hearth build

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Cueman

New Member
Feb 7, 2011
39
RI
I'm fastening my cement board to the floor over the Micore300. Has anyone heard of the screws compromising the integrity of the Micore's insulating value where the screws penetrate to the sub-floor? They are coated metal screws for cement board, but could they conduct enough heat down to the wood to cause a problem?
My instinct is the answer is NO, and there would not be enough heat down to the point of the screw to endanger the sub flooring. But metal is a great conductor of heat, and the screws will take whatever heat comes through the stone and mortar I suppose. I could NOT screw the board down directly under the stove footprint as an option, but I'd weigh that with the need to have the board tidy and snug for my tile work.

Thanks.
 
My first gut thought is...............unless you are conveying 450ºF thru those screws, you won't have spontaneous combustion (or so I'm told). Do you really think you're getting temps (I personally doubt it, since the floor beneath my stove is surprisingly cool), THAT high? I'm gonna park in the camp of "I don't believe you are doing any damage" on this one.

-Soupy1957
 
If you can avoid using screws you should. The hearth R-value rating is because of heat transfer. Screws will transfer heat directly to the sub floor. Instead of screws I used Henry floor adhesive (http://www.wwhenry.com/) to keep the cement board together and in place.
 
This is the first time I have heard of adhesive recommended in this circumstance. Tom, has this been tested? I would think the fibrous nature of micore would prevent bonding to anything but the top skin of the micore.

The amount of heat transferred via the screws is trivial. My understanding is that you need is a solid, stable sandwich. USG recommends screwing down cement board for a solid tiling base.
 
With your comment I went out and took a look at the product I used it was not Herny (sorry for the misrepresentation) it was the heavy duty liquidnail which is non-flammable when dry. Has it been tested on a hearth application. I don't think so. Granted the the likely hood to transfer enough heat through a screw to start a fire in unlikely. When I build my hearth I decided to sandwich Cement Board with non flammable glue.
 
I haven't checked this for awhile, but the last time I did, adhesives are not recommended for high heat applications like a hearth or wall shield. Liquid Nails heavy duty has a top service temperature of 140F. A hearth can get hotter than that. Some adhesives remain flammable and others change under high heat.

I like Liquid Nails Heavy Duty Construction adhesive, but according to the mfg:
NOT Recommended For: flooring applications, subfloor underlayment, ceramics, mirrors, granite, marble, plastic, vinyl or ceiling tile.
 
Thanks for the input everyone. Here's my plan. I've drawn the footprint of the stove's exact location on the rock board. This in theory is the place with the most heat transfer. I will then screw down the rock from the perimeter, working in toward the stove. I will leave the footprint and about 6" wider unscrewed. The weight of the stove will snug the materials down to the floor joists. And this area is nominal considering everywhere else will have screws about every 8". I used the same approach for the wall protection, but I'm less concerned there because I'm maintaining clearances for unprotected wall, because I can.

Last night I was by a VC Defiant full throttle stove fire set on a field stone hearth. I felt the stone directly under the stove, and they were indeed relatively cool. You could keep your hand there if you wanted (but the rest of you would get hot quick). Bottom line is that heat rises, most stoves have a bottom heat shield, and the idea that your subfloor could possibly smolder from hot screws is ludicrous. I think all of the floor protection recommendations from the manufacturers and local building codes are extremely cautious (that is a good thing with fire). I want to do a lab test and set a stove on plush carpet and see how long the heat transfer in the legs of the stove take to reach the flash point of the carpet. I'm sure its not a matter of minutes, but hours or days of hot stoked stove (if at all) And while I'm at it, why not saw dust, or news paper, or a half full can of gas!

So I've talked myself out of worry on the scews. I did not use adhesive on purpose for the VOC concerns and its messy and will end up on my carpet for certain.

With all the concern about floor protection, its clear to me that the most important part of the stove is the pipe work and chimney. Yearly maintenance and proper burning practices and common sense around a flame are the factors most likely to determine if you have a fire in your stove, or God forbid have one in your house!
 
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