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Community and Q&A

Insulating Platform Framing Inside Balloon Framing

Paul034 | Posted in Green Building Techniques on

Hello everyone, thank you for taking time to read and answer my post 🫠

My 125 year old 2×4 balloon frame house was stripped to studs, sheeted with OSB on the interior, and with a 1″ offset, reframed with a 2×6 studs. The house uses the original cedar lap on exterior of the balloon frame Essentially it is a platform new construction inside of a balloon frame.

My question is, would it make sense to spray the interior-side of the OSB with 2″ of closed-cell sprayfoam before filling the rest of the 2×6 cavity with rockwool and finishing it off with drywall?

I think so, as it would prevent condensation on the interior of the OSB during the heating season, and condensation on the exterior of the OSB during the cooling season.

Steer me toward the light fellow humans! 😎

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  1. Paul034 | | #1

    Anyone want to take a crack at this?

  2. Expert Member
    BILL WICHERS | | #2

    Let's see if I'm understanding what you have here correctly... It sounds like this, from the outside in: cedar lap exterior siding / 2x4 balloon-framed studbay / new OSB / new 2x6 framing, ofsset 1" laterally along the OSB so the new and old studs don't "line up" all the way through / new 2x6 studbay / to-be-installed drywall. Sound right?

    If I'm understanding what you have correctly, you basically have a double stud wall with an inner layer of OSB slightly shifted towards the exterior, since the exterior studs are 2x4 and the interior studs are 2x6. If that's the case, then I see no need for spray foam here: just treat it as a "regular" double stud wall and insulate with batts, or go dense-pack. Do you have any insulation in the exterior 2x4 walls? You really should. If you have insulation in the exterior 2x4 stud bays, I would just make sure the OSB is detailed as an air barrier, then insulate the 2x6 studbays with batts. If there is NO insulation in the 2x4 studbays, then I'd insulate the interior 2x6 studbays with batts, but I'd also use an interior side smart vapor retarder. This could be kraft faced batts, but if you use mineral wool (which is what I would use), I'd run MemBrain over the interior side of the 2x6 studs right behind the drywall, then detail the drywall airtight as an additional air barrier.

    If you had air permeable exterior siding, and were insulating over that, then I'd consider spray foam. Since you have an OSB layer, which can be detailed as an air barrier, I'd skip spray foam. If you have batts on both sides of that OSB, you have a wall that can dry in both directions, which is a plus. An inner layer of closed cell spray foam would eliminate that bidirectional drying ability, which is a downside.


  3. Paul034 | | #3

    First, Bill, thank you so much for such a detailed response!

    I do have some questions, the first one is:

    Why would 125 year old lap siding would be considered not air permeable. Honestly, I think it is very air and vapor permeable and even likely moisture permeable. My assumption was that the OSB, that is stapled to the 2x4 balloon framing will dry to the outside. Thus the air cavity was left unfilled before OSB was stapled onto it.

    Thank you for your response :)

  4. Paul034 | | #4

    Also, if it helps, I'm in Cleveland, OH about half a mile away from lake Erie

  5. Expert Member
    BILL WICHERS | | #5

    I had meant that if you were insulating RIGHT OVER the leaky lap siding, spray foam would make sense as an air barrier, since there really isn't any other practical way to air seal a lap siding wall. Lap siding IS very air-leaky! If it's air-leaky, it's also letting plenty of water vapor through. In your assembly though, you can easily air seal the OSB layer, so you don't need to worry about air sealing the lap siding.

    Drying ability has to do with vapor permability, not the R value of insulation in a cavity. R23 mineral wool (2x6 depth) is still VERY vapor open, and doesn't do anything to impede the drying ability of the wall assembly. Mineral wool also lets moisture go right through to condense on stuff though, which is why you usually use an interior side vapor retarder to limit moisture ingress into the wall from the interior of the home. Something like foil faced polyiso is a vapor BARRIER, poly faced EPS is a vapor BARRIER, those things do not allow water vapor through, at all. They are also good air barriers if you tape the seams.

    If you were to use exterior rigid foam on a regular wall (not your assembly, I'm just using this as an example), the reason you can skip the interior side vapor retarder is that exterior rigid foam keeps the exterior sheathing warm enough to limit moisture accumulation and condensation risk. In your assembly, if you had insulated the exterior studbays, you'd have that same advantage -- the OSB would be kept warm enough to avoid moisture issues. This would be similar to using exterior rigid mineral wool instead of rigid foam, since rigid mineral wool is very vapor open, and allows unimpeded outward drying in such an assembly where rigid foam (polyiso, EPS, etc.) does not.

    Ideally what you'd want with your wall would be batts on BOTH sides of that inner sheet of OSB. That would give you a sort of double stud wall assembly, with high overall R value. Your offset studs help to limit thermal bridges. You would have drying ability in BOTH directions (assuming you don't use an interior side vapor barrier), which is a plus. With a wall like that, I'd use an interior side smart vapor retarder to help limit how much moisture could get into the wall, but you'd need nothing on the exterior side -- unfaced batts would be fine.

    It sounds like you've already closed up the exterior side of the OSB though, so you have limited options to insulate there. You could dense pack the exterior stud bays, or leave them open. If you leave them open, you should air seal the OSB, insulate with batts, and now you MUST use that interior side vapor retarder, since the OSB is essentially your exterior sheathing and will be close to outdoor ambient temperatures. This is similar to a regular studwall insulated with batts and using no exterior continuous insulation. Since your OSB can dry to the outside, it's a safe assembly without any special vapor barrier layer internally.


  6. Paul034 | | #6

    Bill, thank you for another detailed response.

    Unfortunately the balloon frame cavity is not currently accessible. Possibly, in the future if and when I will replace the siding, I can add mineral wool batts to the 2x4 balloon framing from the outside.

    The way I was considering the wall system before is that the OSB layer is essentially my outside wall.

    After reading this article, my logic was that if I spray the OSB with a couple of inches of sprayfoam and finish it off with with a mineral wool batt. This way any moisture that would get into the 2x6 cavity from the inside of the house during the cold season would not condense on the interior of the cold OSB wall (because it would be sprayed with 2" sprayfoam and not transfer the cold from the exterior-side of the OSB into the insulated 2x6 cavity) and leave back out via drywall or whatever means it got in. During the hot season when AC is on, the sprayfoam on the interior of the OSB, would not transfer the cold from the inside of the structure to the OSB, and not form condensation on the exterior side of the osb. Also, in this instance if there was no vapor barrier between the drywall and the 2x6 cavity -- it could dry out using the dry conditioned air from inside the house.

    Where did I go wrong with my logic?

    Also, I want to add that in the wet rooms like bathrooms I decided that filling the entire 2x6 cavity with sprayfoam would be appropriate. These cavities are currently half-full waiting for the final layer.

    Please advise...

    What I had in mind is kind of laid out as the wall detail of this:

    Basically the FaB method:

    1. Expert Member
      DCcontrarian | | #8

      Your wall is capable of drying to the exterior. If you have a vapor retarder to the interior you shouldn't have any issues. Flash-and-batt is usually reserved for assemblies that can't dry to the exterior like roofs.

      I agree with Bill that the space between the two walls should be insulated. I would look at making 4" holes in the OSB so that cellulose can be blown into the cavity. It's not expensive and it makes a good air seal that is vapor open. Then batt on the inside 2x6 wall. If it's balloon framed you should be able to fill the whole wall from the top, unless there's fire blocking.

  7. Paul034 | | #7


  8. Paul034 | | #9


  9. Expert Member
    BILL WICHERS | | #10

    DC pretty much covered it. You don't need a vapor control layer (barrier or retarder) in the middle of the wall, because the wall can already dry in both directions. Some people on GBA have intentionally designed walls this way, because they're more robust. The usual way to go for that has been exterior rigid mineral wool panels, but those are pretty expensive. Your assembly will work just fine too, but you should really try to get some insulation into the currently empty portion of the wall. Dense pack cellulose is one option to do that even with the wall closed up.


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