Radiant barriers, insulation and metal roofs

Guest User
5/10/2002
I'm planning to have a metal (unpainted Galvalume) roof installed on my home in Austin, Texas. The primary motivation is comfort with asthetics being a strong second. The home was built in 1948 with a very low pitch asphalt roof on a 1x4 T&G deck and no insulation. In order to get the most out of my efforts I'm thinking I'd like to install insulation, a radiant barrier and full ridge / sofit vents. Orginally I was going to remove the entire roof, install fiberglass batts on the floor of the attic, CoolPly decking, and the roof on that. My contractor is worried about the cost of removing the deck and hazards of having the roof open for any period of time. He has suggested leaving the deck on and installing radiant barrier backed foam panels on top with the metal panels on top of this. I'm all for the lower costs, but I'm wondering how effective this will be in lowering the radiant load and insulating the house. Any other ideas, comments, cautions, or suggestions?
Todd Miller
Classic Products, Inc.
5/12/2002
I understand that, being in Austin, you do not normally have to worry too much about excessive moisture buildup in homes. However, I am still concerned that what your contractor has suggested does not allow for any ventilation. This could lead to excessive moisture buildup in the foam panels. My best suggestion is to pursue your original idea of removing the old roof, creating a vented airspace, and installing a radiant barrier along with new decking, underlayment, and metal roofing. So much of what we do to our homes is making them tighter -- siding, windows, weatherstripping, etc. That is all good but it also leads to the unhealthy trapping of moisture inside of homes, attics, etc. I would never scrimp on having a wellvented attic. It enhances energy efficiency and helps avoid moisture troubles.
Guest User
12/20/2002
his contractor appears to recommend how i have been building my sheds and now plan to reroof my old house. osb then underlayment felt then 1/2 in foam with AL(radiant barrier) facing out. then corrugated roofing metal. your response was 1)radiant barrier functions with air spac only 2)vapor buildup trouble my question is where does water condense? between metal and foam or foam and tar paper? not that any moisture remains for long outside here in tucson....*grin* my expectations are that the foam is very important in lowering our roof temps.
Todd Miller
Classic Products, Inc.
12/20/2002
Condensation occurs when and where the dewpoint is reached. In the case you described, that would more than likely be in and around the joints / seams in the foam insulation including the back side of the metal over those seams. Also, as you have mentioned, radiant barriers are effective only when they face an airspace. Their efficiency is pretty much eliminated when they are sandwiched between two other materials.
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