Insulation, Radiant and Vapor Barriers - Request Opinion on Roof insulation/vent
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gandtodd
04-16-04, 09:18 PM
Hello, I'm still muddling different possibilities for my reroofing, and trying to get two experts at least to aggree (not an easy task!). To recap- what's already there, is a 4/12 pitch over a timber frame cathedral ceiling, 2X6 tongue and groove decking (this is the ceiling as well as the roof deck), 2 inches of foil faced rigid foam insulation, asphalt shingles. I want to add insulation and install a standing seam roof and vent to minimize ice dams. Here's the latest idea from a metal roofing expert and I ask for opinions:
"Adding sprayed in place foam over the existing 2" should provide you with your interior air barrier as it has a very low perm rating. Do not shave the exterior after it opens up the cells.
By adding 4", the existing shingles are far enough into the assembly that they could be left in place as they will be past the dew point, provided you had a proper foam application. This would provide protection during construction and help reduce costs. I would consider strapping the roof vertically with 2x4 on edge screwed down through into the deck. They make deck tightscrews for this application. Make sure the strapping is over the rafters and no more than 24" on center. Then they can foam in place to the top of the strapping.
Then you are free to strap the roof horizontally with minimum 2x2. If you are installing a vertical rib panel make sure it has large enough flutes to move air vertically over the strapping and check with the manufacturer on the required strapping spacing for proper support. I would still recommend using a reinforced underlay over the strapping directly under the roof covering. Use house wrap tape to seal around all penetratons. Then build in eaves and ridge ventilation and choose an energy star coating for best reflectance on the sun."
This suggestion sounds less expensive than removing everything plus I would end up with much more insulation than just a 4 inch layer. The question is, will I get away with no vapor barrier on top of the deck in this scenario? Thanks, CT
"Adding sprayed in place foam over the existing 2" should provide you with your interior air barrier as it has a very low perm rating. Do not shave the exterior after it opens up the cells.
By adding 4", the existing shingles are far enough into the assembly that they could be left in place as they will be past the dew point, provided you had a proper foam application. This would provide protection during construction and help reduce costs. I would consider strapping the roof vertically with 2x4 on edge screwed down through into the deck. They make deck tightscrews for this application. Make sure the strapping is over the rafters and no more than 24" on center. Then they can foam in place to the top of the strapping.
Then you are free to strap the roof horizontally with minimum 2x2. If you are installing a vertical rib panel make sure it has large enough flutes to move air vertically over the strapping and check with the manufacturer on the required strapping spacing for proper support. I would still recommend using a reinforced underlay over the strapping directly under the roof covering. Use house wrap tape to seal around all penetratons. Then build in eaves and ridge ventilation and choose an energy star coating for best reflectance on the sun."
This suggestion sounds less expensive than removing everything plus I would end up with much more insulation than just a 4 inch layer. The question is, will I get away with no vapor barrier on top of the deck in this scenario? Thanks, CT