Row House Roof: Insulation and Moisture Management
Hi All, thanks for any forthcoming help and opinions! Zone 4, close to 5, NJ Possibly getting the chance to install a new TPO + Rigid insulation on the exterior deck of my row house flat roof in the coming weeks. The company I am working with typically does 2 layers of 1 inch poly iso with offset seams for r10 CI on the roof deck. If I want to add fiberglass below the deck, to avoid condensation is it safe to assume I need to stick around the r13 level of insulation to be safe and avoid condensation issues? Is it safe to go higher on the batt insulation at the risk of throwing the ratio of interior to exterior insulation off? I’d love to really fill in the rafter with something like r38, but don’t want to compromise the new decking or rafters. I could inquire about a 3rd layer of 1 inch, or two layers of 1.5 inch polyiso, but sometimes it’s may be best to stick with what they are most familiar with and do the best around that as to not compromise the installation.
As an update to this, the contractor got back to me with a quote including 1.5″ polysio on the roof deck, so now the quotes are as follows:
1) Mod bit rolled roof, no insulation – 10.6k
2) TPO over 3″ polysio (possibly with batts r19 under it, but need to have that quoted still) – 22k.
A pretty hefty upcharge for the extra scope. Would probably never get a payback on that, but would hopefully lead to a more comfortable second floor. The TPO is better for this roof since the slope is about 1-2%.
Main concerns now besides big price jump are: Thermal transfer through the party wall that makes exterior insulation less effective and I’d need to see a detail for insulating the underside of this large overhang (although it is on the north facing side, so least sun): https://imgur.com/a/OEsy72M
Not sure if people have any updated thoughts or experience between the two.
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