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Moving spray foam insulation below attic rafters

ThirtyWest | Posted in General Questions on

Hello, My name is Shawn. First off, I apologize for perhaps the millionth attic insulation question. I have read everything that I can find on the subject and there are still a couple of things that leave me scratching my head.

I am in midcoast Maine, inland, zone 6A. I will be converting 2 attics in the house into light use conditioned space, mainly for storage for the time being and maybe more down the road. Not knowing what the future holds for them I would like to get the insulation details correct. 

The house was built about three years ago and everything is still in new condition. The first room that I’d like to insulate is a cape style 12/12 pitch roof over a garage. It is comprised of free span attic trusses with an engineered floor that is 2′ deep from upper to lower chord insulated with what was r49 cellulose under the floor(some settling has occurred). The rafters are 2×8.

With the attic above a garage I am concerned with air quality, moisture from snow in the winter, and fire. Sealing of the garage floor will be done in the spring to allow for removal of water and slush via a squeegee and not allow it to be absorbed into the cement only to evaporate later. The ceiling of the garage was air sealed before the cellulose was installed over the drywall but will perform a double check on that before closing the attic floor up tight. 

I plan to air seal the floor after adding cellulose in the remaining floor cavity and install a hrv in that room. 

Moving on to the Attic insulation, I have added 2 layers of r6 comfort board behind the kneewall and will be adding r15 roxul in the stud bay of the kneewall. See Pic below…. 

I feel its best to keep a vented assembly to help remove any bad air that may migrate through the ceiling of the garage. This is where I get a lot of differing opinions and seemingly contradictory information. R49 and depth are the obvious problem. The knee-jerk reaction by everyone I’ve talked to is Spray 5″ of spray foam under the roof deck and be done with it(r33.5ish). I’m just not comfortable with that. I know there is no love for closed cell foam on here. But can anyone see a problem with putting an r23 batt of roxul with a 3″ air gap to the roof sheathing in the rafter bay, and then adding 7/16 Osb under the rafters  
And spraying a continuous layer of 4″ closed cell to the underside of the 7/16 Osb? I would fabricate a standoff bracket out of strapping or 3/4″ osb to minimalize any thermal bridging through the spray foam and still have a nailer for the drywall. A theoretical r49.8. The 4″ of closed cell should be a sufficient vapor barrier and the roxul should be able to dry nicely if needed. In the end the real question that I have would be, is the batt in the rafter bay actually doing anything? I like the idea of it being in there because of its fire retardancy. But I have to think that more cold is coming through the rafters than heat through the closed cell.  Also, If return on investment isn’t there, can it hurt having it in there? Any thoughts on the subject are greatly appreciated. I enjoying reading the responses in this Q&A section.

PS, The 3″ air gap comes from a 1″ furring strip made out of scrap foam adhered to the rafter.

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  1. MattJF | | #1

    Is the roof currently set up with soffit and ridge vents to ventilate every rafter bay?

    R23 rockwool in a 2x8 rafter bay will leave a 1.75" gap, which is good. If you are willing to do foam inside the rafters, go for 4-5" of reclaimed or polyiso seconds screwed and taped. Fur over the foam and drywall or 23/32 OSB screwed through the foam.

    1. ThirtyWest | | #2

      Thanks for the reply. Yes on the soffit and ridge vent. When you say polyiso in the rafters are you saying instead of the roxul? I had thought about that, but I wasn't sure about a fire rating at all for used polyiso and read in another thread that the rvalue drops quick when it gets cold out. That was my original plan.

  2. MattJF | | #3

    When I said inside, I meant inside the room. Often times people don't want to loose space inside the room and deal with the reduced R value.

    Continuous insulation is very effective. R23 in the bays and just 2.5" of polyiso continuous (R14-15) inside for a R37 total will be higher performing than 7" of R49 of ccSPF in the bays. This is a good calculator to play with:

    I said 4-5" of polyiso, but 2-3" is probably all that is really needed on a vented assembly.

    1. ThirtyWest | | #5

      Actually when this was first laid out the plan was 3 layers of 2" polyiso fastened directly to the rafters. If I went back to that idea I would still have that empty rafter bay. Short of fire suppression do you think putting the r23 Rockwool in the cavity would actually be any any real rvalue to that assembly?

  3. 730d | | #4

    I won't get get into the rafters but looking at your picture I believe you should add solid blocking at the transition of floor to kneewall and solid blocking at transition of kneewall to rafters.
    Also I believe you are missing an air barrier on the kneewall unless I dont understand your finished assembly.

    1. ThirtyWest | | #6

      It looks like I left that out. But the osb is going on the front of the kneewall as well, air sealed and then insulation applied to the osb. the comfort board is really there as a thermal break for the upper chord on the floor. We just didn't have a good place to stop. so we went to the height of the rafters.

  4. MattJF | | #7

    I would definitely put the Rockwool in the rafters. At a 20% framing fraction it adds R16.8 to the total wall, so definitely not nothing. It is also fast and easy. 3" of foam is way easier to hit the rafters using screws than 6". You also get more room inside this way.

    Mike is referencing blocking at the kneewall as a code requirement for fire blocking. The air barrier on the inside of the kneewall seems pretty straight forward.

    I don't know if there are any details around on insulating a floor like yours. You want to make sure air can't wash under the floor sheathing, which is a risk if there is any settling. It seems like the ideal setup would extend the air barrier down to the garage ceiling at the kneewall or follow the roofline behind the kneewall.

    1. ThirtyWest | | #8

      Thanks for the info! so, batts will go in. The Comfortboard runs down the length of the kneewall to the drywall. I guess I was thinking that it would be sufficient for fire blocking once combined with the cellulose on the floor. Thats why I didn't use foam for that application. I liked the idea of a vapor open floor with it being over the garage. hopefully any bad air could be drawn out and not up into the room. I will reassess my plan. The floor on the gable ends will be air sealed too. Many thanks guys!

      1. MattJF | | #9

        Vapor is related to moisture, not air movement. An air barrier may be vapor open (allow moisture to pass), but still be an air barrier. Your garage ceiling and interior air barrier should block air from the garage.

        You don't want air infiltrating the space between your floor sheathing and cellulose. A decent amount of air will flow right through your comfortboard and bypass the cellulose if there is any gap between the cellulose and the floor. This is particularly true when wind blows on one wall setting up a pressure differential from one side of your floor to the other. You will want to extend the air barrier (Plywood, good OSB, or foam board) down to the ceiling below to enclose the cellulose.

        1. ThirtyWest | | #10

          Sir, thank you very much for your time and willingness to lend a hand. I will run the osb down to the drywall.

          Edit: Can anyone on here comment on the use of DensGlass sheathing for an air barrier? Seem like it would be easy to work with....

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