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Community and Q&A

Wall cavities – retrofit and improve

solidst8dad | Posted in Energy Efficiency and Durability on


New around here and have been doing a lot of reading on Green building and given my last few electric bills and desire to go solar, I am looking to start making improvements.

We are in a 16 year old “builder special” with all of the fun that entails.  While I have longer term plans to reskin the house, given the construction, I will be permanently stuck with a ventilated attic.  Before summer hits again, I am looking to make improvements to our 3rd floor Bonus, bed and bath which are basically built in what would have been an attic.

We are 1 mile from the border of Zone 3 &4 so heat and humidity are a primary concern from March till October. And winters are fairly mild.

There are a lot of improvements I can achieve on the third floor given the amount of access I have to the walls and ceiling as well as 2nd floor ceilings. However, this is where I get a bit lost.  It seems like wall construction is highly opinionated and I understand well that a bad air sealing choice could cause mold growth and do more harm than good.  

I am considering removing the existing faced fiberglass bats (currently faced side towards interior and moving to Rockwool + 1″ Polyiso w/foil then sheathing with 1/2″ CDX plywood or OSB.   Where I get confused is that there seem to be varying opinions about where to put the foam, adjacent to the drywall in each cavity inside the rockwool the or on the Outside of the cavity.

What is the proper way to retrofit in this scenario? Also same for ceilings (this attic/mechanical space is right above the Master Bath).

I can provide more context (adding dehumidifiers, acoustical sealing of the sill and wall cavities and foaming the penetrations) but this is already pretty long.

Thanks in advance!

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  1. Expert Member
    BILL WICHERS | | #1

    Are you planing on using the polyiso as a "cut'n'cobble" installation, cut to fit BETWEEN the studs? If that's the plan, I'd skip that step. If your plan is to put the rigid foam on the exterior, over the sheathing, then that will work much better, but means replacing the siding. Putting the polyiso behind the drywall on the interior is usually the last choice, and it requires that you have some kind of drying to the exterior for a robust assembly.

    Note that you need a certain minimum amount of exterior rigid foam based on the depth of the studs. 2x6 walls need more than 2x4 walls, for example. You didn't mention what size studs are in your wall...

    It sounds like you have mechanicals in your attic, correct? If that's the case, then ideally you want to insulate and air seal under the roof and not the attic floor. This will bring the attic space inside the building envelope, which will help with efficiency if you have mechanicals (furnace, ductwork, etc.) up there that you can't easily relocate to elsewhere in the home. You can either build this as a vented assembly, or a non-vented roof using spray foam. If you have the clearance and you're doing the work yourself, I'd go with a vented assembly. If you are tight on space (shallow rafters), or want to be able to contract it out and have it done quickly, spray foam is a better way to go.


  2. solidst8dad | | #2

    Thanks Bill!

    For clarity I have 2x4 studs. I had planned on Rockwool + 1" Foil Faced Polyiso on the interior walls that are exposed to the attic. This would get me to ~R21.5 hopefully limiting heat gain form the attic space.

    I will call a couple of local contractors and get quotes on trying to encapsulate the attic spaces, (2x8 rafters, 2x4 outer walls) but I am not sure they will be able to do much for me given the obstructions and limited workspace the construction has created.

    Can you link me to an articles on a properly designed vented attic? I would like to look into that too while I am researching. Exterior walls are likely a 2025 or 2026 project. (the vinyl is not holding up the the full south facing I have)

  3. adrienne_in_nj | | #3

    You wrote that you have "interior walls that are exposed to the attic." Do you have kneewalls? If so, you may find these articles to be helpful:

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