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Community and Q&A

Strategy to insulate interior wall exposed to roof cavity

dgeist | Posted in Green Building Techniques on

My home is a brick colonial style in climate zone 3. Our main roof is 4-side hip and is encapsulated and contains our upstairs HVAC system. A secondary elevation is over the garage and adjoins the wall of the two south-most bedrooms. This one is 3-side hip with a flat top (architect bad a math) and is ventilated with soffit vents and a powered fan tied to a HA system. It’s controlled to run at 50% during daylight hours unless raining and goes to 100% when temps are over 85F but it still doesn’t perform well enough to keep the temps from rising into the 120-130F range in the summer and 85-90F in the winter.

The two bedrooms with walls adjacent to the garage on the second level are noticeably warmer during the days and noticeably cooler at night, summer or winter. The house is 1970s era and does contain SOME fiberglass in the wall, but the (stick) framing is exposed on the garage roof side. A portion of the attic floor is actually over a 200 sq/ft extension to the kitchen on the main level and the kitchen ceiling/roof floor is also basically uninstalled.

I’d like to seal up my leakage and prevent the thermal loss/gain as much as possible without throwing money at the problem. ( I would have foamed that attic if I didn’t plan on possibly expanding over the garage in a few years and ripping it all out).

My thoughts are to seal (from inside the garage roof ) leaks, gaps, and transitions between the walls/floor and the occupied spaces on the other side with caulk/foam then fill the cavities with either fiberglass or rockwool batts, then possibly cap the insides with sheets of fitted polyiso to provide an additional air and heat barrier between the living spaces and the adjacent bedrooms (taped and sealed). We use a portion of the space for storage (yeah I know…I shouldn’t) so I don’t think blown insulation is an option.

Does this strategy sound reasonable? Are there any common “gotchas” that I should watch out for with this kind of setup? The walls and ceilings have no poly vapor barrier due to my climate and the inside is between 45 and 55 RH 24/78/365. If the polyiso becomes a functional air/moisture barier, will the interior drywall allow enough “drying” of the space to prevent things like condensation on the room-side within the cavity? I assume this will be okay as long as I can prevent moisture from the vented attic from getting into the assembly.


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  1. GBA Editor
    Martin Holladay | | #1

    This is a wordy post and some of your descriptions are hard to follow.

    To get to the gist of your question: Are you simply trying to insulate an uninsulated wall? That's what it sounds like to me, but I may be confused.

    It sounds like there are bedrooms on one side of this wall, while the other side of the wall faces and an unconditioned attic above the garage. Is that right?

    If that's all we're talking about, then all you have to do is seal air leaks, fill the stud bays with insulation, and install a continuous layer of rigid foam on the attic side of the wall. It's usually a good idea to tape the seams of the rigid foam, and to cover the rigid foam with a layer of drywall to reduce the fire danger.

  2. dgeist | | #2

    Thanks Martin and sorry for the confusion. Yes, it's an uninsulated 2nd floor wall with bedrooms/drywall on one side and open to a roof cavity on the other. Your description is good. Would faced polyisocyanurate board be acceptable over the insulation (assuming it qualifies as a fire block where I am)? Your reminder may motivate me to use rock wool over fiberglass for that very benefit

    For about 10 feet further than the upstairs wall of the attic cavity, the kitchen also extends on the main level. Would I be best suited to insulate this portion traditionally (air-seal all the kitchen ceiling drywall penetrations then add batts or other insulation above in the framing)?

  3. GBA Editor
    Martin Holladay | | #3

    To determine whether you can leave exposed polyiso facing your attic (without a layer of drywall for protection), contact your local building department or fire marshal.

    Q. "Would I be best suited to insulate this portion traditionally (air-seal all the kitchen ceiling drywall penetrations then add batts or other insulation above in the framing)?"

    A. Again, your wording is a little confusing. But I think you are asking how to insulate a ceiling with an attic above. It sounds like you have the right idea. Here is a link to an article with more information: How to Insulate an Attic Floor.

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