DIY hearth construction details

[beefstew]

I'm working on a wood stove install in my basement. The plan is to put the stove in a corner, and build a 1.5" air-gap wall with steel studs, faced with concrete board (dura rock). After that, I was thinking about building a wall agains the dura rock out of limestone (from the hills around me) and mortar. Because of height restrictions, the floor was going to be limestone tile (~1/2")

Stove is a modest Drolet (El Dorado?)

So, questions:

(1) How thick does the limestone wall need to be to be an effective heat storage medium. ~4" thick is easy to source.

(2) To be effective, I remember that there needs to be air holes at the bottom and top of the air gap - ho many and what area should they have?

(3) If I just lay the tile under the floor with thinset, I'm worried that (a) they'll be connected to the floor, which is always near 55F, and (b) that temperature gradients might make them crack out of the mortar. Has anyone tried building some sort of insulation in under the hearth or laying a slab of mortar cut with vermiculite under the hearth surface? Unfortunately the room height is 86" and the required floor-ceiling clearance is 84", so I can't build a substantially raised hearth.

 

[Simonkenton]

For heat storage, the thicker the masonry wall at the back, the better. A four inch thick wall would store quite a bit of heat. An 8 inch wall would be even better.
That is a good idea about insulating the floor. You only have 2 inches to play with. I can't think of a way to put an insulated floor in such a small space.

As for the air holes at the bottom of the wall, let's see. You could set a brick sideways, it is 2 3/8 inches high, 8 inches long. Lay down a brick, leave an 8 inch space, lay another brick. The more air intake the better but I think that 8 inches of brick and 8 inches of air would work quite well.

 

[clemsonfor]

Can you use oh heck what's that insulation???? Can't remember.

 

[jeff_t]

I wouldn't count on the limestone being very much of a heat sink. Are the basement walls insulated?

If you are protecting a combustible wall, the manual says the gap at the bottom must be between 1-3", and not less than 3" at the top.

You can reduce the top clearance by up to 50% with a simple sheet metal shield, if you want to do something else on the floor. Clem is referring to Micore. I don't think I'd worry too much about it. I think whatever you put down isn't going to insulate enough to make a difference. If you were building a raised hearth, it might be a different story. Just my thoughts. I've never put tile on a basement floor.

 

[clemsonfor]

Nah I'm thinking of Roxul Not sure why I could remember it now? All I could come up with last night was rock wool??

Can you split a bat and lay in the gap for extra fireproof r value

 

[begreen]

I wouldn't worry about the floor at all. The heating of it is going to be gradual and the slab will diffuse most of the heat. Use a latex modified thinset for a better bond.

For the wall, make the gap at the bottom at least 1" high over the hearth floor, per each stud cavity, so say a minimum of a 1" x 14" gap. It's ok to take the stove right down to the hearth at the studs. At the top the same thing is needed to exhaust heat. Make at least a 1" x 14" vent hole at the top of each stud cavity. The object here is to allow a free flow of air to convect naturally behind the wall.

PS: Will the metal stud cavities be insulated?

 

[jeff_t]

For the wall, make the gap at the bottom at least 1" high over the hearth floor, per each stud cavity, so say a minimum of a 1" x 14" gap. It's ok to take the stove right down to the hearth at the studs. At the top the same thing is needed to exhaust heat. Make at least a 1" x 14" vent hole at the top of each stud cavity. The object here is to allow a free flow of air to convect naturally behind the wall.

The manual actually gives the dimensions in my post above, if indeed it is an Eldorado(?).

 

[begreen]

Yes, I wasn't trying to correct your post, just providing more specifics. 2 or 3" will work fine too. If the walls behind the stove are concrete and the metal studs attached to them with Roxul as an insulation there is no need for the air gap at all. The entire wall system at that point is non-combustible.

I also see the manual provides guidance for a ceiling heat shield if the desire is to put the stove on a raised hearth which will bring is closer to the ceiling.

 

[jeff_t]

I see 1" top and bottom all the time, but this one says 3" at the top.

I, too, was wondering if the walls behind are finished or concrete. It's a pretty tiny stove, so I assume it is take the chill off in a basement family room or something similar.

 

[beefstew]

The basement room is finished - block wall w/ 4" studwall + insulation + sheetrock on the interior (obviously). My plan for the hearth is to build a 1 5/8" steel-stud wall on the inside of the sheetrock, face that with dura-rock, and then build the ~5" limestone wall inside of that. This would yield 1.5" airgap and ~5" of masonry - I hadn't thought of putting sheetmetal (Al?) on the front of the steel-stud wall, but it would be cheap and easy.

I wouldn't count on the limestone being very much of a heat sink. Are the basement walls insulated?

With regard to Limestone, here's what I was thinking. It has specific gravity of ~2.3 and specific heat capacity of about 0.2 BTU/lb F. I think this works out of a heat capacity of 30 BTU/ft^3 F, or if I build a 4' high hearth, 4' out on each corner wall, 5" thick, a total hearth heat capacity of 400 BTU/F. Then, if I heat the hearth to an average temp of say 150F (realistically, too hot), this would be a dT of about 90F, which would store ~36,000 BTU. Locally, boxelder is the easiest wood to source, and @ 17.9MBTU/cord and 2890lb/cord, ie 6200BTU/lb, the hearth would be able to store the heat of about 6lb of box elder. This certainly isn't equivalent to a masonry heater, but its at least more heat storage than the air in my basement. Does it seem realistic?

 

[Davidram]

I did a similar install in my basement. The first year I used a commercial heat shield gapped from the drywall. It worked fairly well.
For the permanent install, I cut out the drywall and adhered stone veneer directly to the block wall, my wife wanted the 5 extra inches in front of the stove. This was not so good. The veneer stone never got warm since the back side was the infinite heat sink of the earth. I still contemplate digging up the foundation and insulating the back side of the wall.
If I had it to do over, I'd insulate the block wall with 1" of rock wool and build the stone wall up using wall ties fastened to the block every 16". There is no need for steel studs or an air gap. Four inch stone seems right to me. The results will be amazing, The limestone will heat to 140 deg and be a giant radiator. Radiating soft warm radiant energy to every corner of the room even after the fire dies. This is like a soapstone stove on steroids.
On the floor I placed 1/2" concrete board and tiled over it. The rest of the floor got carpeting. Raising it up that 3/4" allowed the carpet to finish flush. This worked well.

 

[begreen]

If you bolted the stone panel to the block wall, can it be detached? If so I would put 1" of micore plus a sheet of 28ga steel behind as a thermal break and heat reflector.