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Rigid foam over fiberglass wall batts in attic

Nigel Campbell | Posted in Energy Efficiency and Durability on

I’m amazed at the info this site has. Hopefully someone can give me some insight on a couple of problems I have:

I have a two story home in Texas. In my garage attic, I have 60 x 10 feet of vertical walls facing the hot attic space. These walls are of 2×4 construction with vertical unfaced fiberglass batts held in place with a thin mesh.

1) The interior walls on the backsides of these attic walls are warmer to the touch than other walls in the room. There are many voids in the fiberglass batts and I can see the backsides of many of the outlet boxes.

I’d like to improve this insulation by using leftover 4×8 sheets of faced 1 inch polyiso rigd foam boards from another insulation project. I’d like to seal the outlets and then use a double layer of 1 inch 4×8 sheets over the entire 600 sq feet of walls with the reflective surface facing the attic and finishing it up with seam tape. This should add R10-12 to these walls.

My question, will I have any moisture issues using this approach?

2) My 2nd issue is that I have 8 feet of the attic walls sitting over the top of 2×12 un-insulated floor joists. Those floor joists extend 25 feet into the interior of my home. I thought there was insulation in that 8×25 space but there isn’t. The fiberglass batts in the wall above it extend past the bottom of the wall cavity and fit loosely into the cavity of the 2×12 floor joists hiding the fact that there is no insulation in that void.

Do I just roll up some fiberglass batting to block the opening of this void? Do I ignore the fact that this 8×25 foot void is directly below my 2nd story and only has sheetrock separating it from the unconditioned garage? Or is it worth it to blow insulation into this space?

TIA,
Nigel C

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Nigel,
    Q. "Will I have any moisture issues using this approach?"

    A. No. Your instincts are good. The builder who insulated the wall you describe was ignorant. The rigid foam will greatly improve the thermal performance of the wall.

    Q. "I have 8 feet of the attic walls sitting over the top of 2x12 un-insulated floor joists. Those floor joists extend 25 feet into the interior of my home. I thought there was insulation in that 8x25 space but there isn't."

    A. This joist bays lack an air barrier. The standard solution is to install rectangular blocking in each joists bay, directly under the wall above. This rectangular blocking must be installed in an airtight manner, and must be aligned with the air barrier in the wall above (that is, the proposed layers of rigid foam that you intend to install). There must be no seams or leaks -- either at the perimeter of each piece of blocking, or at the transitional area between the blocking and the air barrier of the wall above.

    You can use solid 2-by lumber for the blocking if you want, but most builders find it easier to use foil-faced polyisocyanurate foam for this purpose (because it is easy to cut and easy to tape). Seal the perimeter of each piece of rigid foam with caulk, canned spray foam, or high quality tape.

    If there is any horizontal section of these joist bays that are exposed on one side to unconditioned space, then the joist bays need to be insulated.

    For more information on the work I have described, see Two ways to insulate attic kneewalls.

    -- Martin Holladay

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