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Insulation for retrofit and thermal bridging questions

sbwga | Posted in General Questions on

Hello all,

First time posting and I am hopeful that I can convey my questions accurately. I recently purchased a 1997 home in NC and found out this summer that our home insulation is horrific. One primary reason is design. We have a two car garage (west facing) with a bonus room overhead which are located on the opposite end of the home from our HVAC units. The left, back, and front wall of the garage are open 2×4 stud unfinished. The garage door is uninsulated. The garage’s ceiling and common wall to the living space is finished.
The bonus room above has knee wall construction, and soffits that vent to the roof ridge so essentially due to the poor construction, we have 1/2 in drywall separating the interior from the exterior in the bonus room.
I will be spending this fall sealing and insulating the attic, bonus room, and garage. I wanted to ask a few questions about my plans.

1- I would like to use 3.5 inches of polyiso in the wall cavities and seal around the gaps with a closed cell foam. I am wondering if it makes any difference to install horizontal furring strips on the existing studs to create a .75 or 1″ air gap between the Polyiso and the interior drywall? Will that help break most the surface area of the thermal bridging? Typically I have seen battens / furring strips installed on the outside to create an air gap between the siding and the poly but I do not have that opportunity here.

2- In the bonus room area, I intend on installing blocking at all air gaps (ceiling / floor joists), air seal all penetrations / gaps, and utilize polyiso in the stud cavities between the flooring and 1″ ventilation baffles that I will build from wood so that I can seal each panel to the bottom of something rigid.
Does the polyiso act as a radiant barrier as well or would I need to install a different material across the studs for that function?
Additionally, I will be building each access door with insulation, weather stripping and positive retention hardware.

3- In my attic, I intend on removing all old blown in fiberglass insulation, air seal all gaps in the ceiling, install blocking at all air gaps and seal, and then I will blow in 12″ of cellulose. My only issue is that second floor unit is located in the attic and all my ducts are flexible and not well insulated. Therefore my HVAC unit is sending cold air into a very hot attic and drastically reducing my unit’s ability to cool the home. Are there any suggestions other than replacing all of my ducts with metal ducts and duct insulation?

Any and all advice is much appreciated. It’s going to be a very busy (and expensive) Fall.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brian,
    Q. "I would like to use 3.5 inches of polyiso in the wall cavities and seal around the gaps with a closed cell foam. I am wondering if it makes any difference to install horizontal furring strips on the existing studs to create a .75 or 1 inch air gap between the polyiso and the interior drywall? Will that help break most the surface area of the thermal bridging?"

    A. If you have the opportunity to put something between the studs and the drywall, you should install a continuous layer of polyiso there -- not furring strips. A continuous layer of polyiso is much better at reducing thermal bridging than an air gap.

    It sounds like you are a fan of the cut-and-cobble approach. If you haven't read it yet, you should read Cut-and-Cobble Insulation.

    Q. "Does the polyiso act as a radiant barrier as well or would I need to install a different material across the studs for that function?"

    A. If you are using foil-faced polyiso, the foil facing acts as a radiant barrier -- but only if it is facing an air space. If the foil facing is tightly sandwiched to a layer like drywall, the foil can't act as a radiant barrier.

    Q. "My only issue is that second floor [HVAC] unit is located in the attic and all my ducts are flexible and not well insulated. Therefore my HVAC unit is sending cold air into a very hot attic and drastically reducing my unit's ability to cool the home. Are there any suggestions other than replacing all of my ducts with metal ducts and duct insulation?"

    A. Yes. You can transform your attic into an unvented conditioned attic. For more information, see Creating a Conditioned Attic.

  2. sbwga | | #2

    Martin,
    Thank you for the prompt response.
    1- Excellent, I will make plans to do that in lieu of the battens. I was aware that that air gap was less effective.
    2- The foil face will be facing into the "exterior" knee wall space so if I understand you correctly, then I will be able to utilize foil facedpolyiso in this manner.
    3- After reading the link regarding a conditioned attic, I may just opt to change the ductwork from flex to metal and insulate. I have a gas furnace as well as the aforementioned insulation issues. It may be more time/cost effective for me to just make sure my ducts are sealed than to worry about the ventilation channels, replacing appliances, etc. I will have to run the numbers.

    Thank you again for your feedback. It is greatly appreciated.

  3. sbwga | | #3

    1- Was "not" aware

  4. Expert Member
    Dana Dorsett | | #4

    Since you have the studwall cavities available, fill them with the cheap stuff. At a framing fraction of 25% (typical 16" on center stud spacing) R13s in a 2x4 cavity with air barriers on both sides is the performance equivalent of about 1.5" of polyiso (about R9-ish). You can then drop to 1" or 1.5" on the continuous polyiso under the gypsum.

    For a NC climate R13 + R6 (1" polyiso) would beat IRC 2015 code minimum performance for exterior walls, and R13 + R9 (1.5" polyiso) would beat code min by quite a bit.

    Where local code allows it to remain uncovered, a bare foil facer facing an open attic would add another ~R1 to the effective performance. Having an ignition source like a gas furnace up there might require a fully time rated thermal barrier, such as half-inch wallboard.

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