Acheiving a hearth with sufficient protection..

[Matt in ND]

Hey all,

I am trying to wrap my head around hearth and combustible clearance dimensions. My wife isn't very excited about the pre-made hearth pads, so we're looking into alternatives.

Here is what the install guide for the QF Isle Royale says:

In US installations, it is necessary to install a floor protector of 1 inch ( " k " value= .45 ) thick metal clad millboard or equivalent

I am a bit confused by this. I've read up on K values and R values and this seems like a strange way to specify things, because the k value is normalized to 1" thickness already. Do they want 1 inch of material with an aggregate K of .45? Would they be just as happy with 1/2" of material with a k value of 0.20? Would they like 10 inches of material with a uniform K of 4.5? Do they need 1" of minimum thickness no matter how low of a K value we achieve?

According to what I've seen, putting a 2" slab of solid granite down would not be anywhere close to meeting the requirements, because granite has K values in the 2 - 3 range...and even dividing those conductance values by 2 (2" of material), we're still nowhere in the ballpark of 0.45...

Am I understanding things correctly?

The install location will be over carpet, over a floor that bounces noticeably when my kids jump. So I'm also concerned about putting 500lbs of rock under 450 lbs of stove..

Thanks,
Matt

 

[Grisu]

Hey all,

I am trying to wrap my head around hearth and combustible clearance dimensions. My wife isn't very excited about the pre-made hearth pads, so we're looking into alternatives.

Depends how much you want to spend. Have you looked at some of those here: http://www.americanpanelhearth.com/products (Just an example, there are other manufacturers who can also make custom boards).

Here is what the install guide for the QF Isle Royale says:

I am a bit confused by this. I've read up on K values and R values and this seems like a strange way to specify things, because the k value is normalized to 1" thickness already. Do they want 1 inch of material with an aggregate K of .45? Would they be just as happy with 1/2" of material with a k value of 0.20? Would they like 10 inches of material with a uniform K of 4.5? Do they need 1" of minimum thickness no matter how low of a K value we achieve?

According to what I've seen, putting a 2" slab of solid granite down would not be anywhere close to meeting the requirements, because granite has K values in the 2 - 3 range...and even dividing those conductance values by 2 (2" of material), we're still nowhere in the ballpark of 0.45...

Am I understanding things correctly?

The install location will be over carpet, over a floor that bounces noticeably when my kids jump. So I'm also concerned about putting 500lbs of rock under 450 lbs of stove..

Thanks,
Matt

The Isle Royale has a very hefty requirement. You should go by the k-value and even with mineral board you will look at at least 1" of thickness. Here is a list of common hearth materials:
https://chimneysweeponline.com/horvalue.htm
A k-value of 0.45 means a r-value of somewhat above 2. See here: https://www.hearth.com/econtent/index.php/articles/k_values_what_does_it_all_mean
There should be a formula in the IR manual to convert the two.

 

[begreen]

That's odd, I thought the IR has an ember protection only hearth requirement. Are you looking at the old manual (2006)?

This is from the current manual:
FLOOR PROTECTION: Floor protector must be noncombustible material, extending beneath heater and to the front, sides and rear as indicated. The floor must be noncombustible or otherwise adequately protected from radiant heat given off by the unit and from sparks and falling embers. A layer of thin brick or ceramic tile over a combustible floor is not sufficient.

There is no harm however from exceeding the requirement. Yoder Hearth Classics makes premade pads that will be fine for this application or you can make your own. I would suggest adding a support column or two to the floor if it is poorly supported.

 

[Grisu]

BeGreen, did you also read the next paragraph? http://hearthnhome.com/downloads/installManuals/250_5763.pdf

"In US installations, it is necessary to install a floor protector of 1 inch ( " k " value= .45 ) thick metal clad millboard or equivalent at least 16 inches ( 4 0 6 mm) in front of glass and 8 inches ( 2 0 3 mm) to both sides of the fuel loading door. Open the door and measure 8 inches ( 2 0 3 mm) from the side edge of the opening in the face of the appliance."

 

[Matt in ND]

I was consulting this document:

http://hearthnhome.com/downloads/installManuals/250_5763.pdf (page 11), dated march 2013

from here: http://www.quadrafire.com/Products/Isle-Royale-Wood-Stove.aspx?page=Downloads

 

[begreen]

Thanks for that update Matt. Looks like they have a third printing. Mine is dated 2011. Looks like someone made a serious omission in that year.

You could set the granite on a layer of 1" Micore and that would meet the requirements.

 

[Matt in ND]

Ok, so the bottom line seems to be that I need 1" of the engineered stuff, having a k-value of 0.45 or less (e.g. Milcore 300 is 0.43, I think), and anything I put on top of that is not going to meaningfully contribute to the heat protection. So for instance, I could lay a 1" chunk of contiguous or grouted countertop granite (for looks, hardness, and non-combustability) on top of 1" of engineered board (for heat resistance) and be in good shape?

For the pre-made hearth pads, do they specify their R value, or k-val + thickness? It appears that the Isle Royale needs an R value of 1/0.45 or around R=2.22...

 

[begreen]

Essentially yes that is correct. Confounding factor is the carpet. I would remove the carpet from that area and if necessary lay down a sheet of plywood or metal of sufficient gauge to provide a known, flat and level surface to build on.

Yoder Hearth Classics's standard type 2 thermal protection is R= 1.61. By adding a thermashield base its protection goes up to R=3.69.
http://www.hearthclassics.com/