will always dry into the vented crawlspace. The potential issue is how much moisture/frost can collect over the winter where the bottom of the batt meets the foam, and whether it can create mold conditions within the joist bay, which is climate & stackup dependent.