Wood – What’s the height limit of a wood framed building

As I understand your question, you do not want to run the inside wall all the way to the ceiling so that you can keep the wall space above the new knee walls. If so you will certainly want to cap the space at the top of the knee wall to leave a good grounding for a finished surface. One way that you can do this is to attach a 2x2 to the inside of the exterior wall and then bridge across from the exterior wall and the top of the knee wall. The bridging plank can be cut to width from a standard sized plank such as a 2x10 or 2x12. Installation in this manner will both support the top of your knee wall and cover the gap between the two walls.

In terms of finishing you could choose to cover this area with drywall or a nice looking board that you would finish to suit your needs.

We only worry about two things: 1) Live loads, and 2) Dead loads.

1) Live Load (loads that come and go): In your case, there isn't a second floor, so you're just concerned about "snow load". (If there was a second floor, then you'd add "people Load". In the U.S., its 40 lbs. per square foot.) You'll need to determine what your building code requires. You can call your local building department and they'll know. (It'll be something like 40 lbs. per square foot, or higher if you live in a snow zone.)

2) Dead Load (loads that are there all the time): You can add up the weight of all the roof components, or use 15 lbs. per square foot, unless you have concrete tile roofing.

Now, the Load on your header is based on those two factors added together and multiplied by one-half the span on your header. So, let's say you have roof trusses that span 28'. Just add 40 psf to 15 psf to equal 55 psf TOTAL Load. Then, multiply 55 psf by 28' and divide by 2 (because half the load goes the other direction away from the header.) which equals 770 lbs. per foot on the header.

Now you can check the tables for the span of your header (width of your window). If your window is 6' wide your header should be 2 - 2 x 10's. (Table 2.)

Simple, huh...