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Flat roof with 2×10 rafters: Can I use dense-packed cellulose and close the vents?

user-1130926 | Posted in Energy Efficiency and Durability on

I have a flat roof constructed with 2x10s, there are no overhangs but there are round vents if the fascia. I live in zone 5. the ceiling currently is sheet rocked with R-22 kraft aced Fiberglass. The insulating contractor suggested that drilling holes and adding dense pack cellulose would be the best method.I would have to close up the vents at the fascia. would this work? should the dense pack be on top of the fiberglass or below? The GWB is attached directly to the rafters and are pitched about 1-1/2″ in 12′. there is a 5ply hot tar roof above that looks to be in good shape that was installed in 1999. there are a couple of hi-hats in the ceiling that I will remove and use a surface mounted LED or track light in there place. What are my options? can this become an unvented roof without using Closed Cell foam? Any help would be appreciated.
Thanks in advance

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  1. GBA Editor
    Martin Holladay | | #1

    No, I don't recommend that you seal up the vents and install dense-packed cellulose -- either above or below the existing fiberglass batts. That would be risky.

    Here is a link to an article that explains all of your options: Insulating Low-Slope Residential Roofs.

  2. Dana1 | | #2

    Dense-packing over the R22s would yield about R35 inside a 2x10 cavity. In zone 5 at least 40% of the total R needs to be above the roof deck, which means you'd need something like R24 EPS which is ~ 6" or ~R30 polyiso (labeled, before derating for climate) which is about 5", to hit that performance level in mid winter.

    If you use reclalimed rigid foam from commercial building demolition or reroofing the cost of 6" EPS or 5" polyiso is remarkably affordable, and greener than any virgin stock insulation of any type.

    If you're not going to re-roof and doing it all from the interior, 2" of closed cell polyurethane on the underside of the roof deck protects the roof deck. If that's followed by dense packed cellulose, with a smart vapor retarder (Certainteed MemBrain, Intello Plus, etc) detailed as an air barrier between the cellulose and gypsum is safe. The smart vapor retarder keeps the cellulose dry enough in winter to perform well and not grow mold. That ends up at about R40 center-cavity, which is below the IRC's R49 code-minimum for zone 5, but it's definitely higher performance than R22.

    Doing just the smart vapor retarder without the 2" foam won't cut it in a zone 5 climate.

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