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Air permeable insulation in unvented attic

Mark2golf | Posted in General Questions on

I am designing my new home. I am building in zone 2 (Orlando) and plan to build an unvented attic. 
I plan to use rockwool or fiberglass batt against bottom side of roof deck (both insulations are air permeable). IRC R806.5 5.2 allows this. I will comply with all requirements including vapor diffuser port and conditioned attic with 50cfm/1000 sqft.

Quesiton: How do I install the 9” of Rock wool or the 10.4” of fiberglass to get the required r-38 value. I don’t have a roof truss design yet. It roof truss is deeper than the insulation (ie 9”Rockwool and 10” rafter) or if roof truss is not as deep as insulation (ie 10.4” fiberglass batt and 8” rafter).  Also – regardless of rafter depth, is it important that insulation contact underside of roof deck or can there be an air gap (such as could be the case with 9” of rockwool and 10” rafter)?
Thanks
Mark

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Mark,

    Just to clarify. Will you have cathedral ceilings or some type of attic space?

    1. Mark2golf | | #2

      I will have an attic space and plan to locate Air Handler in that space.

  2. Steve Knapp CZ 3A Georgia | | #3

    Be sure to read this article from Building Science America: https://www.buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned.

    If you attic space, it would be easier/cheaper to put the insulation on the attic floor. You could blow in quite a bit of fluffy insulation (more than R-38) for a reasonable amount of money.

    If you are still designing, I would try to get the HVAC out of the attic. You could run your ducts in a plenum truss or use split minis that don't require ducts, for example.

  3. Expert Member
    Armando Cobo | | #4

    Installing your HVAC system in a conditioned attic with a vapor diffusion port is a good idea, but I would not use trusses in this application since you'll have a lot of uninsulated voids between the webs of the trusses. I would use 2x10, 2x12 or TJI rafters.
    I like to use 14" TJIs so I can get R49, which is in the new 2021 Code, and is not that much more money. Rockwool, fiberglass and cellulose are all good choices for insulation, but do pay attention to the details on the vapor diffusion port. Also, TJIs are straight vs. all the dimensional lumber we get today.
    Here is a detail I use...

  4. Jon R | | #5

    > can there be an air gap?

    I believe that 5.2.8 says "directly below", which I interpret as no air gap.

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