ATTIC

Energy wise I would imagine that if you add the insulation to the other roof sections in the dead space behind the knee walls that it would improve the energy performance.  However, I don't believe that it is required by code as long as you insulate as you described. 

Depending on where the house is a 1926 stick framed house probably doesn't have deep enough rafters to put in code required insulation in the roof as it is.  the new Residential Code requires R-38 insulation which would require a 2x12 depth for batt insulation.  Rigid or foam can be done with less depth.  The existing rafters can be sistered with 2x12s to provide the required depth if necessary if this is being brought up to code. 

Another thing to consider is that if this is not allready a legal living space, it appears it may or may not be as you described it, and was originally an attic space it would require a permit to finish it as a living space.  The attic floor joists may or may not meet current code requirements for use as a floor.  If they are undersized then they may feel 'bouncy' when its used as a living space.

Thanks for the information.  The house is built from some large old growth timber.  I don't have measurements right now, but the rafters are deeper, I would imagine, than most homes.  Actually in this attic it was used as living space but probably not since who knows when.  It has an original installed and working radiator for heat just like the rest of the house.  I haven't measured the distance on center the floor joists are because that would require I lift the flooring up which is nailed down roughsawn timber like the rest of the houses construction.  The floor has zero bounce, they just don't build houses like these anymore.  I am pretty sure the insulation will fit.  We had to shim the rafters to make them all even with each other which gave it a little more depth also.   Again thanks for the response.   

The answer to your question is as follows:

The rafters above the attic living space should contain insulation between the rafters from the ridge to the Knee walls. The knee walls should be insulated and the floor areas behind the knee walls should be insulated out to the exterior walls. This is known as the heating envelope.

The attic rafter and knee wall areas will require a vapor barrier to the heated side of the house and the floor areas in the side attic cavities will require a vapor barrier also, but be sure the barrier is face down against the 1st floor ceiling. In outher words, the lower level ceiling goes beyond your knee walls and out to the exterior walls. So the heated side is the 1st floor ceiling. There is no real benefit to insulate the remaining attic rafter areas that extend beyond the knee walls since they are outside of the heating envelope

Hope this helps!

Schuerman,

     You answered exactly what I was looking for.  I am sorry if I was a bit unfocused or unclear on my question.  Your answer is as I expected, but I was uncertain.  Thank you very much!