Flash and Batt in the Hot Humid South
GBA Editor
| Posted in Energy Efficiency and Durability on
Does anyone have any experience with the flash-and-batt or flash-and-fill insulation method in climate zones 2,3,4 ?
In my area (Zone 3 Hot Humid) we see companies installing this system with 1/2 inch closed-cell spray polyurethane foam. Is one half inch enough for this climate area? Your thoughts are appreciated.
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Replies
David,
One half inch works just fine in hot climates. That thickness will work up to climate zone 4. The reason is that the winters are not cold and long in hot climates. Therefore the inside face of the spray foam will rarely be below the dewpoint temperature of the interior air.
Joseph Lstiburek
Joe,
The flash (1/2 inch closed cell foam) puts the vapor profile to the inside surface of the structural sheathing-- is this something to be concerned about? Being a hot humid climate, would it be better to use foam board on the exterior to help reduce the vapor drive to the interior?
Brett,
I'm not sure whether Joe will be back to answer, so I'll take a stab at it. As Joe said, there's no reason to worry with 1/2-inch of closed-cell foam using the flash-and-batt system in your climate. If there is any inward solar vapor drive, the spray foam will go a long ways towards slowing it down. If the moisture content of the sheathing is temporarily raised, it will dry quickly.
Is exterior foam "better"? Certainly it's better from a thermal performance perspective -- because it interrupts thermal bridging through the studs.
David,
It is important to know what your complete wall assembly is; you may not want to use any kind of flash. Also, is this new construction? If remodel, are you removing the existing cladding?.
David,
It sounds like you may already have 100% structural sheathing.
You could create an air control layer at the structural sheathing ala ZIP or similar
Eliminate Spray foam and instead use rigid insulation oustide the sheathing as Brett suggested.
Get better performance and maybe even save a few dollars?