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Community and Q&A

Kraft or unfaced batts with flash-and-batt insulation

Cornelius Q | Posted in General Questions on

So two quick questions. I’m having 2-2.5″ of closed cell sprayed today on my walls, based partially on some helpful information from a prior post. 75% of my walls are 2×6 with the remaining being 2×4 walls, I’m located in zone 5 (on the border of zone 6). My question is whether I should fill the remaining 1.5 or 3.5″ with kraft faced or unfaced batts (since I’m doing that part myself). There’s no difference in cost, so if the kraft faced is better, then I’d definitely go that route. But, I didn’t know if that could potentially be worse for whatever condensation reason.

One other unrelated question. The closed cell they are spraying is apparently some sort of water based product, the contractor said it’s a newer blend that isn’t nearly as bad as far as fume go, etc. I’m not familiar with that product (or really any spray foam product for that matter). I was just wondering if the R-value as good and are there any downsides to a product like that (without knowing the specifics of the blend of course). I know people on here were talking about how bad the typical closed cell is for the environment and how it loses R-value over time, just wondered thoughts on what my contractor might be using. Thanks!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    With the huge dew point margin you would have in the walls there is no difference.

    If you did the right thing and used only 1" of closed cell foam there would be a slight difference favoring kraft faced, even though 1" of closed cell foam would have sufficient dew point margin for 4.5" of mid density fiber insulation in climate zone 5.

    Even water blown foam (Icynene MD-R-210 at about R5/inch, Aloha Energy at about R6/inch etc) uses more than 2x as much polymer per inch. In a zone 5 location the value proposition would be to install 5.5" of half pound open cell foam (which uses less polymer than 1.5" of closed cell foam), and install an inch of unfaced Type-II EPS on the exterior, with a rainscreen gap to the siding, and half-perm "vapor barrier" latex paint on the wallboard. The assembly can still dry in both directions, won't load up the sheathing with moisture over the winter, and outperforms the flash'n'batt on a thermal basis. Even cheaping out with 1/4" of fan-fold XPS or perforated 1/4" fan fold foil faced EPS on the exterior would beat the performance gains of the 2" of closed cell foam, on both a thermal and moisture basis.

    1. Cornelius Q | | #2

      It seemed like from a previous post, the 1" of closed cell is on the verge of condensation issues in my climate zone, so I opted for 2". Since I'm on the border of climate zone 6, it was my understanding that 1" would be really cutting it close in extreme temps. It was my understanding that the 2" would reduce that potential condensation issue, are you saying that's not correct? They're literally spraying it now, so I don't have a choice to change it now, but I'd still like as much information as possible. Thanks!

  2. Expert Member
    Dana Dorsett | | #3

    > "Since I'm on the border of climate zone 6, it was my understanding that 1" would be really cutting it close in extreme temps. It was my understanding that the 2" would reduce that potential condensation issue, are you saying that's not correct?"

    Yes, 2" reduces the moisture accumulation potential compared to only 1", but not enough to matter if it is air-tight to the interior and has a vapor retardent paint or kraft facers (which are "smart" vapor retarders) on the interior.

    Which vendor/manufacturer and product did you go with on the water blown 2lb foam?

  3. GBA Editor
    Peter Yost | | #4

    If you have paid for 2 inches of spray foam, measure to make sure you have that depth consistently before doing your cavity insulation.

    With no price difference, I would go with the faced batt; little to no penalty for this from a moisture management perspective and it will be easier to install, frankly.

    Lastly, measure and manage your wintertime interior relative humidity (ideally 35% at 68 F).

    Peter

  4. Jon R | | #5

    If you want increased R value (say in your 2x4 walls), consider adding foam (strips or continuous) on the interior side.

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