Constructing air returns...


Can someone please give me or point me to a decent description of what construction methods are allowed for air returns in a residential application? If you're going to refer me to the code, please cite volume, chapter and verse for me...
Specifically, are we still allowed to box in the space between a couple of floor joists as part of a return air run? If not, what else *can* be done?
Details if you want them:
The house is a '50's vintage wood frame bungalow in southern Michigan. The original gravity furnace and supply ductwork were replaced with a gas forced air system. The original returns were left in place and look like they will be adequate.
We're planning to finish our attic into two rooms. Each gets a supply and return. The RA grilles go on a hip wall, and are ducted between joists to the middle of the floor and drop through part of a closet for one of the rooms, and two stud bays in an interior wall for the other.
These stacks will be made from galvanized, like everything else in the system, and safely enclosed. Happily, they'll come out right on top of the main return air duct in the middle of the basement ceiling so I can connect them up directly.
It's the horizontal run in the attic floor that I'm not sure how to build.
Before you ask, I've already spent a lot of time doing my homework for this project. I've done the heat loss calculation and confirmed that the furnace is well sized to the new load. I've verified the duct sizes and airflows for the system, and in fact I'll be changing one supply run that the original installer made undersized. The work is going to be permitted and inspected, and I'm not going to do a thing until I know I'm doing it safely and right.
If you can shorten my search for these particular answers I thank you...
Mike D.
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On 29 Apr 2007 19:36:37 -0700, snipped-for-privacy@eskimo.com wrote:

If the joist bays are open enough for you to box them in, aren't they open enough for you to shove rectangular metallic duct in there?
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I wrote:

Goedjn wrote:

If that's all that's allowed, yes, I can do it that way. I'm asking because boxing them in would be easier. Fabricating ducts for those particular locations would be more complicated due to construction details that it would take some extra carpentry to get around.
What are we allowed to do?
Thank you,
Mike D.
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