I am building a house and am about to have it insulated.  My roof is standing seam steel over an ice barrier material adhered directly onto the CDX roof sheathing (i.e. non permeable materials and, therefore, no drying to the exterior).  The rafters are 2x12 and I am planning on installing 2" of closed cell foam directly onto the lower side of the CDX although I may want to up this to 3".  I understand that I need at least an R value of 10 for the CCF for my home in Seattle (zone 4C) so I may need 3".  I have several questions:

1)      Should I up it to 3" to ensure no condensing on the inner surface of the CCF or is 2" enough?

2)      What material should I use to fill the rest of the rafter bay volume?  Previous posts have indicated possibly using wet cellulose but I'm unfamiliar with this material.  My insulation contractor is simply recommending a blowing in fiberglass material.

3)      Lastly, I'm a bit worried about the presence of the CCF in relation to the in-ceiling canned lighting I've got installed.  Should I worry about any possible fire issues of having the heat I would imagine canned lights create in close proximity to the CCF?

Thanks for any helpful advice.  I'm probably a week or two away from insulation and want to get it right.

Cheers,

I'm doing the same thing up in Skagit right now. It seems sort of unconventional, but everyone I've run it by seems to be pretty confident that it will work.

1) Everyone seems to think 2" is plenty. According to the energy and vapor wonks, 1-1/2" would suffice here on Puget Sound.

2) Look up Blow-In-Blanket System (BIBS). It utilizes a netting stapled to the inside behind which is blown glass wool fiber.

3) Your can lights should be some distance from the foam and separated by the secondary insulation.

Have you managed to put the underlayment on yet?
And what are your metal roof plans?

If by the "underlayment" you mean the frost/ice barrier the answer is yes. Both the frost barrier and steel are already installed.

Thanks for the response. I have heard about BIBS but was not familiar with the material. Is there a specific product for BIBS I should be looking for?

Given, that the frame bay is 12" deep and, its a guess, but the cans are about 8-9" deep this would put the CCF quite close to the cans (which was driving my question). It sounds like I should keep the CCF back from the area around the can and only install the secondary insulation in that area (including the area directly between the CDX sheathing and the top of the can itself)?

It sounds like I should keep the CCF back from the area around the can and only install the secondary insulation in that area (including the area directly between the CDX sheathing and the top of the can itself)?

Didn't mean for it to sound like that. The answer given to me by architect, contractor, insulator and electrician is that if you use the proper cans, it is not an issue. Don't break your spray-in layer of foam. It has function. What does the foam mfr say about clearance to cans?

What "frost/ice barrier" did you use for underlayment and what metal roof did you go with and what are your thoughts on them?

This dicussion makes me appreciate the decision that I made to use track lighting instead of the cannister lighting that the builder wanted to install. I could not bear the thought of poking a bunch of holes in my main air and thermal barrier to put in cannister lights. You go to all that trouble to build a tight house and then poke holes in it!

Cannister lighting may have been a good thing for the late 1960's when energy was cheap but now I would avoid installing cans in a new home unless they were fully within the thermal envelope.  Even then, the light quality is not great and the area lighted by each can is limited.  Since this is the case, numerous can lights are usually installed to light a given area which can add to the cooling load especially in the South.  Installing compact fluorescents in existing can lights will save some energy.

Maybe in the next few years LEDs will replace fluorescents to save even more energy.

Posted By Sporto on 26 Oct 2011 11:23 PM
I am building a house and am about to have it insulated.  My roof is standing seam steel over an ice barrier material adhered directly onto the CDX roof sheathing (i.e. non permeable materials and, therefore, no drying to the exterior).  The rafters are 2x12 and I am planning on installing 2" of closed cell foam directly onto the lower side of the CDX although I may want to up this to 3".  I understand that I need at least an R value of 10 for the CCF for my home in Seattle (zone 4C) so I may need 3".  I have several questions:

1)      Should I up it to 3" to ensure no condensing on the inner surface of the CCF or is 2" enough?

2)      What material should I use to fill the rest of the rafter bay volume?  Previous posts have indicated possibly using wet cellulose but I'm unfamiliar with this material.  My insulation contractor is simply recommending a blowing in fiberglass material.

3)      Lastly, I'm a bit worried about the presence of the CCF in relation to the in-ceiling canned lighting I've got installed.  Should I worry about any possible fire issues of having the heat I would imagine canned lights create in close proximity to the CCF?

Thanks for any helpful advice.  I'm probably a week or two away from insulation and want to get it right.

Cheers,

In order:

1:  If you go over 2" with most closed cell foam you effectively block drying toward the inteior as well.  If you want more than ~ R12 in the foam layer, use a higher-perm closed cell foam such as Icynene MD-R-200 (not MD-C-200).  At R5/inch it's lower R per inch than other closed cell foams but that hardly matters with the thermal bridging of the rafters dominating the heat loss.  At 3" (R15) it's rated at 1.3 perms, which is slighly higher than even 1" of most closed cell foam. If you're feeling rich you could go for 6" (R30) and the roof deck could still dry.)  Your are correct, according to the IRC, in climate zone 4C (western WA) anything over R10 is adequate for insulating to code min, so 2" of ccSPF (R12) would be OK even if you're exceeding code-min R slightly (which you are), but 3" of MD-R-200 would be somewhat better, since it would have about 2x the drying capacity of generic 2" SPF, and would have fewer condensing hours at the higher-than-code center-cavity R.  Cost-wise 3" of the higher-perm closed cell foam is comparable to 2" of generic 2lb foam, but get quotes.

2:  If it's a finished ceiling with 1/2" or thicker gypsum board, you can dry blow the fiber behind pre-installed netting (BIBS or Blow In Blanket Systems).  Cellulose is preferable to fiberglass in that it will buffer and redistribute any actual condensation that might occur at the face of the foam, eliminating the possibility that it would find it's way into the rafters.  If you go with fiberglass, use a new-school product such as Optima or Spider, and blow it to 1.8lbs density to reduce convection within the fiber layer, and a more stable R-value.  Specifiy to the contractor that the fiber has to be rolled flat, flush with the rafters, as that will keep the gypsum from being bowed out on day-1 (and with wet-sprayed cellulose or wet-sprayed Spider, pretty much forever.)  If cellulose the density isn't as critical- it's sufficiently air-retardent at 1.5lbs open-blow density, but 2.5-3lbs density would be sufficient to keep it from settling even if dry blown in that stackup.  If you use wet sprayed products, at 8-10" depths allow at least a week of drying time before closing it in with gypsum, and a month from the time of insulating before painting the gypsum.  Keeping the room heated and using dehumidifiers to keep it under 50% RH during the initial drying period is cheap insurance too.

3: Give the foam 3" clearance from the fixture, and use only insulation-contact air tight fixtures.  (3" of cellulose qualifies as an ignition barrier for SPF.)

With framing factors included, 2" of ccSPF + 9.25" of cellulose delivers a whole-assembly R of about R36 (or ~R44, center-cavity.)  If you used 3" of the mid-density Icynene for fewer condensing hours it changes the whole-assembly R value upwards by about R0.2.   If (at significant expense) you did 6" of MD-R-200 and 5.25" of cellulose it only raises the whole-assembly R to ~ R38. This is why putting even a modest amount foam outside the sheathing is a much better deal than putting it between the rafters/studs.  R10 outside the sheathing with an all-cellulose cavity fill would be code-legal, and would yield R44 for a WHOLE ASSEMBLY-R (~R49, center-cavity) for similar money.

Posted By Alton on 27 Oct 2011 01:01 PM
Cannister lighting may have been a good thing for the late 1960's when energy was cheap but now I would avoid installing cans in a new home unless they were fully within the thermal envelope.  Even then, the light quality is not great and the area lighted by each can is limited.  Since this is the case, numerous can lights are usually installed to light a given area which can add to the cooling load especially in the South.  Installing compact fluorescents in existing can lights will save some energy.

Maybe in the next few years LEDs will replace fluorescents to save even more energy.

There are dedicated LED can-light fixtures that are pretty good (Cree LR6, and LR4), but I haven't seen sloped-ceiling versions of them. 

At current efficiency rates compact fluorescents still hold an edge over most LEDs, but the Cree downlights are comparable or even have a slight edge in efficiency, and have better color rendering, making them nicer looking  than typical CFLs.  They're dimmable with relatively cheap dimmers too, whereas decent dimming on CFLs requires special ballasts & dimmers and 4-pin base lamps, which adds up to a bit more than the LED solution.  They're getting there!