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IMC 602.2.1 Exception #5

602.1 General.

Supply, return, exhaust, relief and ventilation air plenums shall be limited to uninhabited crawl spaces, areas above a ceiling or below the floor, attic spaces and mechanical equipment rooms. Plenums shall be limited to one fire area. Fuel-fired appliances shall not be installed within a plenum .

602.2 Construction.

Plenum enclosures shall be constructed of materials permitted for the type of construction classification of the building.
 
http://publicecodes.cyberregs.com/icod/imc/2009/icod_imc_2009_6_sec002.htm

Just to make a clear distinction, my focus is on the material surrounding and bounding the envelope of the plenum space. For the purpose of this discussion, I do not care much about the non-plenum-rated junk that mechanical and electrical contractors try to stick into the plenum space (plastic pipe, wires, foam insulation, ect.).

MtLogCabin, is it your interpretation that if the type of construction allows us to build the entire building using only 3/4" plywood, then the bounding box to create the "plenum enclosure" could also be constructed using 3/4" plywood? In other words, no ductwork or drywall would be required?

And what if there are absolutely no return air enclosures such as a large pole-framed agricultural building? In that scenario, the supply air is distributed/ducted around the perimeter of the building and the return air runs wild throughout the wood framing until it reaches the central roof-top unit. Strict interpretation would suggest that the wooden roof truss is vulnerable where exposed to the return air unless the horizontal return air path is enclosed on all four sides with drywall per Exception #5.
 
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MtLogCabin, is it your interpretation that if the type of construction allows us to build the entire building using only 3/4" plywood, then the bounding box to create the "plenum enclosure" could also be constructed using 3/4" plywood? In other words, no ductwork or drywall would be required?
Click to expand...
Yep

PLENUM. An enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement, and thereby serve as part of an air distribution system.
 
Would the material not be subject to the flame and smoke of 25/50 requirement? Seams that the requirements conflict with each other....unless a wood truss meets the 25/50 rule (does it??).
 
An answer to my own question:

From the commentary

602.2.1 Materials exposed within plenums. Except as

required by Sections 602.2.1.1 through 602.2.1.5, materials

within plenums shall be noncombustible or shall have a flame

spread index of not more than 25 and a smoke-developed index

of not more than 50 when tested in accordance with ASTM E

84.

Exceptions:

1. Rigid and flexible ducts and connectors shall conform

to Section 603.

2. Duct coverings, linings, tape and connectors shall

conform to Sections 603 and 604.

3. This section shall not apply to materials exposed

within plenums in one- and two-family dwellings.

4. This section shall not apply to smoke detectors.

5. Combustible materials enclosed in noncumbustible

raceways or enclosures, approved gypsum board

assemblies or enclosed in materials listed and labeled

for such application.

.:. Materials located within a plenum, regardless of

whether the plenum is constructed of or bounded by

combustible or noncombustible materials, must be

noncombustible or must have a flame spread index of

25 or less and a smoke-developed index of 50 or less

when tested in accordance with ASTM E 84.

This section addresses those items that are installed

within plenums, not the materials that bound

and create the plenum space (see Section 602.2).
 
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