Air barrier necessary for polyiso ceiling insulation?
dfvellone
| Posted in General Questions on
I’m in zone 6.
I’m planning on insulating my second floor ceiling with cut and fill reclaimed polyiso. It’s kraft faced. The second floor ceiling that I’ll be insulating against is pine. My plan is to cut the foam board leaving enough room so I can seal the first layer well. The final layer will cover the joists.
My question is whether I still need an air barrier, or if the permeability of the stacked layers of polyiso along with the sealing detail will be adequate.
Thanks, Daniel
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Replies
The best answer about adequate will come when you fire up the blower door.
>"My question is whether I still need an air barrier, or if the permeability of the stacked layers of polyiso along with the sealing detail will be adequate. "
It reads as if you may be conflating air permeability and water vapor permeability. It's hard to make cut'n'cobbled foam between rafters fully air tight, especially if counting on it for the long term, even though polyiso is pretty vapor-tight.
Even if it appears perfectly air tight right after installation, the seasonal dimensional changes of the joists puts stress on the sealing materials. Almost nothing sticks well enough to the cut edges of polyiso to guarantee a good bond for decades.
Using broadsheet flexible goods such as housewrap or even 6 mil polyethylene behind the pine ceiling detailed is cheap insurance. If the pine ceiling is already in place (sounds like it is), at the very least install an air barrier under the continuous top layer of foam that covers the joists, and tape the seams of that top layer.
I am getting a quote for a 2" skim of spray foam over the backside of the ceiling assembly that I could place my polyiso against. Would this installation still be subject to movement and subsequent failure of a seal?
My second option with the installer is to skim the top surface of my cut and fill covering all the insulation and joists. All this is subject to the price which, given the initial estimate for sprayfoam to r49 throughout , I don't anticipate being able to afford.
Aside from the viability of the two options above, will covering the polyiso with polyethylene as well as taping the top layer get me the results I need? I was under the impression that polyethylene in wall and ceiling assemblies wasn't a good idea.
Thanks, Daniel
+1 on counting on the taped continuous layer of foam as your effective and durable air barrier.