# above ceiling plenum



## bptp32

Type VB Construction; fully sprinklered Mixed use Bldg; two story structure;  the first floor day care center has two forced air furnaces which are trying to utilize the above the drop ceiling space as a return air plenum.  The space contains exposed wood framing, electrical wiring, exposed karft facing on the insulation; alarm wiring and CPVC sprinkler piping.  The architect is stating that this is all OK.  I am citing Section 602.2of the 2007 Mech Code of NYS.


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## Coug Dad

The plenum should be sprinklered if there is exposed wood.  I do not belive it can be utilized as an air plenum because of the exposed wood.


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## beach

I agree with Coug, no wood in plenum. I believe the CPVC has to be a certain distance away from the return air vents in the drop lid, according to the cut sheets....


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## klarenbeek

602.2 of IMC would allow plenum enclosure to be made of wood in this case, because the building can be constructed with exposed wood framing.  The way I interpret that, is while the plenum edge can have exposed wood, wood framing can't extend into the plenum, such as a partition wall extending above the ceiling to the lid, with the drywall stopping just above the ceiling.  Any exposed wiring would still have to be plenum rated.  I'm guesing that the kraft faced insulation needs to be covered per manufacturer's instructions. Plastic sprinkler piping needs to be a wet system and listed & labeled per UL 1887, see IMC section 602.2.1.2.  I'm assuming NYS code is based on IMC


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## Coug Dad

I'm afraid I disagree with Klarenbeek.  While Section 602.2 appears to leave the door open for exposed wood, Section 602.2.1 seems to prohibit wood except in one and two family dwellings.

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 125ºF (52ºC) 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 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 noncombustible raceways or enclosures, approved gypsum board assemblies or enclosed in materials listed and labeled for such application.


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## klarenbeek

The plenum _enclosure _can be constructed of materials permitted for the type of construction classification of the building. That means if the building can be of non fire resistance wood construction, the plenum enclosure can be also.  Anything that extends inside the plenum needs to meet all plenum rating requirements.  I don't necessarily like it, but thats what the code says.  Check the commentary on it also.


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## mtlogcabin

I agree with klarenbleek. If it is a Type VB construction then the combustible structural components of the building are permitted within a plenum. However it is very difficult to have a plenum in a Type VB construction and meet the requirements of Section 717 Concealed Spaces.


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## TJacobs

I assume the above-ceiling space has sprinklers above the ceiling and that the plenum space also meets 602.3?  If so, I am afraid I would agree with klarenbleek.


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## Coug Dad

It would be great if someone could post what is in the Commentary.  The code treats construction and finishes separately.  For example, I can build a wood stud wall.  However, the interior finishes would still be regulated by Chapter 8.  602.2 specifies what materials are allowed to create the plenum but 602.2.1 limits what can be exposed in the air stream.  It is not clear in my mind how 602.2 trumps the restrictions in 602.2.1.  Thanks!


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## High Desert

klarenbeek is right. Plenum enclosures and materials within the plenums are two different things. The plenum enclosure itself can be of wood if wood is permitted in the construction type in accordance with Section 602.2. Then the material within the plenum are regulated by Section 602.2.1.


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## mtlogcabin

*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 condi­dons 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.

:•A building of noncombustible construction as defined in the IBC is permitted to have only noncombustible

plenums constructed of noncombustible materials. The intent of requiring noncombustible plenums in buildings of noncombustible construction is to main­tain consistency between the materials used in ple­num construction and the materials used in building construction. Materials classified as noncombustible in the IBC are used to construct the plenum. These are materials that have been tested to the requirements of ASTM E 136.

In general, noncombustible plenums are those formed and bounded by noncombustible materials and contain few, if any, combustible materials within the plenum space. These plenums represent a rela­tively low risk for fire spread and smoke generation be­cause of the limited amount of combustible materials present. Although some combustible materials may be exposed within the plenum space, the types and amounts of these materials are strictly limited by Sec­tion 602.2.1. The area of a noncombustible plenum is limited only by the size of the fire area containing it.

A building of combustible construction is permitted by the IBC to use combustible or noncombustible building materials for specified components of the structure. In these buildings, either noncombustible plenums or combustible plenums can be used. Again, the intent is to require that the plenum be constructed of or formed by materials that are consistent with the materials allowed for building construction.

A combustible plenum may be constructed using any approved combustible or noncombustible material that is permitted for combustible construction in accor­dance with the IBC. The flame spread and smoke-de­veloped indexes of all materials within a combustible plenum must also comply with the applicable sections of the IBC. Combustible materials, such as ducts, pipe, wire, tubing and insulation coverings may be ex­posed within the plenum only in accordance with Sec­tion 602.2.1.

Note, however, that in buildings that are not permit­ted to have concealed spaces, the use of a plenum is

not feasible. See the IBC for the definitions of the dif­ferent construction types.

The airstream and surface temperature restrictions on the use of gypsum boards in plenums are intended to prevent premature deterioration of the gypsum board because of condensation on the plenum walls or excessive drying of the gypsum board (see commen­tary, Section 603.5.1).

*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 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).

The code recognizes that the plenum space is often used to accommodate components of other building systems, such as electrical, plumbing, fire protection, communication and mechanical. For this reason, the code permits exposure of limited types of combustible materials used for these systems within the plenum. A flame spread index of 25 or less and a smoke-devel­oped index of 50 or less are viewed as acceptable val­ues for combustibles in a plenum because of the mini­mal risk associated with those materials. Any hazard associated with smoke production in a plenum is also reduced by the air handler shutdown requirements of Section 606.

Previous editions of the code used the term "ex­posed within plenums" when addressing this issue. However, some designers and installers used that lan­guage to install plastic pipe and other combustible ma­terial with some insulation wrapped around it, claiming that the material was no longer exposed. If the wrap­ping material were to become damaged, loosened or destroyed in a fire, the combustible material could be exposed to the fire and produce hazardous smoke that

would be spread to other parts of the building through the plenum. The word "exposed" was deleted in the 2006 edition of the code to close this loophole. Note that Exception 5 still allows electrical wiring and cable to be installed in a plenum when enclosed in noncombustible conduit.

There is no stated limit on the aggregate amount of these combustible materials that may be exposed in a plenum. For example, a test of electrical wiring using the procedures of UL 910 accounts only for the amount of wiring present within that tested assembly. Once installed in the plenum, however, additional combustible materials, such as insulated wire, pneu­matic tubing and insulation, may also be present. The aggregate effect of exposing all of these materials within a plenum is usually not known.


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## Coug Dad

> A combustible plenum may be constructed using any approved combustible or noncombustible material that is permitted for combustible construction in accor*dance with the IBC. The flame spread and smoke-de*veloped indexes of all materials within a combustible plenum must also comply with the applicable sections of the IBC. Combustible materials, such as ducts, pipe, wire, tubing and insulation coverings may be ex*posed within the plenum only in accordance with Sec*tion 602.2.1.


So I can have combustibles exposed in the plenum if they meet the flame and smoke for a Class A, B, or C interior finish as applicable?  I do not have a major concern if the building is sprinklered, which would require the combustible plenum to be sprinklered.  What if the building is not sprinklered?  This would create a large concealed combistible space that would allow fire to grow undetected.


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## High Desert

Coug Dad, when you have a ceiling plenum the cfm typically is greater than 2,000 and smoke detection are required in accordance with Section 606, so there is an early warning system.


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## mtlogcabin

Coug Dad if the building is not sprinklered then the attic would have to be draftstopped every 3,000 sq ft and floors every 1,000 sq ft so if it is a large building a plenum probably will not be able to meet the other code requirements.


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## bptp32

thanks so much for all the help.  the exposed kraft facing on the insulation, the exposed romex and the fire alarm wiring will all need to be addressed.  The only option for the contractor, at this point, would be to hard pipe the cold air returns and avoid using the concealed space as the plenum.  thanks again


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## technomaze

I believe the CPVC has to be a certain distance away from the return air vents in the drop lid, according to the cut sheets. The plenum should be sprinklered if there is exposed wood. But you check you local property section.


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