Replies

1. GBA Editor
   Martin Holladay | Feb 01, 2011 05:52am | #1

   Bob,
   1. You mention that you plan to install 3/4-inch polyiso on top of your studs. Are you sure you don't mean rafters? I can't tell if you are describing your walls and your roof, or whether you are describing only your roof.

   2. You are apparently describing a roof assembly with many layers, apparently including polyiso, plywood, closed-cell foam, and EPS. To be sure we all understand, can you list these layers in order? I can't tell whether the polyiso is above or below the roof deck.

   3. Most experts advise that it's a bad idea to sandwich plywood between two layers of foam. If you like foam, it's best to put it on only one side of the plywood -- either between the rafters or on top of the existing sheathing.

   4. If you haven't read it, there's an article that discusses all of the questions involved in insulating a sloped roof. Although the article is focused on conditioned attics, most of the principles also apply to cathedral ceilings like yours: Creating a Conditioned Attic.

2. 
   user-939142 | Feb 01, 2011 05:10pm | #2

   apologies; i tried my best to make it readable, and i do appreciate the input

   i guess the issue is, how do you best insulate when you have to put insulation on both sides of the main roof deck/sheathing? my rafter size is too small for all the R needed, but there still needs to be something there to get enough R total

   [going from the top/outside]
   >metal roofing
   >underlayment
   >ice watershield at eaves and other choice areas
   >new outer roofdeck, 1/2" plywood or similar
   >1x4 lapping (or similar) with air space [used to batten down foam] [air space provides under deck venting]
   >continuos taped foil sheating [possible radiant barrier] and possible secondary drainage plan
   >EPS foam insulation sheets, 2 or 3 layers, 2" thick
   >some sort of underlayment layer/barrier on deck, if needed
   >existing roof deck, 3/4" board
   >spray foam insulation filling stud cavity OR cellulose filling cavity
   >poly iso or similar rigid board insulation over/outside rafters
   >drywall

   should the existing/old/current roof deck get some sort of underlayment covering? seems like the foam on top, staggered and taped/sealed should be enough

   is the new upper roof deck on top of the 4-6" of foam, is it usually attached to wood strapping on top of the foam that is secured through the 6" of foam to the studs? or direct through the foam?

   what about something greener than using all this foam on the outside of the current roof deck? i haven't seen any other solutions other than layering foam

   while i understand the idea of not using foam on top and bottom, the cellulose then needs a breathable barrier. the article you reference has cellulose being sprayed into the cavity with a foam sheet as the barrier sttached to the underside of the rafters, which puts foam on both sides of the roof deck again, with cellulose in between.
   what is the ideal under rafter barrier to hold cellulose in place in the cavity, when foam is on top of the roof deck?

   does an air space with a foil back side only serve as a radiant barrier if the space is sealed? ie a vented space like mine underneath the top roof deck wouldn't work

   up for any ideas...
   i've thought about something crazy, like building a large raftered area on top of the existing deck, and filling it with cellulose, but there would be serious thermal briding issues

3. GBA Editor
   Martin Holladay | Feb 01, 2011 05:29pm | #3

   Bob,
   You wrote, "spray foam insulation filling stud cavity" -- again, although you wrote "stud," I'm going to assume you meant "rafter."

   I will repeat my earlier advice: Put foam on just one side of your roof sheathing, not both. I would advise you to add the foam above your roof sheating, not below.

   You can fill your rafter bays with cellulose blown behind air-permeable netting (or blown behind drywall). That way you won't have any interior polyiso.

   Add as much rigid foam on top of your roof deck as you need to achieve the R-value you want. I wouldn't waste my time including a radiant barrier -- if your insulation is thick enough, the R-value of your radiant barrier plus its associated air space will be slight.

   However, if your above-sheathing foam consists of foil-faced polyiso, you get your radiant barrier.

4. 
   wjrobinson | Feb 01, 2011 07:06pm | #4

   BSC doc Building America Special Research Project: High-R Roofs Case Study Analysis

   http://www.buildingscience.com/documents/reports/rr-1006-ba-high-r-roofs-case-study-analysis/view?searchterm=30%20november%202010