Faced vs unfaced batts in ‘flash and batt’ approach
This is a follow-up question to one I posted previously: https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/98409/flash-and-batt-question-can-i-re-use-existing-batts
In that post Dana Dorsett pointed out the need for certain types of vapor retarders depending on the zone. We are zone 4 and are insulating two areas: a cathedral ceiling in a bedroom (3″ of closed cell, then either re-use R30 faced batts or use new R19 batts to fill the remaining 6.5″); the insulation will be covered with drywall and painted with latex paint. The second area will be the attic rafters (also 3″ of closed cell followed by batts) where we don’t plan to cover the insulation with drywall. 2 questions:
1. Do we need to use faced batts for the cathedral ceiling in zone 4 (the existing R30 ones are faced, but was wondering if this were critical when getting new R19 ones)? My sense from Dana’s reply to the post above is yes.
2. Will it make a difference in the attic if there is no drywall covering the insulation? I got the sense from Dana’s explanation that if we weren’t covering with drywall it wouldn’t matter.
Just wanted to make sure I understood the response to the prior post. Thanks.
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As long as you follow the guidelines in this article: Combining Exterior Rigid Foam With Fluffy Insulation, you don't need an interior vapor retarder or vapor barrier (and in fact it's usually better if you don't have an interior vapor retarder).
In your climate zone, the spray foam layer should have a minimum R-value of 31% of the total R-value of the entire roof assembly. It sounds like you will be OK with your ratio.
Note that it is a code violation to leave kraft facing exposed; if fiberglass batts have a kraft facing, they must be protected by a layer of 1/2-inch drywall. (Kraft facing is highly flammable.)
So if you were thinking of leaving kraft-faced batts exposed in your attic -- don't. Use unfaced batts there.
Thanks Martin. I was trying to understand if what you are saying is in agreement with Dana's response to my prior question. In his reply he wrote:
"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."
Dana is saying that in Zone 4, all you need on the interior side is latex paint. I certainly agree that the latex paint won't hurt. But the latex paint isn't really required -- so if you don't have any latex paint in the attic, you don't need to worry.
You DO need both an interior side air barrier, and when at the IRC minimum R-value fraction you'll need at least Class-III vapor retardency vapor retarder on the interior side. The kraft facer will give you sufficient vapor retardency, but they can't be made reliably air tight.
When you have a lot of margin on the R-ratio the vapor retarder becomes less relevant, but air tightness still counts.
An R30 squished into 6.5" will perform at about R24, and you're looking at about R18 for the 3" of foam, so you have a ratio of about R18/R42= 43%, well above the raw minimum of 31%, so no need to sweat the vapor retardency issue as long as it's air tight.
Unfaced batts that aren't covered are the opposite of air tight, and can be an indoor air quality issue if much air is moving through or by the fiberglass, picking up glass particulates.
Thanks both. The cost of covering the insulation in the attic with drywall would be pretty pricey and we don't use that space (though it houses the upstairs HVAC system which is one of the drivers for conditioning the space). Just hoping we are ok leaving the batts exposed. Are there other options short of drywall?
If the foam is sprayed with intumescent paint the air barrier doesn't have to be timed thermal barrier against ignition, but it can't be flammable itself.
The perforated type aluminized fabric radiant barriers would be sufficiently air retardent for this application, and usually come with sufficient fire ratings to leave exposed in an attic (which is their primary market, after all). Use a perforated type, which will have a grid of pin-prick holes to bring the vapor retardency well above Class-II levels, which would give a drying path for any moisture that did find it's way into the cavity. This vendor has it at 13 cents/ft^2 if you buy a 1000 square foot roll:
It's rated at 6.9 perms too, which is the upper third of Class-III vapor retardency, which is about where you want it. I've not used that exact product, but it has the fire ratings, and is similar to other perforated RB that I've used.
It'll deliver a modest thermal performance boost due to the "ooohh- shiiineeey" stuff, but you're really paying for is the air-retardency and fire rating.
Thanks Dana. I was told that foam being used was of a formulation that would not require the use of intumescent paint and that we would be fine from a fire (and inspection) perspective not covering with drywall. I'll look into the product you mention.