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Anything wrong with this plan to spray foam the attic deck?

user-7147194 | Posted in General Questions on

Hello GBA folks, my name is Chris

We are remodeling a house and trying to incorporate some energy efficiency measures.

The ceiling on the top floor has an immense number of penetrations, can lights, a/c ducts and sound system speakers. Currently the attic insulation is a combination of blown and batt fiberglass that ranges from r-0 depth(prior projects?) to some thing like r-30 depth(I think)

Our contractor has suggested to remove all the fiberglass and then wrap the cans and speakers with fiberglass batting and then spray 2” of  closed cell foam over  everything on the attic floor including the a/c ducts to try and get a good seal.

Then re use the  fiberglass that was previously removed, and top up to r50 ish with blown in cellulose.  Baffles would be put in to keep the blown in insulation out of the soffits.

Is there anything wrong with the plan?

Other information that maybe helpful: roof has  complete coverage of ice shield applied to roof decking and has concrete tile roof on top of that.  Climate zone is on margin of 5/6.

also, there are three bathrooms that vent directly into  attic.  Currently there are no signs of condensation, but I am worried that could occur with a better insulated/colder attic.  Does each of these  vent ducts need to be carried up through the roof  to vent outside?

Any insights or suggestions for modification are welcomed.

Thank you.

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Replies

  1. Expert Member
    Peter Yost | | #1

    Hi Chris -

    1. Spray foam insulation involves manufacturing the product at your job site. It's essentially outdoor chemistry. It's tricky; I suggest your read this article I wrote with spray foam guru Henri Fennell on getting it right: https://www.buildinggreen.com/blog/foam-place-insulation-7-tips-getting-injection-and-spray-foam-right.

    2. Consider replacing your can lights with LED cans; it's the right time to make this move to a much better lighting technology.

    3. Yes, your bath exhaust fans need to be ducted, but not through the roof; exit them at a gable.

    Peter

  2. GBA Editor
    Martin Holladay | | #2

    Chris,
    As long as the spray foam installers are conscientious, this plan will work. The most important aspect of this job is to get a good air barrier. Closed-cell spray foam is an excellent air barrier -- so as long as the spray foam installers know what they are doing, and don't miss any areas, you should end up with a tight ceiling.

    Another approach worth considering (especially since you have ducts in your attic) is to convert your vented unconditioned attic into an unvented conditioned attic. For more information on this approach, see this article: "Creating a Conditioned Attic."

  3. Expert Member
    Dana Dorsett | | #3

    >"Is there anything wrong with the plan?"

    Sure!

    The 2" of closed cell spray foam is an environmentally (and cash) expensive way to get an air seal and even more expensive R value. It is one of the least green insulation materials in common use, due to the HFC blowing agents and high polymer weight per R.

    Using 4" of OPEN cell foam would use half the polymer delivering slightly higher R, achieve as-good or better air tightness, and it's blown with H20 (water) instead of HFC245fa, an extremely powerful greenhouse gas. It would cost a bit over half what it takes to do it in 2" closed cell.

    But even the open cell foam solution is overkill for air sealing.

  4. user-7147194 | | #4

    Thanks for the speedy reply Peter, Martin and Dana,

    Dana, the thought with closed cell is that it is a better vapor barrier. For this application does it matter? I l!ove saving money and if I can save money and do less harm to the planet then I’m on board!

    Martin, I thought about your suggestion for an unvented attic, but was concerned about making an “foam” sandwich equivalent with the impermeable ice shield on the top of the entire roof deck and then foam on the undersurface . Is this a valid concern? Also an unvented attic would cost more with more foam over a larger surface area ?

    Also, the roof is a complex shaped hip, no gables are available to vent out of. What is the next best option?

    Thanks again for helping me examine the options (and save some money too!)

    Chris

    1. Expert Member
      Dana Dorsett | | #5

      >"Dana, the thought with closed cell is that it is a better vapor barrier."

      With vented attics at the zone 5/6 boundary you don't need or want a vapor barrier at the attic floor- it's all about air tightness. Standard latex ceiling paints is more than adequate for managing vapor diffusion drives.

      In colder areas where vapor diffusion alone can potentially create a problem, half-perm "vapor barrier latex" primer paint on the ceilings would be a heluva lot cheaper and greener than 2" closed cell foam (which also about 0.5 perms)

      For an unvented insulated roof deck in zone 5 requires at least 40% of the total R to be closed cell foam against the roof deck, or any type of insulation above the roof deck. In zone 6 it takes a minimum of 50% of the total R. In either case it still requires at least a class-III (1-10 perms) vapor retarder on the interior side of the fiber insulation or open cell foam to prevent frost/condensation in the more vapor permeable insulation, and a class-II vapor retarder (0.1-1.0 perms) would be even better. So for zone 6 you'd be looking at 4" of closed cell foam (about R24-R28) and R23 rock wool to hit the code-min R49 range. For zone 3" - 3.5" (R20+) and R30 rock wool would get you there.

      At 4" most closed cell foam is about 0.2-0.4 perms, which is not a true vapor barrier (but Ice & Water Shield is) and the roof deck can still dry toward the interior, but only very slowly, taking months or even years. That's fine as long as the roof deck is under 18% moisture content when the foam is applied.

      There are closed cell foam products blown with HFO1234ze, which has an extremely low global warming potential. The HFO blown foams run about R7/inch, but are 25-50% more expensive per inch than the R6-R6.5/inch foams blown with HFC245fa.

      Note: Vapor barrier latex does not reach the same half-perm vapor permeance if applied directly onto open cell foam that it reaches when painted on gypsum board.

  5. punkdr | | #6

    Thanks to all that commented. Here is the next update:

    I met with the insulation contractor again and got prices back for the various options. Basically it was about $6K to spray foam the attic deck with open cell spray foam, which I thought makes no economic sense. for about 1/4-1/3 of that price I can get the a/c ducts in the attic sprayed with a little over 1" of ccsf (per the article from Lstiburek) This plan would move the existing fiberglass off and from around the ducts ( the ducts have superficial surfaces exposed to varying degrees currently),spraying the ducts with closed cell sf, and then putting the existing fiberglass back into place. finally blow cellulose over the whole assembly to get deeply buried ducts. I would then attack rest of the ceiling penetrations with a caulking gun from the interior side, to try and achieve better air tightness.

    This plan makes better economic sense. Does it make building science sense ?

    Thanks for any critiques or insights.

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