I may be overthinking this, but have to ask. Couldn't find the answer on USG's website.

I'm installing a hearth in my log home for a wood stove. Behind the stove will be an air space, a layer of 1/2" Durock, then 2-inch thick manufactured stovne veneer. My question is about the Durock. It is Class A fire rated, but it isn't fire I'm concerned about. Durock has some sort of mesh material in it. I don't know what it's made of, but it cuts easily with a utility knife. Will this material melt from the constant heat where the the masonry thimble passes through the Durock, and somehow compromise the installation? Do I need to hold the rock away from the thimble some distance? Any advice would be appreciated. Thanks.

 

Your cement board from Durock is entirely non combustible with fiberglass mesh for reinforcing. Install the unit with adequate space from wall as suggested by manufacturer and use the corrrect cement board screws. The same applies for the thimble which can not come in contact with combustible framing. take advantage of trim rings to hide spacing which can vary from the pipe manufacturer and be aware OF local codes. Always remember, you probably know you need to check with your insurance company to add a coverage when installing a wood burning device. Be prepared for an inspection from your insurance provider as well.


bs5

 

Thanks for the response. I am aware of everything you said, including the insurance company issue (I've already talked to them). I had the chimney built and the thimble installed by a professional and it has passed inspection. I am not concerned with the thimble's proximity to combustible materials, but rather, the Durock's proximity to the thimble that will pass through it. Won't that fiberglass mesh melt? Or does that not matter since it's effectively embedded in the Durock?

 

Melting point of fiberglass:

"Fiberglass cloth has excellent heat resistance at relatively low cost. Fiberglass cloth retains approximately 50% of room temperature tensile strength at 700°F (371°C); approximately 25% at 900°F (482°C); with a softening point of 1555°F (846°C) and a melting point of 2075° F (1121 °C)."

 

Passing a Chimney through the Wall

Here is a very good article. it also has some links that will allso be useful. Perhaps you have seen it. When I built chimneys and I built many brick and block ones through the wall with a section of brick and solid masonry about 16" x 24 " high. Now days that does not pass muster. Later we used a store bought steel with ceramic donut surround for a chimney thimble. The company name escapes me but at the time it had all the approval including UL. I don't know what your mason provided for you. The standards get better and more stringent all the time. Insurance companies have spent skillions of dollars on wood stove caused fires. I burned wood for 25 years until my wood stackers all grew up and left me as wood chucker in charge. Now days it's just nice to go to the thermostat and turn it up. Good luck with your project.

I'm sure the fiberglass has some point at which it breaks down but I do not know what it is. I would think if there is any failures it will be with some other material or workmanship. Nothing can boast 100% efficiency. That is the way I see it. I' have seen codes spelling out 1 0r 2" framing clearances to masonry chimneys. So many do not meet this criteria expecially where it goes through the roof and receives flashing.

bs5

 

Thanks for all the good information. I had no idea fiberglass had such heat resistance. As for my wall penetration, the mason cut a 24x24 square hole in the log wall and filled in around the thimble with brick and mortar. It passed inspection.