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Rigid insulation with OSB in rafters?

hyasinthgurl | Posted in General Questions on

So, you guys have been a big help so far. I have another question, spurred by what looks like a good deal on craigslist for new polyiso insulation.

The catch is that this insulation has OSB on one side and paper on the other. I am interested in putting it up within my catherdral ceiling rafters and under the rafters.

How bad would this be to put up? If it is not good, are their any cheap workarounds? The price is half the foil faced at the big box store.

There would be a 1″ space between the insulation and the roof, but it technically isn’t vented (1950s 1.5 story ranch).

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  1. GBA Editor
    Martin Holladay | | #1

    The name for the product you are describing is nailbase. The best place to install it is on top of your existing roof sheathing.

    It really wouldn't make much sense to cut it into thin strips and insert it between your rafters. However, if you want to install a continuous layer of nailbase under your rafters, that would be possible (although it would be awkward, due to the weight of the product).

  2. hyasinthgurl | | #2

    Thanks Martin, it doesn't sound like a good fit.

    How about fiber faced polyiso inside the rafters (with foil under the rafters)? Found a deal fiber faved also...

  3. GBA Editor
    Martin Holladay | | #3

    Q. "How about fiber faced polyiso inside the rafters?"

    A. It's possible, but it's fussy, time-consuming work. It only makes sense if you do a meticulous job of air-sealing around the perimeter of each rectangle of rigid foam, using canned spray foam, high-quality tape, or caulk. What you save in materials cost (compared to spray foam) you eat up in increased labor.

  4. hyasinthgurl | | #4

    Weekend time I can spare for the rigid, money for spray foam...not so much

  5. wjrobinson | | #5

    Pieces equate to air leaks. You''re building lobster traps. If you catch said lobsters, they are green before you eat them. Green with mold.

    Possibly if you install all the foam first, then spray foam all of it... But using the litttle spray cans from HD... not my idea of something to be done or to be involved in. All the stores now sell the big froth packs, go that route or get your work top sprayed by a sprayfoam company.

    If you do this... take lots of pictures and video to share here. Then maybe your evidence will change my mind to favorable.

    Good luck.

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