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Finalizing insulation in converted conditioned attic

Chowtime | Posted in General Questions on

Hi all:

I recently converted the formerly vented attic above my attached garage to an unvented and conditioned attic. This attic houses my geothermal air handler, whole house dehumidifier, and will also provide some conditioned storage. My goal is to have this space be as energy efficient as possible even if the payoff period is pretty long.

I contracted to add 6 inches of closed cell foam to my 9.5 inch roof rafters (which are spaced @24 at center). Because the attic sits above an insulated but unconditioned garage, I have ~R-60 of blown in cellulose on the attic floor / garage ceiling. Since the cavities have 9.5 inches of rafter space, I had originally planned to add R-15 Rockwool batts to the cavities and, potentially, cover all of it with R-6 Rockwool Comfortboard 80. I also added HVAC supplies and a return to heat and cool the attic.

While the spray foam contractors did a good job (to my unexpert eye), the closed cell foam is a bit uneven in depth at different parts of the attic. It looks like I got more than 6 inches in many areas. This isn’t surprising given some of the hard to access spots. There is also a not-insignificant amount of foam on the rafters themselves. See attached pictures for details.

My question: What would you recommend to “finish” this project off from a roof rafter / cavity perspective? Here are some of the reasons I’m pausing on my original plan:

1. I’m concerned my R-15 Rockwool batts won’t fit and may be very inconsistent as it varies with the depth of spray foam in the cavities? The Comfortboards may be even worse with consistency. Looks don’t really matter, but I’d like a consistent roof envelope

2. I could shave the foam to make things more even and to expose the rafters, but perhaps that is counterproductive or even dangerous given closed cell dust?

3. Given I will have a return in the attic (albeit with a Merv filter), would the Rockwool create a lot of unwanted dust/fibers that will be recirculated in the home?

4. Should I just pay the contractor to come back out and spray foam the remaining 3ish inches and be done with it at R-60ish in closed cell?

Many thanks in advance for your advice!

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Replies

  1. Malcolm_Taylor | | #1

    Chowtime,

    With the foam you already have I don't think there is any useful purpose to adding more. The energy savings of adding either batts or more foam are in the weeds.

    If you aren't convinced by that I would either:
    - Use easily compressed fiberglass batts and cover the underside of the rafters with house-wrap to contain the fibers, and as a warm-side air-barrier/vapour retarder.
    - Install a 1" layer of foam board on the underside of the rafters, which will reduce thermal bridging.

    Depending on what code you are under you may be required to provide an ignition or thermal barrier under the foam.

  2. Chowtime | | #2

    Thanks. I was hoping the Rockwool batts + Comfortboard would provide the ignition barrier I need vs. putting intumescent paint on it?

    1. Malcolm_Taylor | | #3

      Chowtime,

      I’m not an expert on the IRC, but my understanding from this article is that just an ignition barrier is required, and the Comfortboard alone provides that.
      https://www.greenbuildingadvisor.com/article/thermal-barriers-and-ignition-barriers-for-spray-foam

  3. Chowtime | | #4

    Follow-up. I got the R-15 Rockwool batts at cost from a friend so I’d really like to use it vs going with nothing or something else. However, after talking to my insulation contractor, he seems to think adding the Rockwool R-15 batts on top of the closed cell foam could create a moisture issue where the Rockwool would meet the closed cell foam in the cavities. He either wants to add a vapor barrier on the side, i.e. the “end state” would be: vapor barrier —> Rockwool R-15 Batts —> 6 inches closed cell foam —> roof sheathing. Or he would recommend dense packing cellulose between the rafters.

    On my side, I’d prefer to go forward with no inside vapor barrier on top of the Rockwool. I’d also prefer not to drywall since it’s just a storage area. Of course I also don’t want to make a big mistake.

    Can anyone let me know if my approach would work or if I am making a mistake and what I’m missing?

    Thanks so much in advance.

  4. walta100 | | #5

    The way I see it the real problem is the old insulation laying on the attic floor. I know you said you would add supplies and returns but the question really is will it happen and will they stay open? If you actually conditioned the attic so it is more or less the same temp as the rest of your home R49 foam covered R15 rock wool should be fine and the surface will be well above the dew point.

    Now if you or the next owner go for the free lunch and decide to leave the old insulation on the attic floor and not install and operate enough supplies and returns all bets are off. The attic will likely have a temp very close to its dew point and the foam will likely get wet often making mold and rot likely.

    I do like the idea of an air barrier over the rockwool even if it is just a layer of house wrap.

    Walta

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