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I’m fixing up an old uninsulated shed. The roof — built of metal sheeting on plywood, nailed to the 2-by-4 rafters — needs to be insulated. What is the best way to insulate the roof that avoids condensation, which might lead to mould or rot?

I can think of two options:

  1. Install batten insulation between the rafters, and then cover this with a vapor barrier.
  2. Skip the batten insulation and tack polyisocyanurate boards onto the underside of the rafters, leaving an airspace between the rafters.

Option 1 risks trapping condensation between the insulation and plywood. Option 2 doesn’t seem to offer very much insulation.

Are there other options to consider?

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  • What about spray foam insulation?
    – jwh20
    Commented Aug 6, 2020 at 11:46

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You need to prevent air from getting to the metal. If no air circulation there is no moisture will condense on the inside. Spray on insulation works well but is fairly expensive, Fiberglas works but needs to be in contact to prevent air movement. This is one area that I compress the insulation some to make sure it stays in contact. This looses some of the R value But eliminates the sweating. Those are the 2 methods I have used.

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  • The metal sheeting is nailed to the plywood, and from the inside of the shed, I only have access to the underside of the plywood. Unless I take off the metal sheeting, which I don’t wish to do, I don’t have direct access to the metal. Not ideal, I agree, but a constraint around which I need to work.
    – Michael
    Commented Aug 6, 2020 at 15:53
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    Sealing the ends with foam may be your only hope nothing you do under the wood will have any affect as long as air can circulate in the corrugated air space when the metal is cold it will condense water vapor sealing the ends should stop the air circulation. When you were talking about insulating below I thought you were removing the decking.
    – Ed Beal
    Commented Aug 6, 2020 at 16:19
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You can try anti-condensation materials for insulation. It will help to keep the moistures away from the roof.

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    Welcome to Home Improvement. Could edit your answer to expand on what "anti-condensation materials" you're referring to? The OP did already mention using a vapor barrier, is this something different?
    – FreeMan
    Commented Aug 17, 2020 at 16:01

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