R and K values - Concrete Hearth extension on Jotul Kennebec C 450

[oconnor]

I am feeling really stupid right now. I came across some info in my manual that I didn't take in before, that seems to indicate that I am way out of spec on my install when it comes to hearth protection. So, for anyone to whom I have posted that "all you need to do is read the manual", I apologise - Mea Culpa - I have done the same.

I was just reviewing my manual for the Jotul C450, and came across the hearth protection requirements - says "Hearth protection must be noncombustible insulating board, having an R value of 2.92 (0.343 K-value) such as Kaowool® 2600 #15 or the equivalent mortared masonry material." I have a 2" slab poured on top of the joists and original fireplace floor, so under the Kennebec, I have at least 5 inches of cement, but from 6" to 20", I have 2" thick.

According to the table I found here on Hearth.com concrete has an R value of .095. Based on that, I would need 30.73 inches of concrete to meet the R value needs, and that makes no sense to me, but that is what my math tells me.

Can someone point me to some definitive K or R values for sand mix concrete (portland cement and sand), or explain to me how what I have is OK.

What doesn't make sense is that every other Jotul insert manual, and the Jotul stove manuals I looked at, only require the non combustible floor covering vice any specific R value - why would the Kennebec be different?

Am I guilty as charged, or just guilty of over-reading?

 

[badger1968]

I was just reviewing my manual for the Jotul C450, and came across the hearth protection requirements - says “Hearth protection must be noncombustible insulating board, having an R value of 2.92 (0.343 K-value) such as Kaowool® 2600 #15 or the equivalent mortared masonry material.” I have a 2” slab poured on top of the joists and original fireplace floor, so under the Kennebec, I have at least 5 inches of cement, but from 6” to 20”, I have 2” thick.

According to the table I found here on Hearth.com concrete has an R value of .095. Based on that, I would need 30.73 inches of concrete to meet the R value needs, and that makes no sense to me, but that is what my math tells me.

Can someone point me to some definitive K or R values for sand mix concrete (portland cement and sand), or explain to me how what I have is OK.

What doesn’t make sense is that every other Jotul insert manual, and the Jotul stove manuals I looked at, only require the non combustible floor covering vice any specific R value - why would the Kennebec be different?

Am I guilty as charged, or just guilty of over-reading?

Several questions . . .
What is the required hearth pad dimension?
Do you have to have the same R-value throughout? Often you need a higher R-value directly under the stove.
Is the stove itself completely situated over the old fireplace floor?
Does said floor extend completely to the ground?

Your concrete mix may qualify as "cement mortar" which has an R-value of .2 per inch. But, I can't say that definitively.

In any case, if the stove itself is completely over non-combustibles extending to the earth, then you are in pretty good shape. Make sure you have to have 2.92 over the entire hearthpad dimension. If so, you may have to do some work. Micore, at 1.1 per 1/2 inch will get you there quickly. That's what I used for my heartpad (alternating w/ Durock) to achieve a ridiculously high R-value like yours (2.5).

 

[oconnor]

Several questions . . .
What is the required hearth pad dimension?
Do you have to have the same R-value throughout? Often you need a higher R-value directly under the stove.
Is the stove itself completely situated over the old fireplace floor?
Does said floor extend completely to the ground?

Your concrete mix may qualify as "cement mortar" which has an R-value of .2 per inch. But, I can't say that definitively.

In any case, if the stove itself is completely over non-combustibles extending to the earth, then you are in pretty good shape. Make sure you have to have 2.92 over the entire hearthpad dimension. If so, you may have to do some work. Micore, at 1.1 per 1/2 inch will get you there quickly. That's what I used for my heartpad (alternating w/ Durock) to achieve a ridiculously high R-value like yours (2.5).

Pad dimensions - extend 18 inches in front of stove glass

R value question - unfortunately, the manual just gives the one R value for the required pad - but I agree that the florr under the insert should be where the highest value is needed, and a progressively lower value extending out from there, but they don't specify that in the manual.

Stove position - half over old fireplace floor, half over original hearth. Fireplace is on main floor above the basement - the original hearth extension is a poured concrete wedge that starts at the masonary base of the fireplace/chimney, and runs out 16 inches until it buts against a cross beam (header) spanning between joists. That point (roughly 14 inches in front of the stove glass) is were the first combustible comes into play inside the hearth extension - the cross beam header.

From that point 14 inches out, for a further 6 inches, the floor is the mix described, roughly 1.5 to 2 inches thick, poured on top of the joists, which were boxed in from below to create a form. The form sections can't be removed as they are secured to a steel mesh that was laid first before the top layer was poured.

You are right about the ridiculous nature of the R value, and I have yet to ever get a call back from Jotul on the first question I asked them about liner downsizing, so I doubt I will get any call back when I ask for detailed info on getting clarification on what decreasing R value I can apply to my hearth.

I am fairly certain that in reality I have not got a problem in the safety realm, but I seem to have a problem in the regulatory realm, and it seems inconsistant with other Jotul products.

I would love some more input if it is out there.

 

[begreen]

That is a stiff requirement. The difference between regular stoves and an insert is that there is no ashpan, bottom heatshield or air gap created by the stove being up on legs. The insert sits right on the hearth. I agree, it's hard to imagine the radiant heat being quite so high, but you have to go by the manual. This is what they tested to.

What does your dealer and inspector say? Insurance co?

 

[badger1968]

Pad dimensions - extend 18 inches in front of stove glass

OK. Yep, that's pretty standard--what insurance Cos. are usually looking for.

R value question - unfortunately, the manual just gives the one R value for the required pad

That's unfortunate. And doesn't seem right. But if that's what it says, you need R=2.92 18" from the glass--which is really tough. I can only speculate that the C450 really throw heat forward (my stove does this).

Stove position - half over old fireplace floor, half over original hearth. Fireplace is on main floor above the basement - the original hearth extension is a poured concrete wedge that starts at the masonary base of the fireplace/chimney, and runs out 16 inches until it buts against a cross beam (header) spanning between joists. That point (roughly 14 inches in front of the stove glass) is were the first combustible comes into play inside the hearth extension - the cross beam header.

From that point 14 inches out, for a further 6 inches, the floor is the mix described, roughly 1.5 to 2 inches thick, poured on top of the joists, which were boxed in from below to create a form. The form sections can’t be removed as they are secured to a steel mesh that was laid first before the top layer was poured.

Yep, that's a lot like my hearth situation. Although my extension is brick. Your problem begins at that 14" point where you only have 1.5 - 2 inches of concrete over combustibles.

I am fairly certain that in reality I have not got a problem in the safety realm, but I seem to have a problem in the regulatory realm, and it seems inconsistant with other Jotul products.

It sounds like a safe setup to me, but I'm not qualified to make that judgement. As you point out the crux of the matter is the regulatory realm. Will this be acceptable to your insurance Co.? Your local fire dept./codes? Best thing to do is have them come take a look. The only other alternative is adding additional protection at 14" out. Three sheets of 1/2" Micore (3.3) topped with 1/2" Durock (.2) then tile should do the job. Or you could go 2 sheets 1/2" Micore (2.2) + 1/8" Still Air (.92) + 1/2" Durock (.2) then tile--but that uses steel gapping that I don't know how to construct. But, it's burning season, so I'm not sure if you have that option.

I would love some more input if it is out there.

Jotul owners . . . give this guy some advice! :cheese:

Good luck Brent.