Heathstone Homestead hearth construction

[spangler]

I'm trying to place a Hearthstone Homestead, with 6" legs and bottom heat-shield, in front of an existing masonry fireplace. I have the hearth extension pad pretty much worked out (in my head), using Micore, cementboard, and slate, to get the required r-value of 2.5 out 18" in front of stove -- and raises pad-level 4" to match the existing fireplace hearth level.

The problem is, the front edge of the existing fireplace hearth is > a 4" level of brick (above floor-level) on top of 6" of concrete (below floor-level)< -- sitting on the wood floor-joists in basement below -- which, from what I read in HearthWiki, has an R-value of only 1.37. This would be directly under the belly of the stove. There's only 2" clearance in hearth above connecting stove-pipe to play with, and wondering the simplest way to fix the issue.

I'm wondering if I can place the stove on a (1/2"-?) plate-steel platform -- up 2", and out 8" each side...? When comparing the requirements of the two models of Homesteads -- the model with the 4" legs and no bottom heat-shield needing R-6.6 (!) , compared to the 6" legs w/heat-shield at R-2.5 -- a difference of more than R-2 per inch...

Wouldn't my putting essentially another steel baffle and 1.5" of airspace under the stove reduce the R-needed by a similar amount??

btw - HearthWiki lists 1" of "ventilated air-space" at R-1.43... but, does this mean only vertical airspace?

Thanks for any thoughts.

 

[thinkxingu]

Hello There,
I'm not sure why, but most people miss this, right from the manual:

Hearth Requirements and Floor Protection
Install your Hearth Mount stove with 4-inch legs on one of the following:

A noncombustible floor, such as a slab, cement, or stone hearth. (A noncombustible floor will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or the anticipated heat from your stove.)

or

A floor protector with an R-value of 6.6 or more that you obtain from your dealer. (A floor protector is any noncombustible on the floor underneath and extending to the front, sides, and rear of the wood stove.)

You only need R6.6 if you do not have a "noncombustible floor" to put it on. Your brick and concrete are both noncombustible.

S

 

[Highbeam]

I don't know about that thinkx, the slab must rest on earth and not wood to be classified as a noncombustible floor. Otherwise it is no different from setting a 1/2" thick bed of mortar on my oak floor and calling it a noncombustible. Since there is wood beneath the concrete then you have to consider the R-value of the concrete.

Hearthstone allows the use of "dead air space" in their manual for hearth construction. Using the dead air space is not as efficient as micore but is very effective. The manual allows so much R-value per 1/8" of dead air space and it is stackable meaning you can have inches of dead air space to get the required R-value.

Using the owner's manual will always be better than a hearth wiki or some other source when dealing with your inspector or insurance agent.

 

[spangler]

Thanks for the replies.
Highbeam sees my point, I think.
This still brings me back to some questions about hearth construction, and the Hearthstone Homestead in particular:

Again -- as apparently shown by Hearthstone's own manual -- if by taking the exact same stove, lengthening the legs 2" and installing a thin piece of sheet-metal (the manufacturer's heat-shield) underneath, it reduces the required R-value from 6.6 to 2.5...!!... why would not supporting the whole stove on a non-combustible platform (say, steel-plate) above another ventilated air-space of 2" reduce the required R-value by some similar factor??

Also -- besides that it would lead to some accumulation of dust-bunnies underneath -- couldn't a 1" horizontal *vented* airspace be more effective than a 1" *dead* airspace... (using HearthWiki's R-1.43 vs. R-.97, for what it's worth)...? I would think a vented horizontal space, with cooler air allowed to circulate, would be better than a space where the air is contained and allowed to accumulate heat.

Thanks again for any feedback.
spangler

 

[thinkxingu]

Highbeam, I know what you mean, too, except for the fact that the stove is designed to be a "hearth-mount" which, by definition, means it will be mounted on a hearth. What are standard hearths made of besides brick, mortar, concrete, etc.?

S

 

[Highbeam]

The hearth that I removed was made of brick and mortar setting on top of a concrete footing on earth. If you click on my oak pictures from my sig you can see what's left of it in the crawlspace. Building a hearth above/on wood requires that attention be paid to the materials between the hearth and that wood if your stove sends mucho heat towards the hearth which it appears the homestead does.

The steel plate elevated above the hearth and then wrapped with some sort of trim to create a "dead air space" would certainly get you a bunch of R-value. The ventilated air space seems like it would work better but is not in the owner's manual so I wouldn't be able to justify it to the inspector. The idea of the dead airspace is that it provides an insulative barrier just like the trapped air bubbles in closed cell foam.

I grew to hate the R-value requirements of the heritage when I built my hearth. I consider the requirement to be a negative when considering new stoves.

 

[Dakotas Dad]

I am currently mid build/install with this same stove, with the hearth mount 4" legs. I am over a wood sub floor. I built a "plinth" box framed with steel 2x4's, then insulated with 3.5 inches of rock wool, then two sheets of wonderboard which will give us an "r" value near double the manuals specs. My install is replacing a pre-fab fireplace with a brick chase.

My guess would be that if you can show that your built hearth would attain the proper "r" value to an inspector you should be ok.. the manual states you can build it many ways, just with all non combustibles as long as you reach the "r" value.. note rock wool has an "r" value of 3.12 per inch, if you filled that 1 inch with rock wool, and if you could sqeeze out an inch and a half, you would be very close...

 

[spangler]

Thanks D-D, - Did you use the wonderboard as the top layer of your box?

 

[Dakotas Dad]

Thanks D-D, - Did you use the wonderboard as the top layer of your box?

Yes, two layers, mortared between. I have some pictures on the camera, will post one soon as she moves them to 'puter... make sure you get regular wonderboard, some of the stuff now has styrofoam beads or something to make it lighter and easier to work with, and now is not rated for the heat use we need.

 

[Dakotas Dad]

OK, trying to get pics to load from photobucket, but it keeps giving a "moved or deleted" error.. not sure what that is all about, so just go here and browse.

http://tinyurl.com/ylye7l5

 

[spangler]

Thanks for the pictures.
I guess I'm not too clear on the the steel 2x4's -- I see you put insulation between them, but aren't the 2x4's themselves conducting heat to the combustibles below?

Also, curious as to whether you will have your Homestead out into the room? - or with that iron backside tucked into the hearth...?

I wish I could set mine into the hearth (both for space and looks) but am guessing that a fair amount of heat is radiated off that backside, and don't want that locked behind a surround.

 

[Dakotas Dad]

Yes, the steel will conduct some heat, but there is not enough mass for it to move enough heat past the insulation barrier to matter. ie. as the heat travels through the steel it will meet cool traveling up from below (talks with two engineer buddies 1 ME, 1 CE, both with Master's and 20 years experience) tells me it (heat transfer) will be insignificant in this case. And, as one of them said, I meet the 6.6"r" / non-combustible requirement about halfway through the hearth, everything else is outside the requirement zone.

Yes our stove will be partly in the brick chase, only pretty stone part out in front. We are not using a surround, but a heat shield. You can place the back end into the firebox without a surround. What the surround and or heat shield do is reduce your clearances to the side and rear. and more importantly for us, allow the use of the blower kit.

In our case, we have no combustibles to the rear or most of the sides, just the brick chase you see. We will be building a faux firebox back there, out of, you guessed it, steel 2x4's and wonderboard, with insulation, unfaced fiberglass bat. In the front wall we do have the wood 2x's from the original wall construction, with heat shield and double wall pipe they will be about 1/4" to close, but will have no less then 2 layers of 20g steel, 1" air gap, 2 layers of 1/2" wonderboard, r-13 unfaced fiberglass bat and tile between them and the stove.. We are comfortable with that. The area above the stove will be framed in with.. yep more steel and wonderboard, and more unfaced bats. The front side walls would not even be an issue if we moved the stove 2" farther into the room, but we don't want that. We are going for some aesthetics here besides the heat. We want it to look like an old craftsman style fireplace, with a wood stove set in it.. the tiling and size of the "firebox" drive the stove placement as much as anything.

The owners manual for this deal can be very confusing. But if you have a masonry fire place, and want to do a partial in the box install, look at page 11 of your manual, it shows your min clearances for an install with 4" legs without a surround, and on page 15 the same with 6" legs.. The "plumbing" part is the same.

Do you have any pics of what you are starting with? Might help to see..

 

[spangler]

Hm... Interesting. It reminds me that I have wondered when I read about people screwing their materials to the sub-floor -- thinking that those screws would be transmitters for heat...? I was thinking not to use any screws, but hoping that the weight of the stove and slate would keep everything in place (along with a wood framework around the raised hearth.) I flunked all my science classes, and hope to believe your engineer friends, but still, am kind of skeptical about the steel supports... I would be glad to consider being able to use them to support the slate, but now wondering if they would still be a risk in my 4", R-2.5 hearth extension.

I'm not planning to use a blower, but do wonder about an outside air kit at some point -- though I haven't seen any information on how/where those attach to the Homestead -- have you?

I don't have a camera, but may try to borrow and post pics down the road...

 

[dstapl]

Like Spangler, I want the same Homestead stove with 6" legs installed (needs lower R-factor on the hearth than 4" legs) in front of an existing fireplace. My main problem is an existing 2.5" thick hearth (either a slab of concrete or rock, R~0.5) laid over a hardwood floor (the builder cut lots of corners) - it needs to be R-2.5 for the stove.
I have room to add about 1.5' of height to the hearth so my plan is to use two layers of USG Micore (@ R-1.2 per 1/2 inch) with two more layers of DUROCK above (@ R-0.2), topped with ceramic tile for looks.
My questions are:
1. Can I stack the DUROCK on the Micore and still lay a durable course of tile on top? Other forums seem to say so but the Micore has a somewhat flexible texture because its fiber board. I've found Micore described in this forum and elsewhere as a "ceramic board" (like DUROCK, etc.) but the mfgr (US Gypsum) calls it "Mineral Fiber Board" and descriptions point to a softer consistency.
2. Can I place a 450-500 lb wood stove on four legs on this 'sandwich' (120 lbs per leg) and expect the whole thing to work without breaking the tiles or compressing the layers?

HELP! please

 

[Dakotas Dad]

Comment one.. If I was as unsure about my builder as you sound, I would remove what he had put down and start from scratch, so I KNEW what was down there.
Comment two.. if you use duroc, make sure it is NOT "NextGen" this has styrofoam beads in it to make it lighter for contractors to deal with, but it is not approved for use in a hearth situation, because it is NOT non-combustible anymore.


My questions are:
1. Can I stack the DUROCK on the Micore and still lay a durable course of tile on top? Other forums seem to say so but the Micore has a somewhat flexible texture because its fiber board. I've found Micore described in this forum and elsewhere as a "ceramic board" (like DUROCK, etc.) but the mfgr (US Gypsum) calls it "Mineral Fiber Board" and descriptions point to a softer consistency.

My experience from messing with wonderboard is that if you build it with two layers, with mesh/morter between, the top layer will be stiff enough to support your tile without flexing. The micore may compress some, but it's the flex of the substrate that can damage tiles, and the bonded/mesh sandwich is VERY stiff. You could use steel 2x's placed "flat" insulated between/inside with non batted fiberglass mat for an r of 6 something, and then your 2 layers of backerboard. and be just about 2 1/2 inches.


2. Can I place a 450-500 lb wood stove on four legs on this 'sandwich' (120 lbs per leg) and expect the whole thing to work without breaking the tiles or compressing the layers?

Really the same question but.. your stove itself is only around 380-390 , ours has shorter legs but tipped the scale at 374.. depending on the size of your hearth and it's construction that weight is spread over many square feet.. our complete build is around 700 lbs, but that weight is spread over 20 square feet...


HELP! please