Adding XPS on interior side of flat roof ceiling
St. Louis, MO: Climate Zone 4
I’m about to tackle two projects on my rear addition but was hoping for some guidance before I start tearing things apart.
The first project will be stripping and replacing the rolled roofing as I can see water stains on the drop ceiling where the addition meets the house. Whoever installed this roof made no effort to prevent wind-driven rain from getting back there. Anyways, pretty simple project – hopefully I don’t find anything too exciting when I strip the old roofing.
The second project is to replace the drop ceiling with drywall, replace any dirty/wet insulation as needed, probably fur out the rafters (or joists? whichever is the appropriate term for a very low pitch room ceiling) for more fiberglass insulation, then add 1/2″ to 1″ of XPS to the bottom/skinny side of the rafters/joists to air seal and help with thermal bridging before drywalling.
Hopefully this makes sense. Think of it like this, if you’re looking straight up at the ceiling: drywall, XPS, cavity insulation, roof deck, roof felt/IWS, rolled roofing.
For context, currently, there is foil-faced R19 fiberglass insulation installed in between the rafters/joists (16″ oc) and more kraft faced R19 fiberglass insulation laid perpendicular to the rafters/joists on top of the drop ceiling. I’m not sure if the drop ceiling and additional insulation was an afterthought, but they didn’t do a great job ensuring the additional insulation completely covered the top of the drop ceiling. I can see how that job would’ve been a PITA though.
Can anyone let me know if they foresee an issue with that plan? I know XPS is typically installed on the exterior of the roof deck, but in this case, honestly, it’s just a lot more work + materials to do it that way and I can’t see any benefit that would justify the added time and expense.
Please let me know what you guys think! Any help appreciated.
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