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Community and Q&A

Using foam sheeting & faced fiberglass batts

Mark W. | Posted in Energy Efficiency and Durability on

I am insulating my garage ceiling and have laid 1″ foam sheeting above the horizontal 2×4 of the trussses. My plan is to put faced fiberglass batts between the trusses up to the 1″ foam sheets. Then I would like to put another 1/2″ foam sheet over the fiberglass batts, sandwiching the faced fiberglass between the foam sheets. Is there anything wrong with doing this? Thank you for your help!

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  1. GBA Editor
    Martin Holladay | | #1

    If you are using unfaced EPS foam, the foam will be vapor-permeable enough that you will probably stay out of trouble. However, the technically correct answer is that this type of ceiling insulation needs to be designed so that the exterior rigid foam is thick enough to keep the interior face of the rigid foam above the dewpoint during the winter. Thicker foam is safer than thinner foam (when the rigid foam is on the exterior side of fiberglass batts).

    You didn't tell us your climate zone. The colder your climate zone, the thicker the exterior rigid foam has to be. This is explained in the following article: How to Build an Insulated Cathedral Ceiling.

    The minimum R-values for the exterior rigid foam layer in this type of ceiling are:
    R-5 foam for Climate Zones 1-3,
    R-10 for Climate Zone 4C,
    R-15 for Climate Zones 4A and 4B,
    R-20 for Climate Zone 5,
    R-25 for Climate Zone 6,
    R-30 for Climate Zone 7, and
    R-35 for Climate Zone 8.

    Of course, you don't have to do it that way. You can just install a gyspum drywall ceiling and blow cellulose on top. That's a lot simpler.

  2. Mark W. | | #2

    I'm in Wisconsin and the 1" R5 foam is toward the exterior (cold). The foam sheets are enfaced pink. I have cold storage above the trusses. That's the reason I am doing it this way. Thank you!

  3. GBA Editor
    Martin Holladay | | #3

    You don't have to install rigid foam on the cold side of your fluffy insulation if you don't want to. But if you do choose to install rigid foam on top, you need to make sure that the foam is thick enough to avoid problems with condensations.

    Wisconsin is split between Climate Zone 6 and Climate Zone 7. If you are in southern Wisconsin (Zone 6), and you are using pink XPS foam, you need to use at least 5 inches of foam (R-25) to stay out of trouble. It's probably easier to just skip the cold-side rigid foam.

  4. Mark W. | | #4

    The R5 is already installed. So are you saying I will have issues? What about putting the 1/2" R3 over the faced fiberglass on the underside?

  5. GBA Editor
    Martin Holladay | | #5

    As I already explained, the R-5 rigid foam on the exterior side of your fiberglass insulation isn't thick enough to avoid problems with condensation. You have two choices: thicken it up (ideally to R-25 of foam, which is a total of 5 inches of foam) or remove it.

    From a moisture standpoint, there isn't any reason to worry about the interior foam layer that you are planning to install.

  6. wjrobinson | | #6

    Attached garage?
    Building inspector OK?
    Fire rated drywall?
    Rigid foam for holding storage loads and people?

    Garages have little moisture....
    Your plan is wrong but may be fine due to lack of moisture sources.

    If I were there I would;
    Use plywood for a small storage portion above
    Use unfaced high R batts
    Then 2" taped rigid foam below
    Then fire rated drywall

    Good luck.

    Ask questions people before you start projects...

  7. charlie_sullivan | | #7

    AJ has good questions. The usual rules might not apply, depending on what kind of garage this is. Add to his questions, is the garage heated? How much? As for moisture, maybe there is not much, but maybe there are snow-covered cars pulled in that drip onto the floor. But maybe there are lots of air leaks in the doors, so maybe it stays dry.

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