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Crawlspace air sealing question

giantsean | Posted in General Questions on

Hi.  Our house is built on a slope w/ a walk-in basement.  We have a breezeway over newer construction that created an ~5′ tall crawlspace of about 160 sf which we framed as a walk in shed.  Ceiling joists are 2×10 and filled w/ R30 kraft-faced fiberglass.  

This has a few drawbacks. The FG is easily disturbed given the activity that happens in the shed, insects and rodents have nested inside the FG, AND the breezeway tends to be cold in the winter due to lack of air sealing.  As such I have been looking for ways to better air seal the space from below.  I priced out CC foam but it is very expensive and hard to find a vendor who will do small lots, so have been thinking about “smart” membranes and plywood.

I have plenty of leftover Certainteed “Membrain” which is supposed to release moisture one-way.  Would it be an acceptable solution to attach a layer of Membrain (pointed out), cover w/ a thin ply like luan or melamine board, and tape/caulk the seams?  Just want to see if I can solve this at minimal cost and without inviting any new problems.  Thanks as always!

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  1. Expert Member
    BILL WICHERS | | #1

    MemBrain isnt' really "directional". MemBrain is variable permeability depending on moisture levels. In this way, it helps to control the moisture content in a wall. MemBrain isn't going to really do anything to help keep critters out though, since any critter can easily chew through a vapor retarder.

    MemBrain is normally used on the "warm in winter" side of the wall assembly, so wouldn't really be appropriate in your application. You could just install some plywood sheathing, and it wouldn't have to be very thick since it's not structural here. I would probably use 3/8" CDX plywood here just because it's readily available, and thinner sizes might actually cost more. I would air seal it though.

    Another option would be to put some foil faced polyiso up. The foil facer is pretty good at keeping insects out, but rodents like mice could still be a problem. If you have a serious problem with very determined mice, I would put a layer of hardware cloth with 1/4" mesh behind whatever you use to cover the underside of those joists. Hardware cloth is like a super tought version of window screen, and mice will NOT be able to chew through it.


    1. giantsean | | #2

      Thanks Bill. I was thinking the plastic/taping combo would help with the air sealing, but if the plywood is sufficient I will go with that. Now I just have to start saving for plywood at today's prices :D

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