Open vs closed cell cavity insulation with exterior foam panels?
Hello all,
We are debating whether open or closed cell is the optimal cavity insulation for our wall assembly.
The wall assembly (inside to out) is as follows:
– 2×6 conventional framing
– Water-resistive sheathing (Zip System), taped at all joints.
– Foil-faced polyiso insulation panels (2″ thick), taped at all joints and edges.
– 3/4″ vented rainscreen.
– Pre-primed/stained cedar siding.
By calculations the exterior insulation is sufficient to prevent condensation at the sheathing – we’re in Zone 5. The argument for open cell foam cavity makes sense so that any water that can find its way into the sheathing can breath towards the interior. The problem is that the counter argument for using closed cell also seems valid. If it is assumed that no significant amount of moisture can enter from the outside than why use a product (open cell) which can absorb moisture on the interior?
A possible complication is that the there are overhanging soffits which were framed before the exterior foam panels were attached. Because of this there is no exterior foam behind the soffit attachment points (mostly horizontal 2×4’s or 2×6’s). I don’t know localized condensation can occur at the sheathing behind this framing. Would this be enough of a reason to use closed cell in the wall cavity?
Thank you.
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Replies
Walls do not breath… they dry to the inside or outside. Since you are installing 2” of polyiso for outsulation (taped and sealed) over the zip panels, which would be good choices, I would use the open cell foam insulation; but even better, I would use blow-in cellulose. You want the wall to dry to the interior of the assembly since it can’t dry to the outside. I would also use the air-tight drywall approach or ADA.
often the choice is dictated by desired R-value and price, closed say is close to double on both
the general rule is not to 'sandwich' wood which may need to dry on both sides by impermeable material. you should assume that no matter how well built, over time leakage will occur.
a good spray foam contractor should know this and push you towards open cell or another permeable solution.
open cell foam will not 'absorb' moisture, it just allows transmission [yes if you sunk a piece to the bottom of a pool it would absorb some]. if moisture can naturally pass to the foam, it can also naturally leave so it is not a problem.
note: if you use some sort of impermeable wall material on the inside, say like vinyl, then using open cell defeats the purpose.
i don't quite follow you soffit issue, but you could probably get up in there and spray foam detail around any potential trouble areas, or use peel and stick membrane, of course not covering up any actual roof venting. if you are concerned about the area, you should resolve it. open cell foam in the wall isn't going to 'vent' any consistent major water leaks.
also blow-in cellulose would be your best option, healthy, green, affordable, air tight if done right
you could also put rock wool bats in between the studs
Bob,
Your Zip System sheathing might be damp on the day it is installed, or it could get wet in the future if a flashing defect allows rain water to enter your wall assembly. Even if these possibilities are unlikely, it's still better to design a wall system than can dry to the interior, which means that open-cell spray foam is a better choice than closed-cell.
As you now realize, it was an error to attach the soffits before the exterior polyiso was installed. If you can't disassemble and re-frame the soffits -- the best solution -- then you'll have to settle for the second-best solution: install rigid foam or spray foam on the exterior side and the top of the horizontal 2x4s and 2x6s that were attached to the wall during the framing of the soffits.
Thank you for the responses.
Martin, we will be settling for the second best option, i.e. spraying the exterior side of the soffit framing. The GC who made the error - not installing soffits on top of the foam as per the details - is no longer involved on the job, so we are left with the best 'retrofit' option.