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Sealed attic with dense-packed cellulose?

Gordon Grant | Posted in General Questions on

Hi,

I am a HERS Rater in NY. Zone 4A.

I have an architect that is speccing a sealed attic with dense pack cellulose in 2×12 cavities. Is that possible? I thought closed cell spray foam was required.

Shingles, Blueskin, 1/2″ cdx plywood, dense pack cellulose, ” 1/8″ plywood  or permeable membrane to seal in dense pack”

There will be blocking at eaves between rafters.

Any thoughts appreciated.

Thanks in advance.

gg

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Gordon,
    If you want to install cellulose between your rafters, codes require either (a) a ventilated air space between the top of the cellulose layer and the underside of the roof sheathing, or (b) the installation of an adequately thick layer of rigid foam above the roof sheathing.

    Option (a) will only work if your roof has no valleys, hips, or dormers. For more information on option (a), see "Site-Built Ventilation Baffles for Roofs."

    For more information on option (b), see "Combining Exterior Rigid Foam With Fluffy Insulation."

  2. Gordon Grant | | #2

    Thanks Martin.

    I see R806.5 in the code for use of the rigid foam for condensation control. Is there a part of the code that mentions the ventilated air space?

    Thanks.

    gg

    1. Expert Member
      Dana Dorsett | | #3

      R802.2 and more likely, R806.3 are probably the parts you're looking for.

      https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction?site_type=public

  3. Jon R | | #4

    Joe Lstiburek wrote:

    "The valley issue can be handled by dropping the valley rafters. See figures from my Cold Climate Builders guide. Hips are handled the same way. This also extends to framing for dormers and skylights."

    Also review SmartVents by DCI and scissor trusses.

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