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When insulating attic rafters, should they be completely filled or should there be room left for air movement?

Zwerg | Posted in Green Building Techniques on

Attic is finished room, had water damage from use dams, old insulation did not fill rafter completely

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  1. jklingel | | #1

    Please elaborate. WTH are "use dams"? Do you mean "ice dams"? What type of insulation was, or will be, put back in? Etc.

  2. GBA Editor
    Martin Holladay | | #2

    I'm going to assume that you meant to write "ice dams."

    The type of roof assembly you are talking about -- an insulated sloped roofing assembly -- can be either vented or unvented. However, it's easy to make a mistake if you get the details wrong. Here is an article that should explain everything you need to know: How to Build an Insulated Cathedral Ceiling.

  3. Zwerg | | #3

    Thank You for responding, and yes I did mean to type ice dams :). I do have a few vents in the attic crawl spaces. The existing old insul is 2-3 inches at best and very matted and brittle. I started to put new r-19 in attic rafters, which are 6 in deep. Not sure to fill channel completely or leave space for air movement.

  4. GBA Editor
    Martin Holladay | | #4

    If you want to use fiberglass batts, you absolutely have to establish a ventilation channel between the top of your insulation and your roof sheathing. To make this channel, you need to install an airtight baffle that leaves a gap at least 1 1/2 inch deep above the baffle.

    R-19 fiberglass batts aren't enough. They won't meet minimum code requirements for a roof. (The code requires R-30 insulation in Florida and R-49 insulation in Minnesota).

    If you install R-19 batts, you are almost certain to continue to have ice dam problems.

  5. Zwerg | | #5

    Thank you Martin, that answered my ? Perfectly. I can stop scratching the hole in my head now:)

  6. Zwerg | | #6

    Would it be useful to apply rigid brd insul over batts after installed, or would visgueen be appropriate

  7. GBA Editor
    Martin Holladay | | #7

    You shouldn't install any interior Visqueen (polyethylene sheeting). However, when you install the drywall, pay close attention to airtightness -- especially at penetrations like electrical boxes.

    A continuous layer of rigid insulation attached to the underside of the rafters is an excellent way to reduce thermal bridging and to improve the R-value of your roof assembly.

  8. Expert Member
    Dana Dorsett | | #8

    If you MUST use batts in this application, use high density "cathedral ceiling" batts (typically R21@ 5.5") or R22-23 rock wool, not low density R19s or R22s. But if you have 2x6 rafters you won't have the venting clearance for anything better than high density R15s

    Air sealing behind kneewall attic crawl spaces is a fool's errand (Ive played the starring "fool" role in that movie several times over, but never again!) In almost all cases with 2x6 rafters you'd be better off going unvented, sealing & insulating the roof deck with 1-2" of closed cell polyurethane foam then filling in the rest of the space between the rafters with the fiber insulation of your choice, as long as it has NO BATT FACERS or other vapor retarders in the stackup. Then when it's time to re-roof, add rigid foam to the exterior to fatten out the R, and reduce the thermal bridging at the rafters.

    Adding full sheets of rigid foam to the interior side of the rafters can be next to impossible from a practical point of view in the tight spaces behind kneewalls, but if you go that route, using unfaced EPS still works in conjunction with up to 2" of polyurethane against the roof deck, and would more than double the R-value at the rafters.

  9. Zwerg | | #9

    Thank you Martin and Dana

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