Installing cement backerboard around a firebox

[jayoungny]

Hi

Thanks to those who helped in my other thread. Figured it was best to start a new one.

I have a firebox that is nailed into framing studs. I am going up with stone (technically cement with a stone veneer), and that will be attached to backerboard with mortar rec'd by the stone manu.

My first thought is any cement backerboard that is non-combustible will do. I have some USG Durock Cement Board that is noncombustible, but someone else told me that the board has to be rated for 450 deg. I previously called USG about something else, and b/c they gave me some misinformation, am a bit leery about asking them about this. We would all like to think that the rep form a company would know everything there there is to know about the products they sell, but such is not the case.

Was hoping someone with experience can guide me here. Is any cement board - like the Durock I have - ok, or must I get something else?

Next, the fbox manual says to seal all the joints with noncombustible material. I was going to use caulk. I have some that the manu says is noncombustible (isn't all caulk?), but now wonder if that needs to have a rating as well.

Thanks in advance.

--edit--

Found some USG lit:

(broken link removed to https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/durock-cement-board-system-guide-en-SA932.pdf)

That says:

• Noncombustible panel
• Assemblies with 1/2" USG Durock® cement board have achieved one- and two-hour fire-resistance ratings

 

[begreen]

The Durock, as long as it is not their shower product, should be fine. If it's the original heavy Durock, it is pure cement board with a fiberglass mesh skin.

Previous thread:

Wood Fireplace - I am going to ask you fine peeps for some assistance

Hi :) My wood burning fireplace is in need of work. There is nothing wrong with the prefab firebox, nor the chimney. The sidewalls are too close to the firebox (min. clearance not met), the hearth is too small, and there is no caulking. The last part is easy - I think. Here is a pic of what it...

www.hearth.com

 

[jayoungny]

Thanks!

This is not the original heavy Durock - I believe fiberglass is combustible, so that wouldn't work.

What about the caulk? Do I need to use this:

FIRE STOP FIRE-RATED SILICONE | DAP Global

Single component, neutral cure, silicone firestopping sealant classified for sealing fire-rated service penetrations and construction joints in horizontal and vertical fire separations.

www.dap.com

 

[begreen]

Thanks!

This is not the original heavy Durock - I believe fiberglass is combustible, so that wouldn't work.

What about the caulk? Do I need to use this:

FIRE STOP FIRE-RATED SILICONE | DAP Global

Single component, neutral cure, silicone firestopping sealant classified for sealing fire-rated service penetrations and construction joints in horizontal and vertical fire separations.

www.dap.com

Captive fiberglass is not combustible. It will start to melt at a high temp (~2000º), way above any concern here. It's not relevant in this context.