Unvented Flat Roof Assembly
Zins
| Posted in General Questions on
Thinking through an assembly for a 1:12 roof that I’d like to do unvented. I’m just becoming familiar with this style and would love any and all thoughts.
-EPDM
-2nd Sheathing
-6″ XPS, probably two layers, top one taped.
-Air Barrier
-Original sheathing
-4″ open cell foam
-Vapor barrier
-Drywall
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I forgot, this is Zone 5.
You probably don’t need an interior vapor barrier with that much exterior insulation.
If you have enough exterior insulation you can avoid OC foam on the inside.
Do you have a way to remove humidity in the attic space? Since your roof assembly is non permeable it might be a good idea to put a humidity sensor up there and monitor for indoor humidity buildup.
Also, consider polyiso instead of XPS. It is 1/4 the GWP and higher in long-term R-value. It’s what commercial flat roofs use all the time too.
In general EPDM is put over a compliant surface IOW ISO foam, which allows it to be glued directly to the paper scrim on the ISO.
IF you dislike the ISO, a solvent resistant recovery board which would be put over XPS and then glue the EPDM to it.
So I would suggest
EPDM
ISO
ISO
air barrier
roof
Or
EPDM
REcovery
XPS/EPS
XPS/EPS
air barrier