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Insulating interior basement walls if exterior has waterproof membrane and rigid board

The exterior basement walls have waterproof membrane and two inch foam board installed. To insulate the interior basement walls, can i use the old school method of studs, fibreglass and poly?

Asked by user-7114034
Posted Jul 11, 2018 6:24 PM ET

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1.

User 7114034,
First of all, can you tell us your name? (I'm Martin.)

Q. "The exterior basement walls have waterproof membrane and two-inch foam board installed. To insulate the interior basement walls, can I use the old school method of studs, fibreglass and poly?"

A. Briefly, no. I advise you to stick with rigid foam or closed-cell spray foam, as explained in this article: "How to Insulate a Basement Wall."

Admittedly, you could probably get away with using fiberglass batts -- assuming that the exterior details are perfect -- but the interior polyethylene would always be a mistake.

The reasons that I don't advise installing interior fiberglass batts against the concrete are the following: (a) It's extremely difficult to inspect the exterior waterproofing details and insulation details, so you can't determine if there are any flaws in the installation. (b) Small flaws in the exterior details could result in wet concrete or cold concrete, and it's not worth the risk of letting fiberglass batts touch wet concrete or cold concrete.

Answered by Martin Holladay
Posted Jul 12, 2018 6:36 AM ET

2.

Moisture wicking up from the footing into the foundation wall can wick toward the conditioned space side of the assembly. If there isn't a good capillary break between the top of the foundation and the foundation sill, putting any interior side vapor barrier (or even thick or foil faced foam board) can increase the moisture content of the foundation sill.

A thin layer of foam board with a vapor permeance greater than 1 perm trapped to the foundation wall would provide a capillary break for the fiberglass, and allow any ground moisture wicking up the foundation wall to dry toward the interior, offering some protection for the foundation sill.

Local climate matters too. When there is no interior side vapor barrier there has to be sufficient R value exterior to the fiberglass to prevent moisture accumulation at the above-grade portion of the wall. It's legitimate to add the R-value of both the 2" exterior foam and whatever semi-permeable interior side foam gets installed when making that calculation. Using the IRC R702.7.1 prescriptive minimums for the fully above grade walls would work (and would have a hint more margin for a foundation wall):

Answered by Dana Dorsett
Posted Jul 12, 2018 1:28 PM ET

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