Rigid Insulation on top of open rafters

David White
4/18/2017
I am building a 25 x 30 shop with an open 14' lean-to on low side. Single slope Roof an 4/12 pitch. 2 x 6 framed walls with 2 x 8 exposed rafters on 16" centers (span will be 14.5' and 12' approx. with a center beam). I was thinking of running 2 x 4 horizontal purlins leaving 4' between purlins and laying 4' x 8' 1" or 1.5" rigid insulation between purlins and screwing metal roof to the purlins. Will this create condensation issues on the underside of the metal which could eventually rot out the 2 x 4 purlins and cause a problem with the roof fasteners?
Eric Novotny
An informed customer is the Best Customer!
4/18/2017
You will need to check code about the foam being exposed to the inside if you are not going to do solid decking. They may require that you sheet or to require a specific foam that is rated for open exposure. See that they say and that will somewhat dictate your design requirements.
David White
4/18/2017
I think a foil-faced insulation is allowed. What is your opinion on the condensation?
Eric Novotny
An informed customer is the Best Customer!
4/25/2017
My concern would be with getting the seam sealed between the sections of foam to make sure that air and moisture aren't getting to the backside of the roof. I would recommend taping those seams and incorporating some underlayment (synthetic polypropylene) to make sure there is no air movement getting up there.

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