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XPS interior closed cell roof insulation (DIY)

Don J. | Posted in General Questions on

Quick question for you building experts!

It’s a 1920s house with wood planks all over, no vents no plywood, a new metal roof installed right over 3 layers of self adhering premium underlayment. No gaps no venting.

I bought 2 in XPS from HD, cut it and placed under roof deck between rafters. Left 1/2 in gap around and used Great Stuff to spray around it and sealed it.
Actually this method appeared quite easy and it’s much quicker to do than I thought once I figured out how to do it efficiently. It takes me about 20 minutes per rafter. Should have done this long time ago….

Now because I am suffering from moisture paranoia I bought some nice and cheap little hygrometers from china (like dozens) and I placed them BEHIND the XPS. Basically between the roof deck and the XPS board. The wire is sealed with great stuff. I made sure there are no gaps around the XPS boards and it should be 99% air tight.

By the way, the XPS is only R10 and this winter was quite cold (DC area)…however this one layer served extremely well even though it’s only R10…couldn’t believe it!

As soon as I spray around the XPS board to seal it, RH jumps from 30 to 50%, reaching mid 80s RH% after 24h…..and stays there forever.
I did half the house last year that way before the winter and those hygrometers fluctuated between 70s and upper 80% RH. Obviously the RH went down during freezing and increases a little when the roof gets hot…well the roof deck only reached up to 90F this year so far.

Is this a measurement instrument problem…or is this RH real??
Will this cause mold?
Will it eventually dry and the RH come down?

Do I need to continue be paranoid about moisture?

Do I need to see a shrink 😉 ??

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  1. GBA Editor
    Martin Holladay | | #1

    There are many possibilities here.

    1. I'm not a fan of using peel-and-stick over your entire roof. Allowing your roof assembly to dry to the exterior, at least a little bit, is often a good idea. Sometimes this can accomplished with ventilation channels above the roof sheathing. Of course, it depends what type of roofing you choose. Old-fashioned metal roofing with corrugations allows quite a bit of exterior drying of the sheathing, as does tile roofing. EPDM -- not so much. All of these strategies for exterior drying only work if you use a vapor-permeable roof underlayment like asphalt felt -- not peel-and-stick.

    2. I'm not a fan of encapsulating your roof sheathing between two vapor-impermeable layers (peel-and-stick above, XPS below).

    3. It's not clear where the moisture is coming from, but it's possible that you still have air leaks at the perimeter of the XPS sections.

  2. Don J. | | #2

    Thanks Martin
    I was hoping the 0.5 perms of 2 in XPS would allow some drying to take place.
    I would have preferred to vent but a dozen roofers looked at it and couldn't figure out how to do it within reasonable budgets.
    The thing with the RH between roof deck and XPS is quite interesting:
    A. the RH moves very little up and down, like 75-85% RH mostly depending on roof deck temp
    B. even on very dry days with 20% RH in the room I can place a fan to blow outside air right on the XPS boards and no drying occurs (when it's sunny and dry outside too)

    I find it strange because 0.5 perms should let up to 3 grams of water through per cubic meter, at least in theory.
    Another possibility I was thinking about: the interior moisture goes in and out of the XPS since vapor seeks balance; hence, no matter what material we put there vapor will eventually get in there.
    Assuming we have 70F 45%RH in the room, it's roughly 50F and 85% RH and that's roughly what I am measuring behind the roof deck.....
    However when the roof heats up the RH remains high at around 80%. I remember John had sent us last year a useful reference to Bill Rose's experiment explaining how wood emits moisture when heated....perhaps that's related?

    As you say the barrier at the top is clearly undesirable but now it's too late to change that :)

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