Peel and Stick Options
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Faulted1
| Posted in General Questions on
Looking for product for covering unvented roof assembly.
Should I use Grace Ice & Water Shield or an alternative product?
Thanks
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Replies
John,
Unless there is something unusual about your roof assembly, I'd stick with 30-pound asphalt felt.
In most applications, the use of Ice & Water Shield should be restricted to just eaves and valleys.
Installing peel-n-stick over the entire roof has the potential for trapping migrating water vapour underneath the membrane. Moisture migration (due to vapour pressure) would have to be non-existent, which is difficult to achieve. The colder your climate the more this is true.
John Nooncaster,
you might find this thread interesting
https://www.greenbuildingadvisor.com/community/forum/general-questions/19452/condensation-problem
http://www.buildingscience.com/documents/reports/rr-1006-ba-high-r-roofs-case-study-analysis
"To meet durability goals in most applications, the airtightness must be provided by a continuous membrane—preferably adhered or sandwiched—on the exterior of the framing. In designs where the air tightness is provided between framing elements, spray foam has been found to be a practical solution to the challenge of providing this airtightness."
Then add this--
"However, all wood‐to‐wood joints in the framing must still be solved with sealants or other means."
R38 of open cell in Zone 4
I worry above the moisture drive from the asphalt shingles...
What about taping the sheathing joints?
John,
Air tightness and vapor tightness are two different matters. It you are installing R-38 of open-cell spray foam, you shouldn't have to worry about air tightness unless you have a very unskilled installer.
As long as you're not installing interior poly -- and you shouldn't be -- there is no reason for you to need a vapor barrier on top of your roof sheathing. If your roof assembly experiences inward solar vapor drive through asphalt shingles -- a phenomenon about which researchers are still arguing -- the roof assembly should be able to dry to the interior.
All of that said, if you really want a vapor-impermeable roofing underlayment, you don't have to use Ice & Water Shield, which is expensive. You can use Tri-Flex 30.