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Community and Q&A

Blown-in cellulose insulation directly against the roof decking?

Kenneth Patchen | Posted in Green Building Techniques on

I have a question about blown-in cellulose insulation for an attic roof. Here in freezing snowy upstate NY, fifteen years ago, we had blown-in cellulose insulation added to the walls of our 103 year old house. Recently a free energy audit, which included IR imaging, showed some settling of the insulation which is to be expected but it also showed several empty areas that the original contractor missed, which was not expected. We now plan to have new insulation blown into the settled and missed areas as well as adding insulation for the attic roof. Here’s the question: the contractor’s plan for the attic calls for placing Tuff R-2 foam board against the rafters with cellulose then densely packed between the foam board and the roof decking, i.e., the cellulose would be packed directly against the roof decking. (We then plan on finishing the attic, installing new electric and drywall and turn the attic into a living space.)

I know that placing fiberglass insulation directly against the roof decking is a no-no as fiberglass insulation requires separation with a spacer to allow for proper ventilation to prevent mold growth, ice damming etc. However, the contractor told us that when cellulose and foam board are installed properly no spacer is needed. He claims the dense pack cellulose with the foam board create a proper barrier so the moisture does not condensate at the bottom of the roof decking. He explained that there is a whole building science field now so they have a much greater understanding now how the different materials respond under different circumstances. 

Does having cellulose densely packed directly against the roof decking without a spacer for ventilation make sense? how does doing so prevent moisturefrom gathering and causing mold, ice dams, etc.

My thanks in advance for your time.


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  1. GBA Editor
    Martin Holladay | | #1

    The method your contractor is promoting has been used in the Northeast for several years. The method is promoted by some cellulose manufacturers.

    The method is also a code violation. Building codes don't allow the use of cellulose between rafters, unless either (a) there is a ventilation gap between the top of the cellulose and the underside of the roof sheathing, or (b) there is an adequate layer of rigid foam insulation above the roof sheathing.

    Some building scientists warn that the method suggested by your contractor is risky, and may lead to moisture accumulation and rot.

    You can make your own decision. However, if your house will be inspected by your local code authority, you should run the plan by your local building authority first.

    For more information, see How to Build an Insulated Cathedral Ceiling.

  2. Expert Member
    Dana Dorsett | | #2

    With it's foil facers the Tuff-R is a true vapor barrier, which means the roof decking has to dry toward the exterior or not at all. If it's #30 felt (or worse, an impermeable membrane such as Grace Ice & Water Shield) with asphalt shingles up there it's moisture trap, but if it's slate, wood shingles or tiles, or metal roofing on purlins it'll usually be fine.

    If it IS low-permeance roof layup on top of the roof deck it's much safer to use a "smart" vapor retarder on the inteior side, and NOT foil faced foam board. If you want to keep the thermal break over the rafter edges you can cut strips of rigid foam and cap-nail it to the rafter edges, and fill between the strips of foam with split-batts (or if the contractor can figure out how to do it, fill it all with cellulose. It's easier to blow the cellulose in netting stapled to the rafters first, THEN apply the smart vapor retarder (eg Certainteed MemBrain or Intello Plus), THEN the rafter edge strips. As long as at least 2/3 of the center cavity R value is on the exterior side of the vapor retarder, the relative humidity of the entrained air at the vapor retarder will be low enough in winter that it will be fairly vapor-tight. In spring when the roof deck releases it's accumulated winter moisture burden into the cavity air with rising roof deck temps, the RH in side the rafter by goes up, vapor retarder becomes far more vapor open and the moisture is released to the interior.

    With 0.1 perm shingles or worse and 0.05 perm foil facers on the foam, there is no drying in any direction at a rate sufficient to be useful- any moisture that gets into the cellulose & roof deck pretty much stays there. As long as it's bone-dry when it all goes together and you NEVER get even a minor roof leak it'll be fine. But requiring perfection over the long term is too much to ask for.

  3. Kenneth Patchen | | #3

    Thank you very much Martin and for your time and comments. You've given us much to think about and it's safe to sat that we won't be following the contractor's proposal to have insulation blown-in directly against the roof decking. I have to say his pitch was very professional and, almost, convincing. My wife is smarter than I am by a long road and raised the flags of caution.
    Thanks again.

  4. Kenneth Patchen | | #4

    Sorry, Dana, meant to include you in the thanks, as above.

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