Double check my r value math

[rwhite]

Here's the deal, I have found and Avalon 996 "extended" insert for $500. I finally found a manual online and it calls for 18" of floor protection and no r value is mentioned. Beings that this is an older stove I worried that specifications may have gotten more stringent since 1990. So I looked at a manual for a Avalon Rainier. It calls for 1/4" noncombustible floor protection with a k value of .72. If my math is correct this equates to an r value of .33 (.25 (thickness)/.72 = .328) Does this look correct? I just worry that specs on the 996 may be out of date and I should build floor protecttion that is a better than called for.

 

[Battenkiller]

The calculations look good to me, but I'd go with something with a lower K-value like Micore 300 (K=0.49)

http://www.usg.com/rc/data-submitta...ineral-fiber-board-300-submittal-en-IW803.pdf

1/2" Micore would triple your R-value. A little overkill is best when you don't have the true clearances.

 

[rwhite]

Overkill would be great but I cannot seem to find micore anywhere in the Boise area. My options are seeming to be limited to hardibacker or wonderboard.

 

[begreen]

Tough call, there are a lot of stoves out there that only require ember protection. And yet Murphy's law will probably mean that the stove you want in the future needs more than that. If you go with a base of plywood covered by 1/2" Durock NextGen and tile you will be at about R=.40 and well covered for this stove. If you want better coverage, double or triple the layers of Durock using longer screws to hold together the sandwich. Each layer will add R=.39.

 

[begreen]

Overkill would be great but I cannot seem to find micore anywhere in the Boise area. My options are seeming to be limited to hardibacker or wonderboard.

Wonderboard is very good, but it has a lower R value of R = .26 for a 1/2" sheet.

 

[rwhite]

Thanks guys. I do want to exceed the recommmeded if possible. Weight is not my big issue as much as thickness as my hearth is flush with firebox. I have plenty of vertical room in the firebox now that I will likely raise the insert (set it up on pavers maybe) a few inches. As far as the current hearth it is 18" wide. Of that 4" out from hearth is solid masonry that extends all the way down to ground level in the basement. The other 14" is setting on what appears to be 2" thick masonry substrate. So I figure if I make a hearth extension that exceeds the recommended r value I am adding some security on top of the existing hearth.

Sorry for so many questions on this install I just want to make sure that it's done right. So there may be some more to come.

 

[BlankBlankBlank]

Tough call, there are a lot of stoves out there that only require ember protection. And yet Murphy's law will probably mean that the stove you want in the future needs more than that. If you go with a base of plywood covered by 1/2" Durock NextGen and tile you will be at about R=.40 and well covered for this stove. If you want better coverage, double or triple the layers of Durock using longer screws to hold together the sandwich. Each layer will add R=.39.

BeGreen, I am wondering about the screws to be used. Durock screws come in three lengths - 1 1/4", 1 5/8", and 2 1/4". If going with three layers of Durock, totaling 1 1/2", then after thinset the screws for the top layer would penetrate through 1 1/2" of Durock and at best 1/2" into the subfloor below. How solid will that top layer be anchored? Wouldn't one want the screw for the top layer to penetrate deeper into the subfloor? (I assume that each layer would be set in thinset mortar and then screwed.)