Certainly sir. The section for construction was added for 2015, 2012 only stated that plenums shall constructed of materials permitted for the type of construction, no mention of 25/50 smoke flame spread compliance, only material within a plenum, so I think in the 2015 code they made it clearer ( as mud ) Hope this helps.
I have not been on here much but I think its hugely valuable forum, not sure why it doesn't show me as a sawhorse, maybe complete lack of activity. Thanks again for your thoughts on this, the last thing I want is to over enforce the code but I do believe in doing it to the best of my ability.
2015 602.2 Construction
Plenum enclosure construction materials that are exposed to the airflow shall comply with the requirements of
Section 703.5 of the
International Building Code or such materials 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 or UL 723.
2012 602.2 Construction
Plenum enclosures shall be constructed of materials permitted for the type of construction classification of the building.
The use of gypsum boards to form
plenums shall be limited to systems where the air temperatures do not exceed 125°F (52°C) and the building and
mechanical system design conditions are such that the gypsum board surface temperature will be maintained above the airstream dew-point temperature. Air
plenums formed by gypsum boards shall not be incorporated in air-handling systems utilizing
evaporative coolers.
602.2.1 Materials Within Plenums
Except as required by Sections
602.2.1.1 through
602.2.1.5, materials within
plenums shall be noncombustible or shall be listed and
labeled as having 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 or UL 723.
Exceptions:
- Rigid and flexible ducts and connectors shall conform to Section 603.
- Duct coverings, linings, tape and connectors shall conform to Sections 603 and 604.
- This section shall not apply to materials exposed within plenums in one- and two-family dwellings.
- This section shall not apply to smoke detectors.
Combustible materials fully enclosed within one of the following:
- 5.1. Continuous noncombustible raceways or enclosures.
- 5.2. Approved gypsum board assemblies.
- 5.3. Materials listed and labeled for installation within a plenum.