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Community and Q&A

Cellulose insulation and poly

J C | Posted in Green Products and Materials on

We are using Applegate cellulose insulation, and the installing company recommends using a regular 4 mil poly to hold the cellulose in place. Is poly common and necessary? We are looking to use the least toxic materials necessary, and we are unsure about the health safety of poly. Insight will be appreciated. Thank you so much!

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  1. Expert Member
    Dana Dorsett | | #1

    It depends on the climate and the wall construction stack-up. In most cases if a vapor retarder is needed it would be better to use 2-mil nylon, which is variable permeance, than 4 mil polyethylene, which is a true vapor barrier. But it may be better to install it in netting rather than plastic sheeting of any kind.

    So, what is your location, and what are the layers of the wall from the outdoor paint to the indoor paint?

  2. J C | | #2

    Thank you, Dana. We are in CT, and these are the layers below, as communicated by the builder. If there are greener and safer options for any other components listed below as well, we would love to receive guidance. We are working with a standard-style builder (not a green builder), and we are therefore doing our own research and substituting green materials for their typical standard materials. The information from this forum is invaluable!

    Cedar Clapboard
    Benjamin Obdyke Home Slicker
    Tyvek House wrap
    1/2" plywood sheathing
    2x6 douglass fir wall studs
    Open wall bays are to be filled with Applegate blown cellulose and possible the Ecoseal system
    **** 4 mil poly sheeting (this is to hold the cellulose in the bays)<--???
    5/8" gypsum drywall
    paper tape, powdered easy sand light weight joint compound, joint compound (these are typically by a co. like USG)
    Trim applications (consists of wood trim, wood glue)
    Trim all gets caulked with a paintable latex caulk
    Primer on trim (comes factory primed)

  3. GBA Editor
    Martin Holladay | | #3

    You don't need interior polyethylene in Connecticut. In fact, polyethylene can cause more problems than it solves, especially if the house has air conditioning, because the polyethylene will prevent the wall assembly from drying to the interior during the summer.

    For more information on interior vapor barriers, see Do I Need a Vapor Retarder?

    Most installers of cellulose prefer to use an interior membrane that is air-permeable; one well-known brand is InsulWeb. The InsulWeb membrane is designed to allow air to escape during the blowing process. For more information, see How to Install Cellulose Insulation.

  4. J C | | #4

    You're phenomenal, Martin. Thank you! Truly grateful for your expertise and guidance.

  5. Stephen Sheehy | | #5

    J.C. :who designed the house?

    Did you have an architect? If so, he or she should have already specified techniques and products that answer your various questions. You are obviously concerned about toxic materials, but the time to address many of those concerns was in the design phase, not in the middle of construction.

    You may want to consider a blower door test before the insulation goes in. Does your contract with the builder include an air-tightness number?

    Even now, if you don't have one, consulting with an architect might be worth the cost.

  6. J C | | #6

    We do have an architect, and cellulose was specified as the insulation in the design phase. However, now that we are approaching the insulation phase, the architect said that Membrain should be used with the cellulose, while the builder said that Poly should be used. We fortunately learned on this forum that neither product is appropriate.

  7. Charlie Sullivan | | #7

    Membrain would be a fine choice as well. It allows drying to the inside in the summer, though not quite as easily as the netting such as InsulWeb does. And it inhibits vapor tranmission outward in the winter, which you don't need but it might help keep the assembly a little dryer in the winter. It also serves as an air barrier, giving you the option of slacking off a bit on air tightness when you get to the drywall.

    One option is to use the insulweb to get the cellulose installed easily, and then put MemBrain over the top of that for air sealing.

    Overall, Membrain is a fine choice, although it's not necessary in your climate. Poly would be risky and a bad idea. MemBrain + InsulWeb, just MemBrain, or just InsulWeb are all fine choices. If your builder is not comfortable with just InsulWeb, he might be more comfortable with MemBrain in addition or instead; depending on how he plans to acheive air tightness, that might be easier with MemBrain than without.

  8. J C | | #8

    Thank you so much, Charlie. It sounds like InsulWeb is ideal. Very grateful for the expertise offered on this forum!

  9. James Morgan | | #9

    Poly is such a bad choice that I'd be looking for a more knowledgeable insulation contractor. Bear in mind that the cellulose has to be installed to a density of at least 3.5lb/s.f. or it is liable to settle over time and leave gaps at the top of the wall, moreover a retaining material which is not air permeable will make it harder for the material to pack properly throughout the stud bay, so I'd stay away from Membrain as the initial retaining material. Insulweb is the way to go.

  10. J C | | #10

    Thank you so much, James. Does anyone have recommendations for Connecticut companies that properly install cellulose?

    Using poly seems to indicate a fundamental issue with knowledge of how to install cellulose. We are relieved that we researched this for ourselves via this forum.

  11. James Morgan | | #11

    Sorry JC, I'm in North Carolina. I suggest you ask the GC to get proposals from two or three other installers to include their installation spec and a guaranteed density on the packed cellulose. Or ask for a recommendation from a reputable local energy rater.

  12. Charlie Sullivan | | #12

    One way to find a good cellulose installer could be to talk to Bill Hulstrunk, the technical manager at National Fiber in MA, a cellulose supplier. He's the one who trains the installers in their network, so he likely knows who's good.

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