Raised hearth construction for basement woodstove installation?

[Danno77]

those cement blocks with the holes in them are 8x8x16, and I thought he said he wanted the hearth 8" to a foot. that's one block tall with a little extra after tile.

Yeah, but you'd have to pour them full to get the mass of the solid blocks or a poured slab.

yeah, that is what you'd have to do to get similar mass, but that's overkill, so I wouldn't.

trust me, this soaking up heat garbage is just that...garbage. if your stove is generating 70k BTUs then there will be 70k BTUs released into your home regardless of how heavy your hearth is. and anything you are gonna gain by having a foot of cement on top of a few inches of cement is gonna be negligible. you can have hot cement or hot dirt, like I said, its gonna be negligible to everything further than two feet from the stove.

 

[Wet1]

+1 to Danno's comments

 

[meathead]

Spoken like people with no substantial masonry mass around their stoves.

Soaking up heat is "garbage"? You need to get this info out to the public - everyone mistakenly thinks masonry heaters (which operate by...uh...soaking up heat) are the most efficient way to heat with wood! The hearth this guy wants to build will weigh in at well over 1,000lbs if it is poured solid - it will radiate warmth long after the stove is cold.

 

[Danno77]

i respectfully assert that you do understand the value of thermal mass, however you don't understand how trying to put more thermal mass onto something that sits in the middle of the floor which itself is a giant solid hunk of cement without air flow under it isn't the same as what you are thinking. if we were talking about first floor on an exterior wall, then I'd agree. It's a basement floor. It's got INFINITE MASS (well, not infinite, but the earth is pretty damned big)

 

[meathead]

That's why i circled around to agree it is best to insulate the hearth from the floor. With the insulation he would have + / - 1250lbs of cement and rock seperated from the basement floor and the Earth and able to hold and dissipate the heat.

To be honest, I'm not positive he wouldn't see some benefit even without the insulation. The stove in my basement backs up to a 3' thick interior concrete wall. The wall is in direct contact with the floor, exactly as his hearth would be. A full 48 hrs (below freezing outside) after my stove has gone out , the entire wall will still be warm to the touch on all sides. I realize that the wall has substantially more mass than his hearth would have - but if factoring the cement floor/masonry mass/earth connection into the equation as "infinate" were accurate, the wall would never warm up in the first place let alone keep my pipes from freezing when I am away for a long weekend.

BTW, aint no need to "respectfully assert" anything - just argue away you won't hurt my feelings. Worst you can do is prove me wrong and teach me something. If I came off pissy or like I was trying to make it personal I appologize (must have come off poorly if you felt I needed to be "respectfully asserted" to). hh: :cheese:

Note my utilization of smily faces to lighten the mood.

 

[jcims]

Boy, this thread lit up while i was out!

OK, so scrap the wood idea. I didn't pick up the difference in application the first time through...thanks for setting me straight.

There seems to be a bit of consensus that an insulating layer of some type between the hearth and the floor is a good idea. The table that CarbonNeutral provided up front shows 1/2" Micore with a decent R-value when compared to the other materials on the list. That seems like a decent candidate, any other suggestions?

For the bulk fill, we're looking at nearly a yard of concrete or block... Both ideas have merit, but block is sounding good to me.

We've got a cultured stone fascia in other parts of the home that we'll probably use on the sides just for sake of consistency. The guys can probably just mortar that right to the block (assuming i can keep it reasonably square) Not sure what we're going to use on the top, will probably be some kind of stone or stone like substance.

Thanks again for all the help and suggestions, folks. This thing is going to get a lot of use and i'm glad i asked for some advice before just going at it.

 

[Wet1]

Well, I was in the same situation. I could have easily went with just concrete block and then tiled over the top and been done with it, but I wanted to put a thermal break under mine so I wasn't heating mother earth.

Since there's potential for moisture between the slab and your hearth, I used 1" foam board over the foundation. This will work as a vapor barrier and keep the heat were I want it. The down side to this is it's a combustible, so now I needed to bring up the R value above it if I wanted fire protection (which I did). I could have used Micore at the bottom, but moisture was a major concern.

I should mention my stove doesn't require anything more than amber protection, but since I was building it from scratch, I wanted to have a good safety margin. Here's the catch 22. If I just used solid concrete above the foam board, it would take a LOT of concrete to achieve the desired R value since it's R value is 0.08/inch. I didn't want my hearth that high and I didn't want to spend that much time, effort, and money on concrete. I decided on using the 4" block on top of the foam. 4" block has an R value of 0.80 to 1.0 depending on where you look (and FYI, 8" block is R1.11 to 1.3). I covered the block with 2 layers of Durock (R.52 total), and then 1/2" slate. Everything was bonded together with thinset. I suspect the total R value is close to 1.5+, and if I wanted even more protection I could have filled the cavity with perlite or added a layer of Micore, but I thought 1.5 would be plenty.

I'd guess there's well over 800 lbs of building material in my hearth, so there's still plenty of mass there. But I'm not sure that mass makes much of a difference. Having the mass is nice if you want or need heat storage, but I don't think I really need it. Not only is there a lot of mass in the surrounding environment, but heat storage is really only needed if you don't have a constant source of heat to keep everything up to temp. My stove can burn 24 hrs and has a built in thermostat, so I always have a nice steady heat source. Heat storage isn't really needed. Even if I had a traditional wood stove that burned in cycles, it would still be providing fairly consistent heat as long as I kept it going. Compared to the mass of everything else in the house and around the stove, pouring my hearth with solid concrete vs what I used would make very little difference in the grand scheme of things... So while I might loose 30 minutes of heat retention in the hearth, I made up for it in safety and hearth aesthetics.

 

[Danno77]

I'm on board with the insulation between the floor and the hearth. At that point the hearth becomes its own separate hunk of "mass" and making it heavier as meathead has proposed will yield the best results, but remember that a concrete block weighs something like 40lbs (from memory, so don't flame me if I'm a couple lbs off). If you have 18 concrete blocks you are talking about a weight of 720lbs. A cubic foot of cement weighs something like 150lbs, so 816lbs or so. I'm not sure why they are so close in weight, but that's what my math shows. Perhaps the blocks tend to be made of something denser because of their general use for structure, while the stuff you buy at a store for concrete is generally used for paving purposes (who goes and buys bags of concrete for a foundation?)

An 8" thick concrete slab WITH another bit of rock on top should give you an R value of 1.5, so I think if you want it up off the floor even more (it depends on stove hearth requirements) you MIGHT be able to throw some 2x4s under there, and then some OSB to pour the concrete on. I'm not sure about the R-value of concrete blocks, I'm sure google has the answer.


So, solid concrete might give you a little more mass, like meathead says. (But, unlike MH, I'm not sure it's gonna be noticeable.)

So, consensus seems to be:
1. insulate under hearth.
2. Give hearth some major mass, (and you should avoid all combustibles, unless you want to complicate things.
3. Make it look pretty.
4. Burn lots of wood.
5. Take lots of pictures.


Sidenote:
Also, just because it would be cool to do, if you have the floor exposed, and you have the opportunity to do it before you lay your final flooring, it would be neat if you could manufacture something within the hearth itself that could turn pipes laid within into some sort of heated floor system. THAT would be sweet. Now, I do remember reading things about collectors WITHIN stoves blowing up from poor design, and I'm pretty sure a collector on the stove pipe would make the people here cringe (along the lines of magic heat), but I'm wondering if you could get any efficiency to warm enough water within the hearth to circulate in the basement, even if it is just a small area in front of the stove, or a couple of pipes that flow under major traffic areas.

 

[jcims]

Well, for better or for worse, this is what i went with:

The block area is 5'x6'. Most are filled, but the 'sideways' blocks will allow for air to be pushed through the base and possibly allow me to capture some of the heat before it goes into the ground. I couldn't find Micor anywhere nearby, so I just went with 1/2" of durock to keep it off the concrete floor (almost nil R-value). There's still a decent amount of mass, about 600lbs of mortar, and according to references i found, about 800 lbs of block (although it's hard for me to believe they are 37lbs each).

When finished, the surface will be about 11" off of the floor. It actually turned out OK, despite my horrendous skills with mortar. Can't wait to get it wrapped with the finish stone and get that puppy lit up!

Thanks again everyone for the information, it has been very helpful

Bob

 

[madison]

No framing, just stone, example in signature link.

 

[jcims]

Very nice! What is the surface made of? ( Nevermind, you already answered that here: https://www.hearth.com/talk/threads/23542/ )