Rain Screen Gap behind CI
SeanRyan
| Posted in General Questions on
Hey All,
Was thinking I had my setup dialed for continuious exterior insulation:
Innie Windows:
– 2×6 wall with zip system sheathing
– Sealed at the sheathing layer with tape and fluid applied. This is water and air control layer
– 2″ Polyiso
– Batten for rain screen gap
– Siding
But, I was just looking around and saw a spec for a gap between the foam layer and the sheathing, in case any moisture gets back there.
Is this necessary? Is there a good way to add this in simply? The recommendation was for a drainable house wrap, but that feels really redundant over Zip.
Thanks,
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Replies
Sean,
My understanding from this https://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh is that it is only necessary if you have impermeable insulation and the sheathing has no way to dry to the interior. But maybe necesary is different that advisable?
If the gap between the exterior insulation and the sheathing is too big, wouldn’t it cause convection loops and possibly render the insulation less effective? I know Risinger put polyiso directly against his WRB on the sheathing but his assembly allows drying to the inside. He made a video where he poured red dyed liquid behind the polyiso and you can see it drain out the bottom, albeit taking a little longer than if it were an open rain screen.
Matthew,
Joe Lstiburek qualified it in the article I linked to:
" But won’t the tiny gap cause a loss of thermal performance of the foam sheathing? Yes. How much? About 5 percent of the thermal performance of the foam sheathing (not the entire wall assembly) with the 1/8-inch gap, less with a smaller gap."