Best Insulation for new roof above old Roof

Seth Capron
Roof!
11/21/2006
I work with a non-profit organization and we're trying to replace the roofs on our three buildings. Two of them have large attics, but the third has a vaulted ceiling with exposed trusses. To preserve the interior space, we were hoping to put a second (metal) roof on top of the current asphalt shingled roof, and to create ventilation and insulation in between. Our slope is approximately 5 over 12. I have several questions about the best way to do this. My general plan had been to use rigid insulation with a reflective side facing an some form of airspace creating with 1x4s or 2x4s, and installing a ridge vent at the top. Are there any advantages to keeping the old shingles under our insulation, or should we just remove them? Would it be possible/cost effective to use fiberglass insulation instead? How large a gap should I leave? If the goal is simply air flow and it does not affect R-value, is a 1 inch ventilated space as effective as 2 or 3 inches? Do I need to close off the bottom and install sofit vents, or can I just overhang the second roof past the existing roof and leave the ends open? Thanks so much!
Todd Miller
Isaiah Industries, Inc.
11/23/2006
1" vented space is regarded as minimal and usually adequate. I would box in the soffits and vent them for air intake to avoid getting insects, etc. in under the panels. I would use a rigid insulation. There should not be a problem with leaving the old shingles in place.
Todd Miller
Isaiah Industries, Inc.
11/23/2006
Also, make sure that the metal roof youi choose is suitable (per its mfr) for this type of installation.
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