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Tom Peck
Junior Member
Username: tom_peck_csi

Post Number: 2
Registered: 08-2004
Posted on Tuesday, September 14, 2004 - 12:44 pm:   Edit PostDelete PostPrint Post

Does anyone know if plywood or OSB have a thermal barrier rating to them? The IBC indicates that 1/2 inch gypsum board provides the appropriate protections (we've even been able to use iso as a base layer/thermal barrier). We have a wood framed project with "flat" roof (ballasted EPDM and EPS) and wood roof sheathing on wood trusses. As always, in trying to keep things economical, we would prefer to not install 1/2 inch gypsum board. Have searched our product catalogs, and some on the web, but have come up empty. Any help would be appreciated.
Ronald L. Geren, RA, CSI, CCS, CCCA
Senior Member
Username: specman

Post Number: 60
Registered: 03-2003
Posted on Tuesday, September 14, 2004 - 01:19 pm:   Edit PostDelete PostPrint Post

Ref: 2003 IBC Section 2603.4

Thermal barriers of 1/2-inch gypsum wallboard "or equivalent" complying with the time temperature curve of ASTM E 119 are allowed. However, I'm not sure if any plywood or OSB panels have been tested for compliance with this requirement.

Having said that, look at Section 2603.4.1.5. This section permits the thermal barrier to be excluded if the structural panel is installed in accordance with the section (exterior grade, edges supported) and the roofing is a part of a Class A, B, or C roof-covering assembly. Plus, the foam insulation must pass either FM 4450 or UL 1256.

Based on the information you've provided, it appears that no foam insulation is provided above the deck, so the roofing need only comply with Chapter 15 "Roof Assemblies and Rooftop Structures," which does not require a thermal barrier.

One thing to keep in mind: if this is part of a rated roof assembly, the components of the assembly need to match those of the approved assembly. And, if a 1/2-inch gypsum board thermal barrier is a requirement, then you must provide it.
John Regener, AIA, CCS, CCCA, CSI, SCIP
Senior Member
Username: john_regener

Post Number: 186
Registered: 04-2002
Posted on Tuesday, September 14, 2004 - 06:45 pm:   Edit PostDelete PostPrint Post

Isn't this a matter of understanding how gypsum board works?

Yes, it is a thermal barrier but the "thermal" is in the context of fire and not the heat of the sun. When gypsum board gets hot, it calcinates. That is, it gives off water. The water has the effect of cooling the board and preventing the passage of excessive heat. After a period of time, such as 60 minutes in the case of Type X or Firecode C gypsum board, the calcination process stops and fire can then penetrate.

My understanding is that conventional gypsum board is "rated" at about 30 minutes.

So, the gypsum sheathing at the roof is to add to the fire resistance of the roof assembly (i.e., prevent fire on the roof from burning down into the building).

Please correct me if I'm wrong.
Ronald L. Geren, RA, CSI, CCS, CCCA
Senior Member
Username: specman

Post Number: 61
Registered: 03-2003
Posted on Tuesday, September 14, 2004 - 07:21 pm:   Edit PostDelete PostPrint Post

The thermal barrier is required to separate foam insulation from fire originating from the "inside" of the building. The classification of the roofing system (i.e. Class A, B, or C) is the relative resistance of the roofing system to external fire exposure.
Tom Peck
Member
Username: tom_peck_csi

Post Number: 3
Registered: 08-2004
Posted on Thursday, September 16, 2004 - 02:22 pm:   Edit PostDelete PostPrint Post

Thanks for the input and commentary ...

FYI, I was able to discuss this issue a bit further with a local architectural rep in this area (MN) for Carlisle. He had contacted the factory and essentially noted that a thermal barrier is required if one is using expanded or extruded polystyrene on wood decking, but no thermal barrier is required if using iso on a wood deck ... this much I was aware of. However, a wood deck does not provide any sort of thermal barrier rating.

And, apparently, ballasted roofs cannot comply with FM 4450 (for wind uplift) since the ballast will tend to migrate (although at some point it will pile up enough dead weight to resist the uplift).

Also, after a quick search for the FM 4450 standard, the standard's title indicates it is for insulated steel roof decks ... so it doesn't appear to be able to apply to a wood deck.

Ron, regarding the comment that no insulation appears to be above the deck ... actually, it would be in this project. And in researching IBC 1508.1, it references back to Ch 26 regarding foam plastics and comliance with FM 4450 and UL 1256 to avoid a thermal barrier (of course along with having a Class A, B, or C roof covering, too) - so I would be back to the initial discussion noted above.

Anyways, I thought I would throw back some of the information I've learned for other's use if needed. Thanks to all.

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