Any consensus on durock for hearths?

[holg]

I was going to build my raised hearth about a year ago--life got busy. Now I am going to go at it again. The last time around, I read the threads on Durock/Wonderboard and other products, and read the info on-line from the manufacturers. Now I have read threads that have been posted in the interim, and it seems like the answer is no clearer on the preferred cement board for hearth building.

My stove is a hearthstone heritage, R value needed 1.2

My plans are, for the raised hearth, from the bottom up:

Wood floor (existing)
Support frame with plenty of blocking
3/4" plywood
1/2" next generation Durock
1/2" Micor
1/2" Durock
Bluestone, mortored in place

I well exceed the R value--I like to be safe.

But should I use something other than Durock because of the reported off-gassing from the stuff they have put in it to make it lighter? Any thoughts would be appreciated? Thanks

 

[Jags]

Just do it. Durock uses pearlited aggregate, not foam beads.

 

[begreen]

There is a similar USG product that uses styrofoam beads that should not be used for hearths. It is for shower bottoms and the like. Durock NexGen has perlite in it which is entirely non-combustible.

 

[bag of hammers]

There is a similar USG product that uses styrofoam beads that should not be used for hearths. It is for shower bottoms and the like. Durock NexGen has perlite in it which is entirely non-combustible.

begreen may be referring to PermaBase - great for shower tile backer, not good for hearths. You can actually see the little foam beads in the cut edges.

 

[holg]

begreen may be referring to PermaBase - great for shower tile backer, not good for hearths. You can actually see the little foam beads in the cut edges.

Thanks to Jags, be green and bag of hammers. The replies sent me back to the internet to figure out what perlite is. I certainly was wrong in thinking it was polystyrene or some other foam pellet. I learned that perlite is a form of obsidian, a volcanic rock, and that perlite is mined. For commercial purposes, it is super heated and cooled, resulting in the little white non-combustible pellets used in lots of stuff. I presume it is the super heated processed form that is in Durock, rather than the original ore form--the processed form allows products with it to weigh less than they otherwise would.

But this leaves me with 2 questions.
1. If perlite is both non-combustible and actually increases the R value because of it's insulating properties, why the hesitancy on the part of USG to put a tag on it that its acceptable for hearths?

2. And considering it doesn't appear to have a downside, in my application for a hearth in which another 1/2" in overall height isn't a concern, why don't I just ditch the Micor (which I will have to travel 240 miles round trip to get) and instead use another level of durock, which I can get locally and transport easily?

(Original plan was 2 layers Durock and one layer Micor for an overall R value of 1.88; possible revised plan with no Micor and 4 layers of 1/2" Durock has an R value of 1.56--My heritage requires 1.2)

Any responses again appreciated, and thanks.

 

[begreen]

Works for me. Get some long screws.

 

[Jags]

But this leaves me with 2 questions.
1. If perlite is both non-combustible and actually increases the R value because of it's insulating properties, why the hesitancy on the part of USG to put a tag on it that its acceptable for hearths?

The original version of Durock WAS hearth approved. The new version simply was not tested for it. It does have to get certified and that is a pricey test.

 

[begreen]

I think we are going to need the heft of large organizations like HPBA, CSIA, and a large construction lobby to make a formal request of USG to certify the NexGen product.

 

[Highbeam]

Because us DIY hearth builders are a small market.

Make sure your floor is up to the task. Durock is like 96# per sheet plus lumber, screws and thinset and tile your up over 500# for hearth pad plus 500# for stove, and then two or three fat chicks backing their rumps up to the hot stove can make quite a floor load. Like parking a VW in your living room if those gals are still carrying their big gulp drinks from 7-11.

 

[holg]

Because us DIY hearth builders are a small market.

Make sure your floor is up to the task. Durock is like 96# per sheet plus lumber, screws and thinset and tile your up over 500# for hearth pad plus 500# for stove, and then two or three fat chicks backing their rumps up to the hot stove can make quite a floor load. Like parking a VW in your living room if those gals are still carrying their big gulp drinks from 7-11.

2 points.
1. The amount of durock sold for hearths must be a pretty small % of the durock sold--probably can't count on USG going thru the certification process unless they have to for some other reason, and then maybe the hearth certification could be an add-on. Of course that's all conjecture on my part--I'm pretty good at talking about what I don't know anything about.

2. And Highbeam, I gotta agree--no need to increase the load on the floor. I did beef up the floor from the underside awhile back in anticipation of this project, but I think I will make the trip for the Micor to keep things lighter. Actually, I can exceed the Heritage's required R value with one layer of Micor and one layer of dorock--so I'll probably go with that on top of my frame support.

By the way--anything to look for when you are picking up a sheet of Micor to be sure it hasn't been affected by transportation and handling that would decrease it's R value? Also, any stickers it should have on it so I am sure I am getting what I think I am--can't say as I have ever bought Micor before.