Replies

1. 
   user-1105327 | Oct 05, 2012 12:35pm | #1

   in vancouver, a very wet city, .5lbs foam like icynene, is permitted for a ceiling insulation detail. the idea being if there is a roof leak, for example, the foam will allow the water to pass through and manifest itself on the interior ceiling panel. this alerts the homeowner to the problem and a repair can be arranged. a 2lbs foam like walltite eco does not allow water to pass through in the event of a roof leak thereby increasing the probabilty that water could collect and rot out the tops of the rafters or crosstrapping. 2lbs foam is not allowed in a ceiling insulation detail in vancouver (rather, it requires a creepy sounding 'alternate solution' where an architect signs off on the detail)

2. GBA Editor
   Martin Holladay | Oct 05, 2012 01:10pm | #2

   Eric Mikkelsen,
   Unlike Erik Olofsson (are you two cousins?), I don't think there is any clear advantage between open-cell foam and closed-cell foam when it comes to roof leaks. No matter which type of insulation you use, once you get a roof leak, you're going to have to repair the roof.

   When it comes to possible problems due to vapor permeance and wintertime moisture accumulation in roof sheathing, closed-cell foam is clearly superior to open-cell foam.

   In most applications, either type of insulation will work well, as long as you install enough foam to at least meet the code minimum requirements for ceiling insulation -- that is, the minimum R-value -- for your climate zone.

3. 
   mikkelsen | Oct 05, 2012 01:32pm | #3

   Thanks Erik & Martin. Erik, that is indeed a very Scandinavian name. Martin, as you hinted, my concern is mostly with the vapor permeance and wintertime moisture accumulation in roof sheathing. Location: central mountains of Idaho @ 5,000ft elevation. Ceiling will be insulated to about R-65. Will there be any issues using the open-cell & also relying on it to do the air-sealing?

   Thanks!

4. GBA Editor
   Martin Holladay | Oct 05, 2012 01:40pm | #4

   Eric,
   In that climate, I would recommend that you choose closed-cell spray foam if you intend to spray the underside of your roof sheathing.

   If you insist on using open-cell foam, you'll have to find a way to cover the interior side of the foam with drywall, and then paint the drywall with vapor-retarder paint, if you want to avoid the problem of moisture accumulation in the roof sheathing.

5. 
   mikkelsen | Oct 05, 2012 02:09pm | #5

   Again, thank you. are there other options that you think I should consider?

6. GBA Editor
   Martin Holladay | Oct 05, 2012 03:11pm | #6

   Erik,
   New house? Existing house? Flat ceiling? Sloped ceiling? With an attic or no attic?

   Does your roof geometry allow your roof to be vented?

   Lots of factors to consider. It's certainly possible to insulate a ceiling or roof with rigid foam or cellulose.

7. 
   mikkelsen | Oct 05, 2012 04:01pm | #7

   10/12 pitch, metal roof. Attic Truss With Conditioned attic space & storage. New house...starting next spring on construction phase. straight gable roof, no valleys.

8. GBA Editor
   Martin Holladay | Oct 05, 2012 04:07pm | #8

   Eric,
   You can put all your insulation on top of the roof sheathing you want, using rigid foam or SIPs.

   You can provide a ventilation channel under your roof sheathing if you want, and insulate with fiberglass batts. (However, I don't recommend this approach.)