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How can I best install rigid foam as I finish my attic?

Tight Attic | Posted in Green Building Techniques on

Hi folks,

I’m finishing off my attic. The plan is to use fiberglass between the old true 2×6 rafters, which will be vented with baffles. I then want to put lots of rigid foam on top of the rafters, as I’m in a cold climate in upstate New York. I’m thinking 4 inches.

How can I find screws to long enough to fasten the furring strips through the foam to the rafters?

Will the foil-faced foam act as an adequate vapor-barrier as long as I seal the seams with aluminum tape?

Need I use any special drywall on top for adequate fire protection?


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  1. wjrobinson | | #1

    Cold climate? Rigid foam inside attic before drywall? Or outside under shingles as you said "on top".

    Rigid foam on one side, taped done in two layers and sealed well around perimeter may work. Moisture intrusion will be a disaster. Whether moisture gets in is to do with many details and how well whoever does following the "plan".

    The more we seal up assemblies, the less forgiving.

    Join GBA and start reading through member information. Use the search box. All that you want to know is here already. Subscribe to Fine Homebuilding and The Journal of Light Construction. Read up at the Building Science Corporation website. Joe Lstiberuk and Straube are great sources. Martin Holiday here is too. So is Dana. Robert Riversong in the past is a huge source of natural building and moisture issues.

  2. user-1140531 | | #2

    J STAIR,

    Instead of using fiberglass between the rafters, I would put foam board in there. If you want all the R-value you can get for the available space, you will get more out of the foam than the fiberglass. Then if you want still more R-value than what is provided by foam between the rafters, you can put foam board over the inside edges of the rafters, and the drywall on top of that. You might consider 1” or 1 ½” foam between the drywall and the rafters.

    If you want an air space on the cold side, the foam can be set with a gap to provide the air space, and then you won’t need any separate baffle as would be the case with fiberglass. I would cut the foam on a table saw for a tight friction fit between the rafters.

    I may be misunderstanding you. I assume you want the foam added to the interior side of the rafters rather than on top of the roof deck. However, in any case, I do not understand what you are proposing for the use of furring strips.

    That last layer of foam board between the drywall and the bottoms of the rafters should have its joints sealed vapor tight.

  3. GBA Editor
    Martin Holladay | | #3

    J Stair,
    There may be a misunderstanding here. If you want to install "lots of rigid foam on top of the rafters," we assume you mean that you are installing it on top of the roof sheathing, followed by new roofing. That plan makes sense.

    If you meant something else -- for example, if you meant that you really want to install lots of rigid foam underneath your rafters -- you should clarify the confusion.

    Suppliers of long screws include:
    Best Materials — Dekfast 6-inch roofing screws and Dekfast 9-inch roofing screws
    Wind-Lock (a source of long screws and plastic hold-down buttons)
    FastenMaster (a source of screws up to 10 inches long)

  4. Tight Attic | | #4

    Sorry about the confusion. I'm talking about putting 3 or 4 inches of rigid foam between drywall and rafters. Furring strips would be used to attach foam to the rafters and then drywall to the furring. Thanks for those sources of screws!

  5. GBA Editor
    Martin Holladay | | #5

    J. Stair,
    Your plan will work, although your fiberglass batts won't do much unless you build site-make baffles that are airtight above the fiberglass batts.

    Foil-faced foam is a perfectly good vapor retarder. It's a good idea to tape the seams with a compatible foil tape to make sure that the installation won't leak air.

    Ordinary drywall is fine.

    More information here: How to Build an Insulated Cathedral Ceiling.

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