Replies

1. GBA Editor
   Martin Holladay | Feb 17, 2011 11:45am | #1

   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.

2. 
   Dovetail7442 | Feb 17, 2011 02:18pm | #2

   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.

3. 
   homedesign | Feb 17, 2011 03:43pm | #3

   John Nooncaster,
   you might find this thread interesting
   https://www.greenbuildingadvisor.com/community/forum/general-questions/19452/condensation-problem

4. 
   Faulted1 | Feb 18, 2011 02:09pm | #4

   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?

5. GBA Editor
   Martin Holladay | Feb 18, 2011 02:20pm | #5

   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.