Should I insulate the ceiling of a room in an attic with an insulated roof?

Question

My walk-up attic is insulated with spray foam along the roof line. The spray foam delivers an R value of about 20.

The attic is finished, and the rooms inside the attic have a drywall ceiling that is attached to 6-inch joists whose center sits about 6 feet below the peak of the roof, and 8 feet above the attic floor. The rooms have knee walls along the sides.

The ceiling above the finished attic has no insulation, and I was thinking of adding some fiberglass batts on top of the drywall. For about $500 worth of material and a day of work (if that), I could double the R value above the finished part of the attic.

But my question is whether this is worth it, given that the attic ceiling only covers the central part of the house. Heat that would rise up along the outer edges of the house would hit the exterior of the knee walls, where the attic that is insulated only with the spray foam. I can't add more insulation to that space (well, I think I could blow in cellulose behind the knee walls, but that would be a much more complicated operation because I'd have to cut up a bunch of walls).

A secondary question is whether I'd encounter issues if I mixed fiberglass batts with spray foam like this. I'm not sure if it would create condensation or other issues. The attic currently has no vents at all (the spray foam guy insisted they were not necessary with spray foam).

Here's an ASCII-art a diagram showing my house. The upper dotted line is the attic ceiling where I am thinking of adding insulation. The lower bottom line is the attic floor. The vertical lines between the two dotted lines show the interior walls of the finished part of the attic.

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   /---\
  /|   |\   
 / |   | \
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Answer 1

To answer a question like this one has to examine the path of heat from the interior space to the exterior, which happens mostly by conduction, but also by convection (air movement) where a space isn't well sealed. In your case, I'm guessing that you have a sealed attic, otherwise the insulation in the rafters would be pointless.

If that's the case, and no air moves from the attic to the outside world (or vice versa), insulating your knee walls and ceiling will provide some benefit. Heat transfer to the attic from the occupied space will be slowed, improving efficiency and comfort (the interior wall surface will be warmer, reducing radiant heat loss from skin, or "draftiness".

Exactly what the financial and environmental benefit will be is difficult to estimate without significantly more detail about your climate, heating system, etc. You probably won't see a cost break-even for a number of years, but the creature comfort improvement may be apparent immediately.

Moisture shouldn't be a concern with fiberglass batts. Just don't introduce a second vapor barrier through either poly sheeting or faced batts. Just use unfaced.