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I have a new home under construction in NE South Dakota (cold Zone 6)

Matt_hofer | Posted in General Questions on

I have a new home under construction in NE South Dakota (cold Zone 6). Exterior walls are 2×6 with 1/2″ 5-ply plywood.

My insulation contractor will be dense packing cellulose in the walls. He uses a 6 mil polyethylene for the netting. My concern is the netting is a vapor barrier, allowing very little wall drying to the inside, only to the outside. Your thoughts and comments would be appreciated, Thanks.


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  1. charlie_sullivan | | #1

    Poly vapor barriers are generally out of favor these days. In your pretty cold climate, you'll probably be OK, but you are missing the opportunity to dry to the inside in the summer, especially if you are running A/C. Rather than debating about it, a good option that can satisfy both camps is to use MemBrain which doesn't cost that much more and is almost magic, preventing outward vapor drive in the winter, but allowing inward drying from the wall to the interior in the summer. It's not very expensive. I'm not sure whether it's robust enough to hold back dense packed insulation, but you can use it over something else that does that job if necessary. (Not poly)

    I'm a little surprised that an insulation contractor would want to use poly rather than mesh like insulweb, because it's harder to dense pack the insulation when the air can't get out. That makes me a little worried about whether he'll do a good job of dense packing.

  2. Matt_hofer | | #2

    Thanks Charlie. According to Certainteed, MemBrain is not strong enough for dense pack cellulose netting. If I use Insulweb netting do I come back over that with MemBrain or just Sheetrock directly over the Insulweb?

  3. GBA Editor
    Martin Holladay | | #3

    You can install MemBrain over the InsulWeb after the cellulose has been installed. Here is a link to an article with more information on this topic: How to Install Cellulose Insulation.

  4. Dana1 | | #4

    With back-ventilated siding and decent roof overhangs the wall would be fine with 6 mil poly on the interior, but it would be a lot more resilient with the InsulWeb + MemBrain approach.

    MemBrain is only 2-mils thick and very stretchy- it's nylon. It would make huge pillows under dense-packing blowing pressure, even if it didn't blow out (which it might).

  5. Matt_hofer | | #5

    Thanks to all. Martin I don't see the link you sent "How to install Cellulose Insulation", would you please resend.

  6. GBA Editor
    Martin Holladay | | #6

    I'm not sure why the link isn't working for you. It's working for me. Even if it isn't visible, it works. Try it again.

  7. Matt_hofer | | #7

    Got it . One last question, in a 7500 heating degree day climate zone do I need the MemBrain vapor retarder or could I just Sheetrock over the Insulweb?

  8. charlie_sullivan | | #8

    Lots of people think it's fine to just sheetrock over the insulweb. If you do that, you'll want to make really sure that the sheetrock is truly airtight. Air flowing out leaks adds much more moisture to the wall than vapor permeability of the sheetrock does. But the vapor permeability of the sheetrock does mean some moisture will go into the wall and increase the moisture level in the sheathing. MemBrain will result in lower moisture in the sheathing. That could prolong the life of the sheathing even if it isn't necessarily a problem without it. Also, you might have a local code official who thinks that a vapor barrier is needed, in which case the MemBrain helps satisfy that official. I would recommend using it--it's cheap, will make your walls last somewhat longer, and will help you achieve excellent air tightness more easily.

  9. Expert Member
    Dana Dorsett | | #9

    In climate zone 6 with plywood sheathing skipping the MemBrain would be a mistake. The walls that get direct sun would probably be OK, but the north side would be susceptible.

    If you painted the sheet rock with a "vapor barrier latex" it would be fine without the MemBrain, and that would be better than poly sheeting (more vapor tight than optimal) or only standard latex paint (too vapor open for the climate) as the interior side vapor retarder.

    If you sheathed it with Celotex or DenGlass / GlasRoc instead of plywood it would be fine without the interior side vapor retarders, as long as the siding is back-ventilated, due to the much higher vapor permeance and much lower moisture susceptibility of those sheathing types.

  10. Chaubenee | | #10

    Dana, are people starting to use Densglass and Glasroc for residential sheathing now,underneath foam exterior insulation?

  11. Matt_hofer | | #11

    I would like to thank everyone for the comments and advice.

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