Membrane placement for cathedral ceiling with external rigid foam
Good day/evening everyone,
It’s another question about insulating cathedral ceilings 🙂 and yes, I have read through the articles How to Build an Insulated Cathedral Ceiling and Five Cathedral Ceilings That Work …but that is not to say I fully understand and can quote them all from memory 🙂
I am zone 4 and planning the insulation for our upcoming build project…the plan is a vented metal roof, and this cathedral ceiling is the main sticking point right now. I feel like I have a pretty good handle on what needs to be accounted for, but I did have a couple questions as I was perusing the articles here.
For reference, in the first attached image (from the How to Build an Insulated Cathedral Ceiling article), we see the rigid foam insulation sandwiched between the two sheets of sheathing and the self-adhesive membrane above the top-most sheathing for the roofing.
My questions come after reading Getting Insulation Out of Your Walls and Ceilings and wondering about whether it would be more beneficial to apply the self-adhesive membrane either on the sheathing below the rigid insulation or on the sheathing above it? This would be under the context that this membrane is being used as indicated in the PERSIST model to act as the air barrier for the entire house.
What is throwing me off is where the article states:
Since the membrane is on the warm-in-winter side of the insulation, it’s exactly where it belongs.
That makes sense, but then we look at Assembly #2 from the Five Cathedral Ceilings article (attached image)…that section doesn’t explicitly mention the membrane, but I presume it is there…wouldn’t it be acceptable if we have our membrane encapsulating the insulation with a vented roof (such as Assembly #2), but where the placement of the insulation is inverted? Meaning, where the rigid foam is above the insulation batts (such as in the illustration in the first image)? I would think that it wouldn’t matter if the rigid insulation is above or below the batts.
If the primary concern is air tightness, then it seems like (for my climate) it might not matter whether the membrane creates the air barrier with the rigid foam inside or out…Assembly #2 seems to be fine with keeping all the insulation within the membrane. All things being equal, I can see some benefit to placing the membrane on the outermost roof sheathing…but the comment about the membrane being on the warm-in-winter side is throwing me for a loop.
[EDIT] I have also read Sandwiching Roof Sheathing Between Two Impermeable Layers and I can say that I do not have any intention of creating those two impermeable layers, though I can see how you could end up there if you place a membrane on the bottom sheathing of the rigid foam “sandwich” and then another membrane on top…that is not what I am proposing or even considering…just wanted to throw that out there for clarification 🙂
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