Zip R for Roof + Wall in Zone 4A
s4d
| Posted in Energy Efficiency and Durability on
Hi, I’m in zone 4A and am evaluating the following wall assembly:
FIBER-CEMENT LAP SIDING + KEENE EASY-FUR MESH FURRING + ZIP R-6 SHEATHING (1-1/2″) + WOOD STUDS (2×6) WITH BATT INSULATION (R-21) + GYPSUM BOARD (1/2″)
and the following roof assembly:
ASPHALT SHINGLES + UNDERLAYMENT + ZIP R-9 SHEATHING (2″) + WOOD RAFTERS (2×6) WITH BATT INSULATION (R-21)
Given that the continuous insulation in Zip R is not technically above the deck, does the “ratio rule” apply? Would either of these assemblies be inviting condensation into the wall system? Would an interior vapor retarder be necessary or beneficial in this scenario?
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Replies
s4d,
It's my understanding that Zip-R is not rated for use as a roof sheathing.
Couple of things:
1. Zip R cannot be installed directly to roof rafters. For a short period of time, I believe Huber was allowing it to be used over a structural roof deck (i.e. Zip R on top of sheathing on top of rafters), but I cannot find a reference to that being approved any longer in their manual. Here's a video I found that alludes to it: https://m.facebook.com/knudsonlumber/videos/using-insulated-r-sheathing-in-an-roof-assembly-ben-bogie-zip-system-building-en/3012138218889728/
2. Regardless of the situation, the ratio still applies, because you have fluffy insulation meeting rigid insulation (or sheathing covered in rigid insolation) and therefore a condensing surface.
That makes sense. Thanks for the feedback!