Open or closed cell inside wall cavity?
Putting forth question for homeowners knowledge .
Zone 4 (Coastal NJ) , brick exterior , Deck Armor used for WRB on exterior walls, high glazing ratio . This is a very large structure at 13,546 sf .
As of now homeowner is planning attic to be outside conditioned space and basement will be unconditioned .
Home will have radiant floor heat , this will be thermally broken from perimeter framing . A/C to be ducted , some in attic , some in basement . These systems will be WELL Insulated . Exterior walls are framed 2 x 6 , double pane LowE windows
Any recommendations for type of foam inside cavities for this man would be helpful . Foam will probably be 2″ with other material to the interior .
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Replies
Richard,
You are describing a flash-and-batt installation. For flash-and-batt, the only suitable type of spray foam is closed-cell spray foam. You can't use open-cell spray foam for flash-and-batt.
Richard,
And by the way, if this is a new home, and the home measures 13,500 square feet, then I assume that the homeowners are wealthy. If so, it's a shame that they would install ductwork in an unconditioned attic.
Every time I see a $1 million or $2 million house with crappy energy details, I just shake my head in wonder.
I absolutely agree Martin . Unfortunately the design was performed as many are , by an architect that does not quite grasp the scope of what he is designing .
It does fall on me to make sure that this place is heated adequately without some expert going in and assuming it needs 30 BTU per ft as many do . So far because of glazing details and orientation it is looking like 18 per foot at design . At least we can make the system responsive and efficient , that's a help .
Sure does say alot for a new approach or at least one that is not catching on fast enough in Integrated project design and delivery .
Thanks for the tip Martin . This is why I refrain from commenting on more building science subjects .
Closed cell foam thermally bridged by studs is a waste of good foam. A 2" ccSPF + R13-R15 fiber solution still comes in at only R14-R15 whole-wall, with the framing fraction factored in.
At the same wall thickness and less money they could build with 2x4 framing and install 2" of rigid polyiso on the exterior, and come in a bit north of R20 whole-wall. Caulking the sheathing to the framing at each stud bay goes very quickly and is every bit as air-tight as spray foam cavity fill. The exterior foam can be taped with a temperature rated 1.5-2" purpose-made foil tape quickly & cheaply too, as a secondary air barrier.
It's higher performance, lower cost, more moisture resilient (since all of the wood is now partially inside the conditioned space, with a faster drying path to the interior). It uses half the polymer (with standard 1lb density polyiso instead of 2lb spray foam), and it's heluva lot greener due to the differences in blowing agents.
In climate zone 4 you could use open cell foam in a flash'n' batt and probably get away with it, but not in the NW zone 5 corner. But there would be no point to it. The open cell foam makes it air tight, but not more air tight than caulking the framing to the sheathing would.
If he's still insisting on flash'n'batt, it only takes ~1" nominal shot of closed cell foam to protect the sheathing from wintertime moisture drives in zone 4 (and it doesn' have to be perfect- a half-inch is enough, really) which would leave ~4.5" of cavity for other material. Damp sprayed cellulose would work, and would be greener than a fiberglass solution (blown or batt). If blown fiberglass, damp sprayed Spider (without blowing mesh) would be easiest. If fiberglass batts a compressed R19 batt (unface or kraft faced) would perform at about R16 at 4.5" depth. Compressed rock wool R23s would deliver about R19. R19 fiber + R6 foam adds up to an impressive R25, but that's a center-cavity value. The thermal bridging of the framing whacks that back to R15-ish (best case, assuming 24" o.c. spacing).
A 2x4 with R13 cavity fill and R12 polyiso sheathing is also R25 at center cavity but since the outer R12 isn't thermally bridged by framing, the whole-wall performance is still north of R20. If you used R15 high density fiberglass or rock wool cavity fill instead of R13 fiberglass or damp sprayed cellulose you'd get another R1 of whole-wall performance out of it, but adding another half-inch of exterior foam would deliver another R3, and may have a comparable up-charge.
With brick siding foam thicker than an inch has an up-charge in masonry ties. Framing with 2x6 with just 1" of polyiso on the exterior and an all-fiber cavity fill is STILL plenty of exterior R for dew point control on interior moisture drives (IRC chapter 7 says 2x6 framing only needs R3.75 see: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm). The sheathing would be FAR better protected from exterior moisture drives of sun-baked dew-wetted brick as well. That too would come in a bit north of R20 whole-wall, albeit at a slightly thicker wall. With even 1" of closed cell foam inside the cavity the drying rate toward the inteior is limited by the ~1 perm foam, and at 2" it would be slower still. With exterior foam the masonry cavity won't be drying path for the sheathing, but it also won't become a wetting path from the moisture drives off the brick. With latex paint as the interior vapor retarder the sheathing has a 3perm+ drying path into a humidity & temperature controlled space, and with foil-faced foam in the masonry cavity a 0.05 perm wetting path from the brick.
With a 2x4 framed/ R15 cavity fill wall + R6 foam sheathing the wall would be a bit thinner than a flash'n'batt 2x6, and slightly higher performance, coming in at about R16-R17 whole-wall, and would have even more wintertime dew point margin.
Too late for any exterior measures Dana . Brick ledge is not wide enough , windows are installed , place has deck armour installed already . Now just trying to help the homeowner / builder make it the best he can at this advanced stage .
Thanks for the help .
If that's the case, a flash-inch of cc foam and damp sprayed Spider or compressed rock wool is probably the best he's going to do on a performance/price basis. It meets code min, but it's nothing to write home about.
If he can give up an inch of interior space adding an inch rigid foam to the stud-edge strips and going to 1" ccSPF on the sheathing with 5.5" of fiber would bring it up to about R20 whole-wall, but it's a bit awkward doing that when if the windows are already in.
I suppose it's too late to keep the ducts out of the attic too?
May still be time to bring that attic inside the envelope . Basement is not so bad for first floor ducted systems , at least not as bad as ventilated attic . Owner has a problem with that however .
Think he'll end up with 1/2 -1" ccSPF and the 21 or 23 Roxul compressed a bit . I had mentioned Roxul to him quite some time ago . House is kind of rural as far as the world is concerned and will require sprinkler system also on a well .