The Correct Blocking of Knee Wall Stud Bays That Meet a Sloped Roof
 

The 2x4 studs in the knee walls of the finished attic room in my 1950's 1.5 story house are cut to the roof angle and tacked to the rafters. The stud bays have no horizontal blocking at the top...
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It appears to me that to properly block the stud bays in order to enclose the insulation batts, a simple 2x4 block will not suffice.
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A horizontal 2x4 block will leave a gap at the corner. An angled 2x4 block will not provide sufficient surface for sealing to the sheathings. A 2x6 trimed to a parallelogram in profile seems to be a solution. Is there better technique?

Option 1 (left) is easiest and the "empty" zone can easily be filled with insulation prior to drywall installation. It gives you nice flat nailing surfaces for both the drywall faces as well.

The 2x6 method would also work, but will be more money and more labor time for minimal thermal gains.

How about option 1 and fill the void with spray foam?

No real need for it there and there is no backing to contain it upon expansion. The 2x6 rafter will have a 4" batt. in it already per that detail, so cutting the batt to the length/angle/etc. to accomodate the void would suffice.

I've always framed knee walls as Option 2, the drywall doesn't require fastening to the top plate unless a special shear/fire wall. Check with your local AHJ. Be sure to air-seal the joint at bottom plate/drywall/floor, per minimum Code.

Gary

The Correct Blocking of Knee Wall Stud Bays That Meet a Sloped Roof
 

Wouldn't one also want to air seal at the top, in order to completely enclose the cavity? It seems to me that option 2 is the least preferable because it delivers the smallest contact surface and therefore the most potential for air leakage both at construction and if later shifting occurs.

I have ripped plates at an angle to fill the front top gap before (similar to 3). My reasoning is the rafters (roof) are directly tied to the studs/plates (wall) with less chance of drywall tape cracking at that joint when the roof flexes with the changing seasons/wind pressures. Solid framing holding the joint rather than paper tape.... You should also caulk the ceiling/wall drywall joint (at drywall install) to help as in ADA: http://www.buildingscience.com/docum...wall-approach/

Be especially careful at any electrical outlets and use a foam pad there, that 1/8" hole around it is about 1-1/2" square; Fig. 4; http://www.buildingscience.com/docum...wall-approach/
Any moisture in the wall would dry to the room side without a vapor barrier (not needed because of f.b. keeping cavity warm), but why let it in anyway?

Gary