Flash and batt question
joenorm
| Posted in General Questions on
This question is about the “flash and batt” method for a roof.
Is absolutely important to have no gaps or sloppiness in the area in-between drywall and bottom side of spray foam?
I understand from an R-value question the answer is yes. I am more concerned from a moisture control perspective.
Would that air in-between the fluffy stuff and the spray foam be problematic?
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Replies
Joe,
You don't want air in this assembly. That means (a) you don't want a layer of air between the top of the batt and the underside of the cured spray foam, and (b) you won't want a layer of air between the drywall and the underside of the batt.
Air layers provide hidden pathways that encourage convective loops and increase the chance of exfiltration.
The easiest way to ensure that you have no air layers in the assembly is to choose thick batts and compress the batts when you install them.
Hi Joe -
Air leakage is two holes and a driving force. Lack any one of those, and you don't have air leakage. However, if air can circulate in a closed loop--called a convective loop--it can move heat and reduce efficiency of the overall assembly.
From a moisture perspective, convecting air carries moisture. I have never thought about how MUCH more moisture is moved across an assembly because of convective loop. But logically, it would accelerate transport of the moisture compared to no air space.
Peter