I’ve been having an interesting time the last couple days checking out various brands of cement boards, I’m finding they are very much not all alike, and probably should not be considered interchangeable. Everything that I mention in the following info comes from either the product websites, or the company “tech support” phone line people.
At this point, I would say that the clear winner for hearth use is Durock, with a distant second for Hardibacker.
I would be hesitant to use Wonderboard, and absolutely wouldn’t consider Permabase.
Durock clearly specifies in it’s descriptive product info PDF that it is acceptable for use in “UL Listed Wall shield / Floor Protectors”, and gives an R-value for the 1/2” sheets of 0.26. The US Gypsum website describes it as “Non-Combustible” - there seems to be no hesitation about labeling it suitable for hearth applications.
Hardibacker has that states it is ”online documentationthat states it is “Non-Combustible per ASTM E 136”, and “1 Hour Fire Resistive per NER 405. It lists an R-value for the 1/4” sheet of 0.13, but gives no value for 1/2”. The FAQ mentions it as suitable for “Fireplace Facing” but makes no mention of use for hearth protection. When I called their tech support line, they confirmed that Hardiebacker was definitely “Non-combustible” and said it could be used under a hearth, but that heat would “go right through it” and said it was only about R-0.15. The tech support guy didn’t sound really comfortable with the idea of using it for hearth protection. However it sounds like it would be 100% OK to use in “clearance reduction” wall shields.
Wonderboard had online documentation that pretty much made no mention of fire resistance, the only thing I could find was the MSDS line on Fire Hazards that said “Will not burn”. I called their tech support line, and was told that Wonderboard has an R-value of 0.2, but that the fiberglass facing was “combustible” so the product would only be suitable if it was actually covered in a NON-Latex based cement and tile - then it would “probably” be OK, but didn’t seem totally comfortable with the notion.
It was an interesting point that he brought up however that it would be a concern about the nature of the cement that was used to put the tiles down with - it needs to be non-combustible, and some formulations contain latex that is.
Permabase in their documentation says it can be used for “fire resistant walls” but lists expanded polystyrene in it’s ingredients on the label, and has a statement in their online documentation that the product should not be exposed to temps over 220*F (105*C). It was listed with an R-value of 0.2, but their tech support line person said it was “combustible” because of the expanded polystyrene (aka styrofoam) beads in the cement mixture.
I think most of them use fiberglass netting to hold the cement together. We have to keep in mind the specific use. A Hearth Board is meant to lower temperatures from a relatively low temperature to an even lower one....say, 300 degrees to 150 degrees. Therefore, the combustibility of the materials is not as important as the R or K value - since fiberglass is good to at least double or more of the temperature that is likely to be found on a hearth board. Even at a really high temp, the fiberglass would be more likely to deform or melt since it was encapsulated.
I have seen hearth boards that use other fiber sand sheetrock, neither of which are non-combustible. Some even use wood around the border. As to the latex which is added to or already in thinset mortars, I think the same thing applies. These hearths are not going to hit temperatures of 500+ degrees except perhaps in the most extreme situation when legs are cut off a stove and someone installs it incorrectly.
Personally, I would use Durock if possible, wonderboard as a second choice and stay away from the others. Elk (a building official) is comfortable with Hardi even though it has paper in it (cellulose), but again...that is encapsulated and he has taken a torch to the stuff and it does not burn. In no case would I use the last one mentioned, since their documentation DOES list a max. temp.
Extensive experience in the field seems to indicate that (at least) Durock and Wonderboard are readily available and work well for this purpose.....so why mess with anything else?
Although not a cement board (and I realize that was the scope of your study), could you do the same research for the Micore 300? It seems to be very popular in hearth construction, but I wonder if the manufacturer has the same reservations as those you spoke with.
I think most of them use fiberglass netting to hold the cement together. We have to keep in mind the specific use. A Hearth Board is meant to lower temperatures from a relatively low temperature to an even lower one....say, 300 degrees to 150 degrees. Therefore, the combustibility of the materials is not as important as the R or K value - since fiberglass is good to at least double or more of the temperature that is likely to be found on a hearth board. Even at a really high temp, the fiberglass would be more likely to deform or melt since it was encapsulated.
I have seen hearth boards that use other fiber sand sheetrock, neither of which are non-combustible. Some even use wood around the border. As to the latex which is added to or already in thinset mortars, I think the same thing applies. These hearths are not going to hit temperatures of 500+ degrees except perhaps in the most extreme situation when legs are cut off a stove and someone installs it incorrectly.
Personally, I would use Durock if possible, wonderboard as a second choice and stay away from the others. Elk (a building official) is comfortable with Hardi even though it has paper in it (cellulose), but again...that is encapsulated and he has taken a torch to the stuff and it does not burn. In no case would I use the last one mentioned, since their documentation DOES list a max. temp.
Extensive experience in the field seems to indicate that (at least) Durock and Wonderboard are readily available and work well for this purpose.....so why mess with anything else?
I agree with much of what you say Craig, it makes considerable sense, but I was looking at a combination of what was said and not said… I’ll admit that I didn’t call the Durock tech line, but the link I gave didn’t make me want to - it answered all my questions and made specific mention of the application. The others seemed much more hesitant when I talked to their tech support folks, which I did because I couldn’t find the answers in the online documentation. Once I had the person on the line, I was listening to not only what they had to say, but how “comfortable” they seemed in saying it. Wonderboard seemed like they were OK, but only if you covered their fiberglass. Hardiebacker was totally emphatic that they were NON-combustible, but seemed concerned about their insulating value…
I actually got started on this because of availability - the Home Despot that I went to had Wonderboard and Hardi-backer, but did NOT carry Durock. The Permabase looked like a reasonable substitute, and the aprons said it was what they had switched to instead of Durock, so I purchased two sheets - now I have to find another place to use them…
[quote author="Corie"] Although not a cement board (and I realize that was the scope of your study), could you do the same research for the Micore 300? It seems to be very popular in hearth construction, but I wonder if the manufacturer has the same reservations as those you spoke with.
I might at some point, but I’m currently trying to deal with a backlog of projects, a bunch of which are sort of time sensitive, so this would be fairly far down the stack…
I read on the Hardi site someplace that they recommended NOT using their product for fireplaces. I think it was in the FAQ section.
From the Hardiebacker FAQ section…
Q. Can I use HardieBacker board as a fireplace facing?
A: Yes, HardieBacker board can be used as a fireplace facing. HardieBacker 1/4’’ board can be used as a component in a 1-hour fire resistive construction when installed in compliance with National Evaluation Report NER-405. HardieBacker boards are recognized as non-combustible, when tested according to ASTM E 136.
Looks to me like they think it’s OK… It doesn’t mention using it in clearance reduction shields, but I would think if you can use it for facing, and they tell you it’s non-combustible....
Q. Can I use HardieBacker board as a fireplace facing?
A: Yes, HardieBacker board can be used as a fireplace facing. HardieBacker 1/4’’ board can be used as a component in a 1-hour fire resistive construction when installed in compliance with National Evaluation Report NER-405. HardieBacker boards are recognized as non-combustible, when tested according to ASTM E 136.
Why I support Hardi backed board is ease if use no nasty rough edges smoother surface easier to drill nail and cut than cement board . Till one has used both extensively, it is hard to pass judgment. The wed site do not expound practical experience in applications After a bout 400 squared of hardi siding I feel I have cut and installed enough to place a value judgment
Hundreds of sheets of tile backer cement board installed, I’ll tke hardi backer board anytime. The only reason I suggested hardi backer board is not price it cost more but for a dollar or two the ease of use is worth it.. With keyman as a wittiness I applied my acetylene torch with the cutting edge to the hadri backer product It did not burn did not smoke and very little blackening I made it cherry red and it still stayed intact. I doubt any hearth pad will be subjected to the heat of an acetylene cutting torch. Ever work with cement boards it is nasty material to work with
I agree, this is interesting - I wonder if someone should call them on it? I got the same text using 1/4” board (on the 1/2” board FAQ) from the page on the product itself Here.... I do wish they’d make up their minds…
Why I support Hardi backed board is ease if use no nasty rough edges smoother surface easier to drill nail and cut than cement board . Till one has used both extensively, it is hard to pass judgment. The wed site do not expound practical experience in applications After a bout 400 squared of hardi siding I feel I have cut and installed enough to place a value judgment
Hundreds of sheets of tile backer cement board installed, I’ll tke hardi backer board anytime. The only reason I suggested hardi backer board is not price it cost more but for a dollar or two the ease of use is worth it.. With keyman as a wittiness I applied my acetylene torch with the cutting edge to the hadri backer product It did not burn did not smoke and very little blackening I made it cherry red and it still stayed intact. I doubt any hearth pad will be subjected to the heat of an acetylene cutting torch. Ever work with cement boards it is nasty material to work with
Not arguing the ease of use question - If I was putting it in the bathroom it would be easy to choose the Hardiebacker. But as JTP is pointing out, we have the MANUFACTURER saying that it both is and is NOT approved on their own website. - Where does that leave me as the “inspectee” if I used the stuff in good faith based on the statement I was referencing - and one of your colleagues says “NO” based on the statement that JTP found? Your test with the torch might be impressive, but does it over-rule a manufacturer saying the product isn’t suitable?
On another topic that is related - my post on it seems to have either dissappeared or didn’t get up for some reason - The VC Encore 2550 manual says:
For installations with the heat shield attached, use a non-combustible floor protector such as 1/4” non-asbestos mineral board or equivalent, or 24 gauge sheet metal
I have been trying, w/o much luck to find out EXACTLY what constitutes “1/4” non-asbestos mineral board or equivalent” in terms of either a specific product, or an “R-value” - Do you have a specific reference that would say?
I tried calling VC - they said “Call the dealer” - when I said I didn’t have a dealer, they gave me a local reference.
I called the dealer - they said “Call the Building Inspector” - In Billerica the FD handles stove install inspections, so I called the FD
The FD said they didn’t have a clue, but what I was planning sounded good - but call the building inspector.
The building inspector said he wasn’t sure either, but would buy off on 1/2” Durock if I showed him a web page printout saying it was non-combustible…
That’s about as useful as the Quad manuals that say [3/8” metal clad millboard].
What I take off of the difference in info form their site is that you CAN use the hardibacker as a FACING material backer (because it IS non-combustible). But, “Because heat will transfer through JH products, they cannot be used around a fireplace [as thermal protection].” Make sense to everyone else?
On a side note, HHT recently started shipping white non-comb sheeting materials with fireplaces that have non-combustible zones around the fireplace. This stuff is pretty hard, easy to screw down, should cut easy, and very non combustible. I don’t know if it would offer thermal resistance, and I don’t know how available it is. But it is really nice stuff.
That’s about as useful as the Quad manuals that say [3/8” metal clad millboard].
What I take off of the difference in info form their site is that you CAN use the hardibacker as a FACING material backer (because it IS non-combustible). But, “Because heat will transfer through JH products, they cannot be used around a fireplace [as thermal protection].” Make sense to everyone else?
Sort of, except that they give an R-0.13 rating to their 1/4” material - I haven’t been able to find anything on their 1/2” material, but doubling the 1/4” number gives 0.26, same as 1/2” Durock. Their tech support guy said around 0.15, but sounded like he really didn’t know for certain. However that is about the same range as Wonderboard, and what was given for Permabase, so I would say that Hardibacker is no worse than any other cement board - thus at least in theory it should work as fire protection if Durock would, either by itself in a low R-value requirement situation, or with Micore, etc. as part of a “sandwich” construction.
It should also be fine as clearance reduction material, since it is non-combustible. Everything I’ve read on NFPA specs for clearance reduction shields say that they have to be non-combustible, but don’t care about thermal resistance - note that plain sheet metal (effective R-0.0) is a valid material…
Goose, I know you have to go with what is called out in the specs here, so take this with grain of sand.
I built my hearth out of Durock. 3 layers to be exact. screwed together then tiled with a 3” airspace below.
The top of the hearth get’s pretty warm, but the lower pieces (The underside) are never even luke warm. The heat does not transfer through to the bottom layer.
Also, I use a left over piece of it to block the fire when doing quick cleanup’s of the door. I left it there (attended of course) for about 5 minutes and the fiberglass did turn a little brown, but it was exposed to a rather hot bed of coals. You can guess what that temp was, but I"m sure it was close to 1000 or more perhaps. The Durock was awful hot after that, so I covered it with foil, and now I have yet to see that piece get much more than warm.
The point here is this stuff is VERY tough and fire proof. The company is being conservative and rightly so, as some yahoo would attempt to build a fireplace out of it if they said it was fireproof.
FWIW Department - I just called Hardi tech support again, and spend a fair bit of time trying to show them the problem - The tech agreed that the Fire Prevention FAQ Reference that said it couldn’t be used was poorly worded and needed to be fixed.
His explanation makes some degree of sense, though Elk can probably clarify a bit better - There are apparently two different types of construction standards - Fire Resistive or Fire Rated, and Non-Combustible. Hardibacker is “Non-Combustible” so it can be used in “non-combustible” applications, but can’t be used in fire-resistive because it transfers heat. This is consistent with the next question about substituting Hardibacker for Type X fire-rated gypsum, where they say no because they transfer too much heat - Hardibacker is R-0.19 and Gypsum is R-0.45 for the 1/2” product.
Thus bottom line is that it depends on the language in the stove manuals - if they say “Non-Combustible” then Hardibacker can be used, if they say “Fire Resistive” or “Fire Rated” then they can’t.
I also got a more definitive answer on the R-value rating - He said their nominal 1/2” product is really only 7/16” thick, which is done to save weight over other cement boards, and has an R-value of 0.19
There is a product called Mircore 300 made by USG that has less than one-quarter of the thermal conductivity rating for Wonderboard (or almost five times the r value, that will allow you to make a much thinner hearth board. It seems to be about $75 for a 4x8 sheet. There is another thread somewhere on this site that has a little table that compares the K and R values of a lot of different materials; very useful.
The trouble is that micore is not available in many places. I could not source a single sheet in the whole state of WA. Told by USG that I may have a very hard time finding it at all. Good ratings though.
