Posted By Sporto on 21 Apr 2011 10:47 PM
I'm building a new home and getting close to needing to decide on insulation.  My contractor was planning on just doing FG batting in walls and roof rafters (no attic, just vaulted ceiling) however, I've been reading about the benefits of doing a hybrid (closed cell foam and FG batting or cellulose) insulation installation.  We're building with 2x6 walls and 2x12 rafters.   We're building in Seattle so pretty temperate.  My understanding is that the spray-in closed cell foam will seal the walls so we won't need to do much sealing inside the bays (just along any other exposed seams not covered) and, if done correctly, won't require a a vapor barrier.

Question:  in the rafter bays, do we need to create a 2" space under the roof sheeting for condensing or can will the combination of the spray in foam and batting replace the need to allow for a gap between the sheeting and insulation to allow for condensation?  (i.e. can we just spray the foam onto the lower surface of the sheeting rather than allowing for a 2" gap per traditional building techniques?)

Any guidance and help is much appreciated

Posted By Dana1 on 22 Apr 2011 04:48 PM

Posted By Sporto on 21 Apr 2011 10:47 PM
I'm building a new home and getting close to needing to decide on insulation.  My contractor was planning on just doing FG batting in walls and roof rafters (no attic, just vaulted ceiling) however, I've been reading about the benefits of doing a hybrid (closed cell foam and FG batting or cellulose) insulation installation.  We're building with 2x6 walls and 2x12 rafters.   We're building in Seattle so pretty temperate.  My understanding is that the spray-in closed cell foam will seal the walls so we won't need to do much sealing inside the bays (just along any other exposed seams not covered) and, if done correctly, won't require a a vapor barrier.

Question:  in the rafter bays, do we need to create a 2" space under the roof sheeting for condensing or can will the combination of the spray in foam and batting replace the need to allow for a gap between the sheeting and insulation to allow for condensation?  (i.e. can we just spray the foam onto the lower surface of the sheeting rather than allowing for a 2" gap per traditional building techniques?)

Any guidance and help is much appreciated

It won't meet code to do the roof deck unvented in Seattle (some inspectors are pickier than others), but it won't cause a problem either. The foam becomes the condensing surface, but 

A: doesn't wick moisture toward the roof deck, and

B:Allows the roof deck to dry (if slowly) toward the interior and

C:Has a very limited number of condensing hours per winter in Seattle's climate.

See: http://www.buildingscience.com/docu...of-systems

But it may not be the best value.

On new construction, unless this is a built-out attic with kneewalls or you intend to finish it as living space some day, it's cheaper/better to air-seal with foam at the attic floor and blow cellulose until it's 3" above the tops of the joists (creating an ~R10 thermal break on the framing).  If this method is used, even R50 is cost effective even in a 5000HDD climate, where as 2" of closed cell foam at the roof deck would already have costing you more than R50 in cellulose on the attic floor, before you bought the first bag of fiber to bring it up to code-min levels for R.

If you ARE committed to going the conditioned attic approach, you can get more bang/buck out of putting 2-3" of rigid insulating panels above the roof deck, with the fiber inside, since that puts cheaper R on the exterior that thermally breaks the rafters for a big performance uptick.  Nailer-deck panels bonded to iso or EPS are available from a number of vendors (Hunter, Atlas) and some variants come with a vent gap between the insulation and the exteiror nailer deck, which allows the nailer deck to dry into the gap.  In Seattle's climate you'd only need 25-30% of the total R to be in exterior foam to make the structural roof deck resiliant to interior side air leaks or vapor diffusion, so even 2" of iso or XPS would allow you to do the interior at R30 and meet or exceed code for center-cavity R, but BEAT the whole-assembly R value of a batts-between rafters/joist solution by a huge margin.

Similarly, even an inch of XPS (R5) on the exterior of the studs provides more whole-wall R value for less money than an inch of closed cell SPF (R6-6.5) in a flash & batt or flash & spray cavity fill, provided you detail the air-barriers correctly, again due to the thermal break it's providing over not just the studs, but the band joist and foundation sill as well.   Taping the seams of the XPS with housewrap tape, and foam-sealing the edges can make it a primary air-barrier as well, but doing that and sealing the structural sheathing as well is worth the trouble.

Spray foam is great for it's sealing properties, and for adjusting vapor permeabilty in some situtations, but it's a very expensive way to add center-cavity R.  Acoustic sealant type caulk is a much cheaper way to seal stud wall cavities to the sheathing/plates and between plates/subfloors etc.  And with Seattle's modestly cool climate it doesn't take much exterior foam to keep the interior of the structural sheathing above the dew point of the interior air, making anything more vapor retardent than latex as an interior paint as a finish unnecessary.  Displacing the bulk of the foam-budget to the exterior is the right thing to do.

Adding furring to the exterior of the rigid XPS through-screwed to the studs as a means of mounting the siding further adds a "rainscreen" gap, enhancing the drying capacity of the assembly even further. (10mm minimum rainscreens are required by code for studwall construction in Canada, and should be in WA, IMHO)  The fastener spacing required for the furring depends on the depth of the foam, the furring/stud spacing, and the type of siding used, but it's pretty straightforward to put as much as 3-4" of exterior foam with lightweight wood siding.

Building studwalls with flash & batt but without exterior foam would be criminal-waste.  If you're going to do one vs. the other, go with exterior foam.  At 2x6 flash & batt/ or flash & spray with a 20-25% framing fraction comes in with a whole-wall R of ~R15, whereas the same studwall without flash-foam but with an inch of exterior foam it's ~R20, a 33% performance boost, for lower installed cost.  (In colder climates than yours it's sometimes necessary to use both exterior and flash foam to avoid condensation issues in winter, or going to awkwardly thick exterior foam an a very thick wall.)

would you recommend to allow for a vented volume between the rigid foam and the roof deck?

Sporto,

E-mailing someone through Green Building Talk forum does not leave a return e-mail address.  So I can not reply to your e-mail without an e-mail address.  My e-mail address is in my signature block.  I write it strangely so bots will not recognize it as an e-mail address and send me a lot of SPAM.  Anyone that wants to e-mail me should simply substitute the at symbol for the word "at" and substitute . for "dot".

I am not up on Seattle weather including humidity in the different seasons so I really can not go beyond what Dana1 has already replied.

Rosalinda,
Thanks for the input.  Question:  If you do install 1.5" rigid foam over the outside of the wall sheeting how do you fasten the exterior siding boards to it?  Do you drive a fastener all the way through the foam and into the plywood?  Or do you install some sort of wooden firring strips to attach the siding to?

sporto

Posted By Sporto on 25 Apr 2011 12:33 AM

Rosalinda,
Thanks for the input.  Question:  If you do install 1.5" rigid foam over the outside of the wall sheeting how do you fasten the exterior siding boards to it?  Do you drive a fastener all the way through the foam and into the plywood?  Or do you install some sort of wooden firring strips to attach the siding to?

sporto