I sometimes see conflicting details for cathedral roof details that have exterior insulation. I am curious about other's experiences. I have detailed exterior roofs many ways depending on firm details, GC and product reps specs.
Option 1
Finish Roof > Battens > Poly Iso Insulation Staggered > Air and Vapor Barrier > Sheathing > Structure
Option 2
Finish Roof > Battens > Drainage Membrane > Poly Iso Insulation Staggered > Air and Vapor Barrier > Sheathing > Structure
Option 3
Finish Roof > Drainage Membrane > Plywood > Battens > Poly Iso Insulation Staggered > Air and Vapor Barrier > Sheathing > Structure
Your images explain things a bit. Look up cold and hot (warm) roofs. It depends where the insulation is in reference to your roof structure. In the third image this is a hot (warm) roof with ventilation.
Hi Chad! Thank you! I think the "Perfect Roof" (if that's the one you're referring to) can be used in any climate condition but it would be "roof vented" rather than "attic" vented. I am not sure if it's considered a hot or cold roof, but please correct me!
Typically at hot roof is any assembly where there is no enclosed, unconditioned space over the insulation. It's just nomenclature though. There is a lot that will impact how a roof is designed - cost, climate, construction type, building codes, ect. There is no 'best' solution that can be applied to all projects.
FYI: The air barrier typically goes on the outside of the sheathing (this can vary), the vapor retarder (not barrier) is typically on the warm side of the insulation. It's location will be determined by the climate you're designing for. The right type of rigid insulation assembly can act as a vapor retarder.
"conflicting details" could be: design goals; budget; locality/climate; zoning constraints for height vs building hgt; local construction preference ; design experience (even inexperience); etc, etc; or some combo of the above.
hard to say with much certainty from this side of the internet, but not all projects can do the "best" solution. Design is a measured consideration of a lot of factors.
What's your opinion on a warm roof that uses foil-face polyiso as the final plane before the battens and roofing vs plywood over battens over polyiso. In the past, I've seen my old principal and GC get into debates about this when it comes to bidding because the GC didn't like the foilface polyiso to be the last layer under the battens+ roofing, whereas my principal was of the stance that as long as the final roofing is installed correctly over the battens, the foilface is more than enough and another sheet of plywood over-design and increases labor and material costs.
Mar 7, 24 3:22 pm ·
·
Wood Guy
First, I prefer the more precise terms "vented roof" and "unvented roof." Calling a roof hot or cold seems to cause more confusion than clarity.
The problem with foil-faced polyiso as the final layer is that the foil facing is often quite thin and fragile, sheets shrink as they age, foil tape between sheets can tear (if it's there at all) and polyiso can absorb water but the foil facing slows drying to nearly nil. The foil facing used on commercial roofing is much tougher but I would still worry about it if it is likely to get wet.
If you have plywood sheathing, roofing underlayment and roofing over the polyiso, and the roof isn't complicated, or poorly insulated and air-sealed in a cold climate where ice dams are likely to occur, then I would be fine with foil-faced polyiso.
If, however, the roofing is metal directly over the purlins, and/or the roof is likely to experience ice dams or otherwise have more than the occasional drop of water reaching the foil, I would use a different system. (Metal roofing installed over purlins without sheathing will condense due to night sky radiation.)
Exterior Roof Insulation - Location of "Control Layers"
I sometimes see conflicting details for cathedral roof details that have exterior insulation. I am curious about other's experiences. I have detailed exterior roofs many ways depending on firm details, GC and product reps specs.
Option 1
Finish Roof > Battens > Poly Iso Insulation Staggered > Air and Vapor Barrier > Sheathing > Structure
Option 2
Finish Roof > Battens > Drainage Membrane > Poly Iso Insulation Staggered > Air and Vapor Barrier > Sheathing > Structure
Option 3
Finish Roof > Drainage Membrane > Plywood > Battens > Poly Iso Insulation Staggered > Air and Vapor Barrier > Sheathing > Structure
Your images explain things a bit. Look up cold and hot (warm) roofs. It depends where the insulation is in reference to your roof structure. In the third image this is a hot (warm) roof with ventilation.
Hi Chad! Thank you! I think the "Perfect Roof" (if that's the one you're referring to) can be used in any climate condition but it would be "roof vented" rather than "attic" vented. I am not sure if it's considered a hot or cold roof, but please correct me!
Typically at hot roof is any assembly where there is no enclosed, unconditioned space over the insulation. It's just nomenclature though. There is a lot that will impact how a roof is designed - cost, climate, construction type, building codes, ect. There is no 'best' solution that can be applied to all projects.
FYI: The air barrier typically goes on the outside of the sheathing (this can vary), the vapor retarder (not barrier) is typically on the warm side of the insulation. It's location will be determined by the climate you're designing for. The right type of rigid insulation assembly can act as a vapor retarder.
"conflicting details" could be: design goals; budget; locality/climate; zoning constraints for height vs building hgt; local construction preference ; design experience (even inexperience); etc, etc; or some combo of the above.
hard to say with much certainty from this side of the internet, but not all projects can do the "best" solution. Design is a measured consideration of a lot of factors.
There are many, many ways to design and build a good roof. You are showing some of them. Do you have a question?
What's your opinion on a warm roof that uses foil-face polyiso as the final plane before the battens and roofing vs plywood over battens over polyiso. In the past, I've seen my old principal and GC get into debates about this when it comes to bidding because the GC didn't like the foilface polyiso to be the last layer under the battens+ roofing, whereas my principal was of the stance that as long as the final roofing is installed correctly over the battens, the foilface is more than enough and another sheet of plywood over-design and increases labor and material costs.
First, I prefer the more precise terms "vented roof" and "unvented roof." Calling a roof hot or cold seems to cause more confusion than clarity.
The problem with foil-faced polyiso as the final layer is that the foil facing is often quite thin and fragile, sheets shrink as they age, foil tape between sheets can tear (if it's there at all) and polyiso can absorb water but the foil facing slows drying to nearly nil. The foil facing used on commercial roofing is much tougher but I would still worry about it if it is likely to get wet.
If you have plywood sheathing, roofing underlayment and roofing over the polyiso, and the roof isn't complicated, or poorly insulated and air-sealed in a cold climate where ice dams are likely to occur, then I would be fine with foil-faced polyiso.
If, however, the roofing is metal directly over the purlins, and/or the roof is likely to experience ice dams or otherwise have more than the occasional drop of water reaching the foil, I would use a different system. (Metal roofing installed over purlins without sheathing will condense due to night sky radiation.)
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