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Inner wall height in double-stud assembly

etting | Posted in General Questions on
I’m building a double stud wall with a structural outer wall of 2x4s 16″ oc with a double top plate and an inner wall of 2x3s with a single top plate, separated from the outer wall by 1″ strips of foam insulation essentially padding the 2x3s out to a 2×4 thickness.  The total heights of the walls are 97″.  I’m insulating each wall with 3.5″ Rockwool ComfortBatts that are 47″ tall.  The batts are designed to compress slightly, both vertically and horizontally, into the stud bays so that they press against the studs and thereby leave no gaps.  This worked quite nicely in the outer wall, as the height of the stud bays is 97 – 4-1/2 (one bottom and two top plates) = 92-1/2″ and the two batts that fill each bay add up to 94″ in height.  For the inner wall with a single top plate, however, the height of each stud bay would be 94″, so the batts would just fit vertically and would probably leave a small gap at the top, as despite their excellent frictional grab onto the sides of the studs, they do sag perhaps 1/4″ if not compressed vertically.
The solution I had in mind is to build the inner wall 96-1/2″ high so that the bays would be 93-1/2″ high and the Rockwool batts would therefore have 1/2″ vertical compression.  It would also make the inner walls easier to tilt into place.  The ceiling drywall would slip into the 1/2″ gap above the inner wall’s single top plate.   Do you see any problems with this idea, especially in regard to the drywall installation?

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  1. Expert Member


    Sounds like a good idea. As the outer-wall will experience more shrinkage due to having more plates, it also insures the inner-one doesn't take up any loads.

    I'm not sure whether it makes any difference whether the drywalls slips over the inner-wall or not. Might be easier to assume it won't and then you don't have to worry about how you attach the-inner wall to the ceiling.

  2. etting | | #2

    Thank you, Malcolm. You raise a good question about how the inner wall would attach to the ceiling trusses, as any nails or screws would prevent the drywall from slipping into the gap. The IRC doesn't seem to say anything about how the inner wall shall be attached to anything. Assuming the ceiling drywall would slip in, I was thinking the inner wall's attachment to the subfloor plus a few long screws through its top plate into the outer wall's top plate would be sufficient, perhaps bolstered later by some screws through its top plate into the ceiling trusses--after the drywall slips in. Does that sound feasible?

    1. Expert Member
      MALCOLM TAYLOR | | #3


      That sounds like a good plan.

      I think screwing the two walls together would be sufficient. I wouldn't worry about attaching it to the trusses afterwards.

  3. etting | | #4

    Thanks again, Malcolm!

    1. Expert Member
      MALCOLM TAYLOR | | #5


      I've forgotten where you are building.

  4. etting | | #6

    Central Arizona.: Zone 2B, but on the cold end of it.

    1. Expert Member
      MALCOLM TAYLOR | | #7

      That's right - I knew it was somewhere hot-ish, but for some reason not.

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