Basement Finish duct work.

[mtCDCcb]

I am working on finishing my basement. I am building a 4'X8' room to enlcose my furnace and water heater. The return ductwork (taken passively from exterior) for the furnace is currently snug against the concrete wall. Should this be re-worked so I can get framing and insulation and drywall behind the ductwork? This will be an expensive thing to do, but can be done. I want to build properly up to code. Basement walls are required to be insulated (R13) so my guess is that we will have to move the duct work in order to do so but I just want to make sure since if will cost extra. Any insight would be helpful. Just wondering if we can keep the furnace/water heater room unfinished.
Thanks,
mtCDCcb

[Jerry Peck - Codeman]

I believe that the return duct you are referring to is probably actually for combustion air from outdoors.

Regardless, though, you raise an interesting question. I recommend checking with your AHJ (local building department) for the following scenario:
- Check on insulating the new walls and ceiling in that 4x8 room. This effectively makes that room outside the thermal envelope. This would, naturally, require that the door to that room to be equivalent to all other exterior doors (weatherstripped, etc.). What you are doing is bringing the basement walls in around that room and insulation that room off from the rest of the structure, the basement, and the space above.

Your AHJ may approve that, or they may not - will not hurt to ask.

The main concern will likely be proper combustion air and fresh air makeup air with a heat exchanger.