Flue clearance from type X drywall

Why would you (is it required?) need 1" of clearance between a type B vent and type X drywall if the drywall is fire resistant? I always write it up, but I am beginning to wonder why.

I believe some of it is to stop transfer of heat to nails or screws


Thanks Larry, I always search the forum first before asking a question, but I searched the HVAC forum and only went back a little less than 3 years. It looks like the issue is open to some interpetation, although it was definatly leaning towards being OK.

Also the paper is not fire proof unless it is treated , Stay with the 1 inch clearance to be safe .

Page 2, first paragraph by manufacturer states “Drywall is not combustible or explosive.”

But NFPA classifies it as limited-combustible.

[FONT=Times New Roman]***[FONT=Verdana]In regard to gypsum wallboard***[/FONT][FONT=Verdana], the product’s paper facing prevents it from passing ASTM E 136. However, because it does have a demonstrated ability to perform in fire rated assemblies, the NFPA has placed it in a special classification called limited-combustible. This category distinguishes gypsum wallboard from other, more highly combustible products. To qualify as limited-combustible, a material must have a noncombustible structural base or core, a surface less than 1/8 in. (0.3 cm) thickness and a flame-spread rating of 50 or less.

[FONT=Times New Roman][/FONT]
[FONT=Times New Roman][FONT=Verdana]On top of those two definitions, one should consider whether there is framing wood within the “clearance to combustibles” distance even if there is drywall between it and the vent pipe.[/FONT][/FONT]
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Good post Larry.

B- Vent manufacturers say NO, and I believe underwriters Lab has not signed off or tested B-Vent to a less than 1" clearance. :smiley:


Thanks larry

Manufacturers requirements always override “codes” (except in cases where codes are prohibitive).

Generally, vent manufacturers require a minimum clearance (depending on the type of vent) to combustible materials **AND **building materials, which means, if it’s not part of the vent system, it must not make contact with any other building materials unless it has a “zero clearance” listing.


Time, the Enemy

  	 Because your heating system “has worked just fine” for years is of  little comfort. Time is working against you! Pyrolysis accomplishes its  destruction in a matter of years or months. In a test conducted by the  NFPA, a stack of wood fiberboard's (1/8-inch thickness)  was exposed to a  heat source of only 228°F. The wood self-ignited in only 96 hours!
  	 	  	 Ordinarily, a piece of 2" x 4" construction lumber has an ignition  temperature of approximately 500°F. Exposed to moderate heat for a  prolonged period, however, this ignition temperature is reduced and the  wood is capable of self-igniting spontaneously.
  	 	  	 Like the NFPA 211 standard, both local and regional building codes  designate minimum installation requirements. To use less than the  minimum is to** invite disaster.**

I have noted of scholarly members much like yourself, Roy Cooke and Marcel Cyr and Mr. J. Pope being three more, a unique quality of posting relevant historical material form NACHI message board posted any time in the past.
Like abracadabra presto magic …Pooof!!! 2006 dates this one.


Are the archives a clue to them magical quest?

Larry…is there some ritual, a native right of passage pre say Nick Gromicko sends you on…like the Maasai Warriors in Kenya sending a young boy reaching maturity into the forest with only a bow and nothing else for days eat off of nature and kill wild beasties to make them into a man?
Thats why kenton was in Africa, its all making sense now!!

Larry… tell me if I am getting warmer…Maybe an incantation or a series of secretive narrates before you enter your computer…no no no a you wave a magic wand over their computer screen, sorry I was stretching with the incantations…ha ha ha , how foolish of me. Sorry.

Either / or… I am satisfied just to be able to see the magic appear when I am lucky enough to be here on the grand INACHI MB.

Psst, Larry, can you tell me


Thanks for the kind words, Robert, but all I do is use the “search” function at the top of this page or I do have some saved reference materials on my computer. :slight_smile:

Thanks Larry.
A member for 4 years and still unaware of all the magic INACHI has to offer every member.
I consider NACHI the black hole of the home inspection industry universe swerling like a vortex. By doing so it is creating its own gravitational force swallowing information at speed of light to feed its members inside.:slight_smile:

One thing I do know as fact Larry is that, ***you and many astute members***are true free thinking home inspection warriors.
Once sent into a home with nothing more than the bare essentials, will surely survive and exit the same way you entered, a true warrior working on behalf of your client and the INACHI tribe.

Thanks buddy.
Just a bit of creative license.:wink:

OK now I am worried, exploding drywall?