The age-old question: open- or closed-cell foam?
n2dirt
| Posted in General Questions on
I know the questions has been asked a hundred times. I’m building in zone 3. I’ll be using Zip System R panels. The living room and kitchen is open and under a cathedral ceiling. Everytime I think I figure out what foam to use, I read another article that tells me the opposite or my builder or insulation guy says something different. I know with the zip system it needs to dry to the inside so I was leaning toward open cell in the walls. The cathedral and the other parts of the roof is where I really don’t know what to do. That’s why I asking the experts here.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Open cell foam uses half the polymer per R of closed cell foam, and uses low impact H2O (water vapor/steam) as the blowing agent.
Closed cell foam uses 2x the amount of polymer for the same thermal performance as closed cell foam, and (99% of the time in the US) uses HFC245fa as the blowing agent, which has a lifecycle global warming potential nearly 1000x that of CO2.
The performance benefit of closed cell vs open cell in a 2x6 cavity only adds about R1.5-R2 to the stackup, and that's assuming you really get the full 5.5" of foam depth, which isn't practical with closed cell foam, but IS with open cell. Is the additional environmental hit going to be worth it? (Probably not, IMHO.)
On the wall assemblies the R-value of the ZIP-R is sufficient for dew point control where the open cell foam meets the interior facer of the ZIP-R- it will not accumulate wintertime moisture.
If you want to buyback that R1.5-R2, using 2" wide strips of 1/2" polyiso on the interior side stud edges and going to 6" instead of 6.6" on the cavity fill reliably gets you there.
On the ceiling assembly the vapor retardency of 11.25" of open cell foam (a full-fill of a 2 x12 rafter cavity, which meets or exceeds the R38 code min) might be enough to be protective of the roof deck from wintertime moisture accumulation in a zone 3 climate, but with as little as R5 of rigid foam on the exterior of the roof deck it certainly is. Alternatively, a smart vapor retarder like MemBrain or Intellow on the interior side of the fully-filled rafter bays would be more than adequate moisture control, even without the exterior foam, at a total price point MUCH lower than with closed cell.
Getting to R38 with closed cell foam would only take ~6" and would leave a 5" cavity, an the thermal bridging of the rafters would be nearly 2x that of a full-cavity fill of open cell. The thermal bridging would lower the whole assembly performance of the closed cell only solution to less than that of the full fill of open cell. The 6" of closed cell would also have to be applied in three 2" lifts with cooling off periods between lifts to avoid shrinkage or fire hazard issues, as opposed to two 5.5" lifts with open cell. That's lower thermal performance at the same R value, higher price.
There is some hurricane resistance improvement for a closed-cell solution, but simply going unvented is already a huge improvement on it's own.
The air-sealing aspect of open cell foam is slightly better than that of closed cell foam (probably due to it's much higher expansion ratio while curing) according to recent years' testing of wall assemblies by the Building Science Corp.
Bottom line: Open cell foam is going to be far greener environmentally, and far greener in terms of what's left in the budget.
Thanks so much for your help. I'll be using the zip r on the roof as well. Looks like open cell for me.