Metal/Polystyrene Over Existing Asphalt Shingle

Guest User
We have a 40 year-old cabin at 8000 feet in southern Utah. The existing asphalt shingle roof needs replacing but only small parts are significantly deteriorated. The cabin has a vaulted roof with no attic space or insulation. The roof decking is 2x6 T&G. The slope is 3" in 12". There are no valleys, only ridges. We want to replace the roof and add insulation. We are considering leaving the existing shingles in place and doing the following: (1) lay 30 lb. felt or a synthetic underlayment, (2) install 2" polystyrene rigid foam, (3) install standing seam metal. Please comment on this proposal and offer any suggestions.
Todd Miller
Isaiah Industries, Inc.
Unless there is currently a vapor barrier in the assembly, I would suggest underlayment and then a vapor barrier over the old shingles. As far as installing standing seam over foam, this is done but this is a pretty low pitch and there will be risk of over-driving fasteners and causing problems. The installers will just have to be very careful to not overdrive the roofing fasteners.
Eric Novotny
An informed customer is the Best Customer!
I would suggest removal of the asphalt. The asphalt will not allow to the foam to have continuous contact with the roof decking and will impact the resultant R-Value you will achieve. If it were my home, I would proceed as follows: - Remove asphalt shingle - Prepare surface and clean surface - Nail/Screw any loose boards and replace any damaged ones - Install peal and stick underlayment (non-granulated) over entire roof surface - This will give you the proper air seal and vapor control at the roof layer - Install Rigid foam (go for more than 2" if you can swing it) - Install new roof

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