“Flash and batt” question: can I re-use existing batts?
We had a tree fall on our house and damage a section of roof that includes a cathedral ceiling/roof. It was previously insulated with R30 batts (faced) under the rafters followed by drywall (this is in part of our bedroom). We are planning to spray 3″ of closed cell foam under the rafters and fill the remaining 6.5″ with fiberglass batts. I realize that the R30 batts are designed to fill an 8-9″ rafter depth but was wondering if re-using them would significantly compromise their insulation properties vs using new R-19 batts. An additional concern is that these R30 batts are faced and my understanding is that we should use unfaced batts if using closed cell foam.
-Rob
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Compressing batts is fine, but whether or not it's moisture-safe depends on your climate zone. With 3" of closed cell foam you have about R18 of air-impermeable foam. If you compress an R30 into 6.5" it's performance will be about R23-R24 which is fine. See:
https://www.greenbuildingadvisor.com/sites/default/files/Compressing%20fiberglass_0.JPG
That comes out to a total of about R42, which makes the R18 foam 42% of the total R. That would be fine in US climate zone 5 or lower with only a Class-III vapor retarder on the interior side (standard latex paint on gypsum board.), but in zone 6 or cooler you would need a Class-II vapor retarder or "smart" vapor retarder to keep the batts sufficiently dry on the less-sunny parts of the roof.
Kraft facers are smart vapor retarders, but unless it's air tight it hardly matters, since air convection can move orders of magnitude more moisture than vapor diffusion.. A layer of gypsum board (even unpainted) can be made sufficiently air tight to make the vapor retardency of the facers relevant.
Rob,
For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation. (While the article focuses on exterior rigid foam insulation, the recommended ratios of foam-to-fluffy insulation also apply to flash-and-batt jobs.)
Thanks Dana. When you talk about air convection I assume you mean from the inside since the spray foam should air seal and prevent movement from the outside (or am I misunderstanding convection). Since there will be a layer of gypsum (and latex primer and paint) coverting the batts, it sounds like the verdict is that I can use the existing R30 faced batts.
The original batts were able to fit into the rafter bays without anything to hold them there while the drywall guys installed the drywall, but now they will be compressed and I wonder if we will need to put in some sort of bracing system to hold them there. I just hope the labor costs for doing that, if needed, won't cancel the savings from re-using the batts.
-Rob