Exterior rigid foam on existing shingle roof?
I’m considering doing all exterior rigid foam on my home. Crawlspace, walls and roof. Continuous and sealed. My roof has 30 year architectural shingle roof on it now that is only about 6 years old. I was wondering if I can just use my existing roof as the WRB and lay down and secure foam right over top of it. I’m in climate zone 4a. I’m thinking two sheets of 3 or 4″ polyiso foam, taped seams, furring strips and metal roof on top. Would I be better off removing the existing roof down to the decking and adding a peel and stick vapor permeable membrane instead? Seems like a waste to get rid of the current roof and i’d think the foam over the shingles would make the shingles last for a very long time.
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Michael,
Here are links to a few relevant articles:
Getting Insulation Out of Your Walls and Ceilings
How to Install Rigid Foam On Top of Roof Sheathing
The main advantage of removing the existing shingles and installing something like Ice & Water Shield is ensuring that you have a good air barrier under your rigid foam. Clearly, you don't want an existing ventilation channel (soffit-to-ridge venting) to undermine your insulation plan -- but I assume that you know that.
If you install your rigid foam in an airtight manner -- with sealed seams and staggered joints between layers of rigid foam -- and you've thought through the question of hidden air channels, then you could leave the asphalt shingles in place.
One more point: most building codes, including the 2012 IRC, require at least R-49 roof insulation in your climate zone. Depending on what R-value per inch you ascribe to polyiso, you'll need about 8 inches of polyiso to achieve R-49.
The economics of stripping/abandoning a nearly new roof are really poor- it's usually better to do it at the time of re-roofing, unless the roof/attic is otherwise completely uninsulated. (But I suppose you've already run those numbers.) To be sure, a new metal roof is going to outlast the remaining lifespan of the asphalt shingles by 2-3x.