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Community and Q&A

Sealing and Insulating a Vented Attic

tneicna | Posted in Green Building Techniques on

I live in Zone 4A (Home was built in 2005) and I’m in the planning stages of converting my vented attic to conditioned/air sealed. Cost is going to be a concern, as achieving R-60 with just closed-cell foam would be very expensive.

What’s the most economical (and safe) way to achieve R-60 with my setup? I have rafters with 2/4s on towards the roof deck (gable ends are much much smaller, about 1.5 inches in depth).

I read some comments in other threads about putting 4 inches of CCF and then topping it off with the rest of OCF (open cell foam) or even another material.

But my stud depth towards the roof deck is not as deep as I expected.

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Replies

  1. Expert Member
    Michael Maines | | #1

    Getting R-60 in a 3 1/2" cavity would require insulation that performs at R-17 per inch. Among commonly available and affordable materials, the highest R per inch in foam insulation (closed cell spray foam or some polyiso rigid foam) is about R-7 per inch, and that decreases over time to about R-6 per inch. To get R-60 you would need 10" of either of those, or 8.5" if you are only concerned with the initial R-value (typically when trying to meet code requirements).

    There are a few ways to add insulation above the existing roof; or to furr down the rafters toward the interior. Would consider either of those approaches?

  2. Expert Member
    BILL WICHERS | | #2

    Is there a particular reason you want to convert to a conditioned attic? If you don't have any mechanicals up there, you're really better off with a vented attic. Vented attics are less prone to failure, and much cheaper to construct in regards to insulation, since you can just pile on more loose fill until you get up to whatever R value you want. The only concern with "deep" insulation in this case is vent baffles out at the eaves, which are relatively cheap and easy to build.

    If you have no choice and have to condition the attic (that "have no choice" part is important here), then I would either try to get some of the R value above the roof using polyiso sheets. 6" of polyiso gets you up to about R39, leaving a much more manageable R21 to put in the rafters to arrive at your target R value. Another possibility would be to build continuous vent channels (minimum 1" deep per code, 1.5" is better) in each rafter bay, the insulate under those. If you did this, I'd build out the vent channels using either 1/4" wafer board or 1/2" polyiso (whichever you can get cheaper), then full the remainder of the rafters with spray foam and finish off with fluffy stuff on the inside. The vent channel gives you moisture protection, the spray foam gives you an air barrier/seal and some R value, the interior side batts give you the rest of the R value.

    My preference by far would be to keep the attic as a vented assembly though if at all possible, then use blown cellulose to get to your R60 target.

    Bill

    1. tneicna | | #4

      Air Handler/Ducts are in the attic. I can't move the air handler into the living area upstairs or downstairs because there is no free area to place it.

      I do not really have a choice in a manner of speaking.

      Here's one option:

      - CCF on the roof deck/etc to to get about ~=R20
      - Remove the existing R30 from the floor
      - See about replacing the roofing shingles etc and use what you said, polyiso, then new shingles, but that would have to be in the near future but not right now.
      - Add a supply/return for the Air Handler in the attic
      - Seal the attic floor

      Another option:

      - Remove the old R30 fibreglass
      - Seal the ducts as best as possible (ie: fix/remove/replace flex ducts with better quality stuff)
      - Air seal the attic floor
      - Cellulose to R60

      1. Expert Member
        BILL WICHERS | | #5

        There is no need to seal the attic floor if you condition the attic, since you are bringing the attic inside the buildings envelope and intentionally 'connecting' it to the living space this way.

        Air sealing the ducts and instulating them is a good halfway point if you can't do the rest of the project right now.

        Bill

  3. 1910duplex | | #3

    We are also in 4a. We had no insulation at all in our walkup attic. We did not have mechanicals in there at the time (now have minisplit system for upstairs there), but it was not possible to insulate at the floor because of the stairwell.

    We didn't get to R-60, but what we did was do about 2 inches of closed cell foam against the ceiling and about the same against the eave walls and then put rockwool batts against them between rafters.

    You are going to have to furr out your rafters a lot, I think. We furred out most of our rafters by 1.5 inches with polyiso foam strips we cut, so that R-23 batts would stay in. But those were 2*6 rafters. In a small part of our roof, there were only 2 by 4's and we did R-15 batts.

    This amount of insulation caused our attic temperatures to stay closer to our second floor temperatures, so for instance, when it's 68 with the heat on, it's 61 in the attic. Or when it's 80 degrees on the second floor, it's about 90 degrees in attic.

    Whether that will help substantially with your bills or comfort, I do not know! Unfortunately, it has not made as much as a difference in comfort as we were hoping for.

    https://www.greenbuildingadvisor.com/article/a-vexing-question-about-heat

  4. Expert Member
    MALCOLM TAYLOR | | #6

    This is really a continuation of a discussion that the OP posted several days ago. For more background: https://www.greenbuildingadvisor.com/question/should-i-use-spray-foam-in-my-unconditioned-attic

  5. dan7210329 | | #7

    I'm just a homeowner, so take this with many grains of salt: If you are considering furring out for insulation to hang off the roof rafters, Heco Topix Therm screws (Small Planet Supply when available) are specially designed to screw to rafters and furring strips but have no threads in the middle so insulation is not compacted by screw threads. Also, consider Rockwool Comfortboard as the lowest part of the insulation assembly as it has sufficient stiffness to rest on 2x2 furring strips. You still want to add an air-barrier too, which will help contain breathing particles of insulation.

    1. Expert Member
      MALCOLM TAYLOR | | #8

      Dan,

      The OP describes the roof as having rafters, but it is actually framed with manufactured trusses. Modifying or adding members to them should only be done with the approval of the supplier.

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