I have a non-vented attic, CC foam at the roof. Does it do any harm to add 6 inches of cellulose at the ceiling level? can it be blown on top of can lights, or is there a fire hazard?

Posted By McFish on 07 Apr 2014 02:08 AM
I have a non-vented attic, CC foam at the roof. Does it do any harm to add 6 inches of cellulose at the ceiling level? can it be blown on top of can lights, or is there a fire hazard?

The temperature in the attic (and the moisture content of the wood in the attic) depend on the local climate, and the R-value of the cc foam at the roof deck relative to that on the attic floor.  The potential for thermal bypassing air currents (and moisture transport air currents) between the roof deck and attic floor are high, so you won't get quite the thermal benefits of that 6" of cellulose that you would if it were tight to the cc foam at the roof deck- you can't add the R-values together to meet code minimums.  Moreover, insulating at both the roof deck and the attic floor is technically code violation, but relatively low risk in most situations, and will have at least some thermal benefit. So, where are you (for climate data), and how much cc foam do you have up there?

If the can-lights are rated for insulation contact AND the sockets populated with nothing higher than the fixture is rated it's fine to blow over them with cellulose.  When in doubt, boxing over the fixtures with an air sealed box leaving 3" of clearance between the box & fixture, then insulating over the box would be acceptable.

Thanks, Dana; I'm in Calif zone 11, ie either zone 3 or 4 nationally. R50 at roof, enclosed attic; just debating if additional several inches at the ceiling is worth it. I'm skeptical of builder's ability to air-seal the ceiling, but with 8 inches CC (2x6 rafters, 2 inch thermal break) I was hoping the cellulose would act as an added air barrier. Walls are OC 2x6, with additional air-sealing and caulking; specced ACH50 under 2. We'll see if they get there. I will blow my own, and the foam guy says he'll give me cellulose at cost. In return he gets pics and performance data for his wwebsite. Design day heat loss for house is 28000 BU/hr without the cellulose based on BeOpt2.1

Cellulose is not an air barrier- ever. It's an air-retarder, and a pretty good one when dense packed into framing cavities, but in an open blown attic floor situation it won't improve the air tightness of the house a bit.

But 8" of closed cell foam is a crime against the planet, due to the high lifecyle global warming potential of the HFC245fa blowing agent at about 1000x CO2.

It's also close to being a class-I vapor retarder at 8" and will trap construction moisture between the roofing and foam if the rafters or roof deck aren't allowed to dry sufficiently before spraying it. It has to be applied in 2" lifts to avoid potential shrinkage/cracking issues (or catching on fire as it cures.)

In a US climate zone 3 you'd be fine getting the same R50 with open cell foam (albeit much thicker) at lower cost, and a tiny fraction of the environmental damage, since water is the primary blowing agent with open cell foams. Unless your installer stipulates that the product uses a HFO1234yf blowing agent (GWP less than 5x CO2) or water (of which there are but a very few products out there, and probably not available in CA) stick with open cell foam.

A better unvented roof would be 2 layers of 2" EPS above the roof deck (R16), with 8-10" of open cell underneath (R30-35), which would offer a comparable or better thermal break on the rafters at about the same interior space and air tightness.

With an R50 roof deck the addition of 6" of cellulose won't create any problems, but it will hardly make a difference in the whole house loads. You'd probably be better off applying the money elsewhere on the building envelope if you're starting off at R50 with thermally broken rafters.

Thanks for the feedback, Dana. I'll see what we can do about switching to OC for the roof deck.

Posted By McFish on 09 Apr 2014 12:54 AM
Thanks for the feedback, Dana. I'll see what we can do about switching to OC for the roof deck.

Icynene MD-R-200 or MD-R210 are semi-open/semi-closed water blown foams that can hit R50-ish at 10", and still have at least some drying capacity toward the interior.  I'm not sure it's worth the expense in this application though, if you have the ~12-15" of space needed get there with open cell.

Half pound open cell foam can be blown installed in 5" lifts without fire or shrinkage risks, so you'd be looking a 3 lifts instead of five for closed cell, and it's typically about 2/3 the cost of closed cell foam per unit-R.

At 12-15" the vapor permeance of half-pound foam is about 3  perms (comparable to standard latex paint on paper-faced gypsum), which would be just fine in your climate, plenty of drying capacity, but on the tighter end of class-III vapor retardency for minimal wintertime moisture accumulation.

There are low global warming potential foams. I looked at doing cellulose under the roof deck and decided against it without foam. First I tried the two tank approach which was fine but would have required two more kits to finish my attic. I have since switched to a fastkick kit which is supposed to have low global warming potential (if you buy into all of that and there is plenty of facts either way and I dont.) So I am going to evaluate after 2 inches of 1.5lb foam. I might then go to open cell, net and blow, or just pack in fiberglass. I am not sure yet. It is hard to argue that foam will seal it up tight and that is why I ultimately went with it.