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Should I revisit my insulation strategy? Insulation company told me to cut out the mid-wall smart vapor barrier

alumniu | Posted in Green Building Techniques on

I recently had the largest insulation company in my area come out to my new construction house to quote insulating the walls. They immediately told me to cut out all of the mid-wall “Membrain” smart vapor barrier that I had put in place on the backside of the inside wall studs because it would accumulate moisture like crazy.

I mentioned the 1/3 – 2/3 recommendation for insulating, as well as the permeance adjustability of the Membrain (to which he was familiar with), but he was adamant that I would have major problems if I continued on the insulation path I was planning on taking, and had me concerned enough to think about cutting it all out.

Can anyone calm my fears of mold and moisture?

Climate zone 5 (northern Illinois)
Double stud wall construction (from outside->in)
Fiber cement siding
1/4″ rainscreen gap
Tyvek housewrap
2×4 outer wall framing
3.5″ to 4.5″ gap (framer spaced east 2×4 framed walls closer than rest of the house)
Membrain smart vapor barrier
2×4 inside wall framing
future drywall
*both stud bays, and middle gap filled with dense pack blown-in cellulose

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  1. GBA Editor
    Martin Holladay | | #1

    Your insulation contractor is mistaken. Find a different contractor, or refer him to this thread on GBA.

  2. GBA Editor
    Martin Holladay | | #2

    Here's a further discussion of the issues raised by your contractor.

    Your contractor told you that the MemBrain "would accumulate moisture like crazy."

    Here is the question you should ask the contractor: "Where will the moisture come from? Are you worried about exterior moisture or interior moisture?"

    If the contractor is worried about exterior moisture, that would either be rain -- unlikely to reach the MemBrain -- or vapor that is driven inward during the summer. But the MemBrain will never be cold enough during the summer to worry about condensation (and in any case it is vapor-permeable).

    If the contractor is worried about interior moisture, I assume the worry concerns either vapor diffusion or exfiltration (air with moisture).

    If the contractor is worried about outward vapor diffusion during the winter, tell the contractor that the MemBrain will never be cold enough to be a condensing surface -- and in any case, the MemBrain is vapor-permeable.

    If the contractor is worried about exfiltration, explain that exfiltration problems are handled by paying attention to air leakage. If you seal air leaks, and perform a blower door test, you shouldn't have to worry about having enough exfiltration to cause a wet wall problem.

  3. Expert Member
    Dana Dorsett | | #3

    MemBrain will never trap moisture, but it IS going to be more vapor open in winter when place 1/3 of the way through the insulation from the interior. If your average mid-winter interior conditions are 35% relative humidity @ 70F, and the average mid-winter outdoor temp is 25F (about right for most of N-IL), the average temp 1/3 of the way from the interior (the MemBrain layer) will be about 55F.

    With 40% RH @ 70F conditioned space air the entrained air in the insulation on the interior side of the MemBrain becomes ~60% RH @ 55F, which is a much more vapor-open zone for MemBrain than if it were on the interior, exposed to 35% air.

    Mind you, the proximate air on the exterior side of the MemBrain is going to average no more than ~30% RH @ 55F, (the dew point of 30% RH air @ 55F is about 25F, the average outdoor temp) which helps. But it's still more effective to put the MemBrain on the interior side of the total insulation, where it will be warmer, at an even lower relative humidity on both sides. If the MemBrain were located next to the wallboard layer it would be 70F, and <18% RH on one side, 70F and 35% RH on the other side, and much more vapor tight.

    Leaving it where it is it'll still be doing something, but not as much as you might think. But NO WAY is it going to cause a problem there.

    With the rainscreen and CDX sheathing you're still pretty safe even without the MemBrain in your zone 5 location, but if you're the type to worry about it use a vapor-barrier latex primer on under the interior finish paint.

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