R-Value of Building a Hearth

amkazen · Jan 27, 2008

HI, I am going to install my recently purchased Englander 12-FP stove over this next week. The first thing to do is to build a hearth and I would like your comments. So, my 2 questions are: did I calculate the R factor correctly and would you remove or leave the laminate flooring and laminate flooring sound deadening underlayment?

The floor is Columbia Colonial Clic laminate flooring and from what I can find I believe I would be safe to use an R value of .5. The laminate flooring sits on top of a laminate flooring sound deadening underlayment that sits on top of 2" of gyp-crete. I have purchased metal studs that I plan on "Liquid Nailing" to the gypcrete as I cannot screw the metal studs to the gyp-crete as there are radiant heat tubes in the gypcrete.

I am considering cutting the laminate flooring our in the shape of the hearth as I am not 100% sure of the R-rating of.5

I plan on putting concrete in between the metal studs and concrete has an R value of .92 per inch. I believe the metal studs are 1 5/8" thick so 1 1/2" of concrete would have a minimum R-value of .92 + (.92/2) = 1.38.

I plan on using 2 layers of 1/2" Wonderboard so that would be an R-value of .2 + .2 = .4.

I plan on the top surface of the hearth being 19" x 20" hearth stone (Cultured Stone) manufactered by Owens Corning and it has an R value of .355 per inch so this would be a total of .355.

Gyp-crete has a K value ranging from 4.75 Btu/(h · ft² · ºF) (.6840 W/[m · ºC]) to 5.15 Btu/(h·ft²·ºF) (.7416 W/[m·ºC]). This equates to a R-value range of 0.19 - .21 per inch. This equates to .19 x 2 to .21 x 2 = .39 - .42

Adding it all together I come up with .5 + 1.38 + .4 + .355 + .39 = 3.025. If I remove the laminate flooring I get 2.53.

The Englander manual says I need a minimum of R factor of no less than 2.

webbie · Jan 27, 2008

concrete has .92 per inch? That sounds high.

I show it is less than 1/10 of that.

I think our fellow board member, the Chimney sweep, has it wrong on this page:
http://www.chimneysweeponline.com/horvalue.htm
(again, that is wrong!)

Notice that an entire concrete block - 8" - with an air space inside is only R=1

I assume the laminate floor covering is rated combustible? If so, you are probably best off starting on top of it.

Here are some suggestions that I would think will suffice:

Remember, AIR is one of the best insulators, being about R=1 for each inch.
Also, Micore - or similar mineral board or even rock wood (available in many drywall supply houses) also has a good value.

So, a hearth consisting of:
The studs glued to a cement board which is glued to the floor
(note: an epoxy adhesive might be the best)
Then 2 or 3 layers of cement board on top - for rigidity as much as insulation in this case.
Then your stone

that would seem to beat the specs by a long shot

If you wanted to add even more to the value:
EITHER - a piece of sheet metal spanning 1/2 way up on the studs - in other words, screwed to the studs as in enclosed diagram
OR, a 1/2" of Micore laying against the floor

thechimneysweep · Jan 28, 2008

Craig is right, our chart was wrong. I've fixed the chart on our website, and E-mailed the manufacturer we got the chart from with a heads-up to fix theirs.

Thanks to Craig for the PM this morning about this, I had skipped over this post, and wouldn't have caught the booboo.

woodburn · Jan 29, 2008

I recently did the same thing. I cut the laminate floor away. I didn't like the idea of all that expansion and contraction taking place under my stove. Not sure it would really matter, but remember- any structure is only as strong/stable as its base!

By the way, not to change the subject, but I have the Columbia Causual Clic laminate floor. Does yours scratch riduculously easily? This floor is pathetic.

Gooserider · Feb 6, 2008

I just caught this thread, looks like someone went in and fixed the chart already, thanks...

I think it or a couple of other values may still be suspect - I *KNOW* the value of 0.26 for 1/2" Durock is accurate, that comes straight from their literature. So why is cement MORTAR 0.1 per half inch, and concrete less than .1 for an entire inch? OTOH, the concrete block number seems ridiculously low, less than bricks or just about anything else on an "insulation per inch" basis...

I would like to go through the entire chart and see if we can get better values for some of these materials - or perhaps ask someone with (we hope) access to the reference books to do it - I'm going to send Corie a PM about this.

Gooserider

amkazen · Feb 8, 2008

0.26 cement board
0.26 cement board
0.26 cement board
0.355 hearth stone
0.355 hearth stone
1.25 air
2.74

Hi,

Attached is a picture of the beginning of my hearth construction. Thanks for the help in getting accurate R values. Above is the total R value for my hearth. I was going to fill concrete in between the studs but doing so drops my R value down to 1.61 due to the difference in R value between concrete and air.

I will post more pics as I progress.

And, I agree, I do not like my laminate floor very much. I have not had the issue of scratches but don't ever get a liquid on it and let it sit for any length of time. I also have not found a decent cleaner for it that does not leave streaks. I have friends who have laminate floors and they love it so perhaps it is the brand, or maybe it is the level (entry level) of floor, and a more expensive one will be better.

Gooserider · Feb 8, 2008

Looks like a good beginning - from the way it looks, I'm guessing you cut out the laminate underneath? It is probably best if you do.

Also why two layers of hearthstone?

One other thing that I would do, it doesn't directly increase the R-value, but IMHO it's a good idea, is to put a layer of thin sheet metal somewhere in the sandwich - when I built my extension I used aluminum roof flashing - I would probably put it on top of the studs under the three layers of Durock. It will act as a heat spreader, and help dissipate any heat that makes it through the pad so that there won't be any "hot spots". I don't know that it is essential, but I think it's a good thing, and it adds minimal cost or work to the project.

Gooserider

amkazen · Feb 9, 2008

Hi,

I think you have a good idea about the sheet metal. I'll see if I can find some tomorrow. The three layers of cement board is just to get to the height I need: the cultured stone I am using on the side (face?) of the hearth is 4 1/2" high so the three layers is just about perfect for the hearth stone to sit on top of the side stone.

Yes, I got to thinking about the laminate floor and felt it was best to remove it.

Tomorrow is the day to get the stones set!!

Gooserider · Feb 9, 2008

Sounds good, keep us posted...

Gooserider

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R-Value of Building a Hearth

amkazen

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