Ok 5/16 vs. 1/2 Durock

[jeffman3]

OK I think I may have blown it. I want to get your opinion before I pull my hair out. I am building a hearth for a Homestead Tribute. (Owners manual states that A R factor of .6 is what is required. So I go the lumber yard for materials, and they have 3'x5' 1/2" 48"x48" 5/16" Durock. My hearth is 48"x48" so I get 5 sheets of 48"x48" 5/16". I know that 1/2 inch Durock cement board has an R factor of .26(USG SA932 09305 cement board systems catalog). So I am thinking that hey, This is great. I can get better R factor rating with out spending any more. 5 sheets of the 48"x48" is 1.56 inches thick vs 1.5 without all the cutting or taping. So I buy 5 sheets of the thinner board. (only have to break off a corner with this stuff, and the price is the same, less waste!)I would have needed 5 sheets of the 3'x5' 1/2" (to make three layers) in order to get an R factor of .78. and then I have all the cutting. You see where I was going? OK, so I am just going over things last night and discover that USG doesn't list an R factor listing for the 5/16 boards. So now I am wondering do I finish this hearth pad or not now? Do I need to go buy another $70 dollar's worth of boards, or is my plan OK to use? what do you think? Has anyone else done this?

 

[webbie]

It is the same material and therefore should have the same R and K values - given the thickness. It would be similar, in my opinion, to buying 1/2 bricks and then using 2 to get the value of one.

In reality it might even be better than that, because each board provide a bit of a resistance to heat, so a bunch of thin layers may be a tiny bit better than one thick one.

I would rest easy.

 

[jeffman3]

Craig,
Thank you I feel much better now. That goes with my thinking but I am new to wood stoves and want to make sure I do this right. I really don't want to try to take this thing apart and rebuild it.
(I can't afford that, time or money). I have another question about thin set. I have a bag of standerd thin set morter(for setting tile on cement floors). I have read some about a latex fortified thin set. Will what I have work, or do I need to get a bag of this other material?

 

[webbie]

Your regular thin set should be fine - sometimes there is latex in the mix anyway, and you can also buy liquid latex to mix in with it (instead of some of the water)........ the idea is (I think) it adds some flexibility to the stuff, and perhaps more water resistance.

You can usually pick that up just about anywhere - even at many paint stores or hardware stores. Probably overkill for a small job and a hearth. Thin set, as it is, is just about the stickiest stuff on earth! Never try to remove a tile after that stuff has set.......

Also, dampening the cement board and tile bottoms will always help when installing.....

 

[jeffman3]

Thank so much for putting my mind, and my wife's mind, rest. I feel better telling her that I have some good advise, from people that know, on issues that I am ignorant about.