Designing a Wood Stove Hearth – Thermal Mass and Heat Transfer
I ordered the Morso 7110 and am planning a tile hearth and brick/tile wall to go behind the stove.
I was trying to keep the stove pipe in the first rafter bay. Just typing this I realize I could move it over to the second rafter bay, too, which frees me up to make a thicker wall behind the woodstove and still meet the required clearances.
My goal is to make a pretty hearth, and at the same time to hold some heat. I’ve acquired reclaimed brick and 6×6 terra cotta tile. I like the look of the tile and was thinking of covering the brick with tile.
The r value (per inch) of the materials are in the attachment (according to this site)
https://www.houseneeds.com/learning-center/pex-tubing-radiant-floor-heating/flooring-r-value-different-materials
Brick 2.25
Tile 1.00
Thinset 0.40
I’d planned to keep the hearth below the stove fairly thin, with just my terra cotta tile over thinset on jetboard (a backer board suitable for this purpose).
1) Is there an advantage to making the floor portion of my hearth thicker? I HAVE noticed that the hearth below my boyfriend’s stove does warm up after a while. If the added thermal mass would be advantageous I’d consider it. However a thicker hearth will be more of a tripping hazard (the hearth is in a corner but next to a traffic flow)
2) Based on the R values provided, it appears that the tile and thinset will conduct heat better than brick. Am I reading the information correctly? My friend had expressed concern that the tile+thinset+brick+jetboard assembly might not conduct heat back to the bricks adequately, but I’m inclined to think it would be fine.
3) Would you suggest I consider making the (back wall) hearth super thick? Like maybe laying the bricks end-out, and making the hearth wall 8+ inches thick? If I let the chimney go through the second rafter bay then I have plenty of space. Or should I stick with the original plan of using the bricks stacked (mortared) flat or on edge and covering the face with tile.
4) are there any concerns for the integrity of the assembly with a sandwich of jetboard (with a 1 inch min air space created by screw+washer+1″ metal pipe spacer), mortar, brick, thinset, tile, grout. This would be about 5 feet wide and 4 feet tall.
I appreciate any opinions or experience!
Thank you all again,
Paula
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Replies
Those r-values are not even close to correct. Wood is about R-1 per inch. They seem to think brick is a better insulator than wood. No. Brick, tile and thinset are all going to be pretty similar, something like R-0.2 per inch. Tile is likely less than brick, but given its thickness the difference isn't worth worrying about.
That's interesting, thanks Trevor. Would you agree that given the application a lower R value is better? I'm basically looking for the opposite of insulation, right? Or is it a different property altogether? Specific heat?
Lower R value is better, but these materials are all good enough that you can consider that problem solved. The specific heat is what you want. There's a limit to how effective it will be, because you will overheat the room before you get the brick under or behind all that hot. You won't be storing a huge amount of heat, but the more material you use the better. So if you don't mind using up the space, and you have the material, I'd say go ahead and put in plenty of thickness.
I don't know how hot they get, but I come up with 70 btu per brick of thermal storage. IMO, not enough to worry about. On the other hand, 1000 gallons of water in a tank in the basement.....
Paula,
I would base any decisions about the hearth and covering behind the stove entirely on aesthetics. The gains from thermal mass just aren't significant en0ugh.
If it's a new house I would suggest leaving the hearth flush with the finished floor around it. If you need a bit more height for the backer and tile, skim that amount off the tops of the joists below. Morso stoves have very small setbacks to nearby walls. My own preference is to just leave them drywalled.
This is very helpful! I will install this hearth before the finish floor in order to minimize the difference between hearth and finish floor. I can see the points being made that the thermal mass is not a significant gain, so I'm considering skipping the brick to simplify the process, and just tiling the wall behind the stove for the look. The wall immediately behind the stove is an interior wall, but I do plan to insulate and air seal the exterior walls carefully, as that can have as much of an impact if not more for the thermal performance of my house generally.
I had the thought of filling the interior wall cavity behind the stove with bricks instead of insulation.
I plan to have a heat pump hot water heater behind THAT wall, and I have heard that they can be a little noisy. Perhaps insulating the walls around the heat pump hw heater would be preferable to keep the hums and thrums out of the wood stove area.
Paula,
That's what I'd do. Fill the walls with Roxul Safe & Sound. Do the same with your bathroom walls and floor, and make sure to seal around any penetrations. Makes a big difference.
Whenever I see "thermal mass" in a question I cringe, it's an area where many people seem to have an intuitive sense that is at odds with how the science actually works.
As others have noted, the heat capacity of your hearth is going to be negligible compared to the heat capacity of your house and the heat loss of the house. I think the effect you're after, though, is that a large warm surface is more pleasant than a small, hot surface. It's nicer to be in a room with an 8' wall at 100F than with a 2' stove at 400F, even if both are releasing the same heat into the room. So yes, your stove will be more pleasant if you have a larger, less-hot surface radiating the heat. You want something that has high heat conductance, which is another way of saying it has low r-value. You don't want it to be very thick -- you don't want it absorbing heat, you want it transmitting it. You want to make it as easy as possible for heat from the stove to get into this surface -- without creating a fire hazard.
You want the wall behind this radiator to be insulated like the rest of the house.
Looking at another Scandinavian company - the Finnish company Tulikivi uses soapstone:
"The specific heat capacity of soapstone is about 1 J/gK and its density about 3 g/cm³, making its volumetric heat capacity 3 J/cm³K. The mineral magnesite has good thermal conductivity and heat capacity. The specific heat capacity for natural stone is usually 0.84 J/gK, making the value for soapstone about 20% greater than average."
https://www.tulikivi.com/en/tulikivi/Properties_of_soapstone
If you find some cheap source of soapstone, maybe worth considering as a design/aesthetic choice (if exposed). Might also consider Durock (USG), as an alternative to drywall (https://www.custombuildingproducts.com/media/993575/TB28%20Durock%20Bulletin.pdf).
As stated above, you probably don't want to compromise wall insulation/sealing (or other beneficial/desirable thermal properties/behaviors - including those required by code). My neighbor imported a large old tile oven (ceramic oven/stove) from Austria, amazing looking thing - if you have $$$ to spend.
We love our setup. Our soapstone wood stove pipe goes into the clay flue, surrounded by a brick wall on all 4 sides. It’s rises two floors. It takes about a day for the brick to warm up, but heats the whole house, including the two upstairs bedrooms. As an added bonus, it’s located in a south facing atrium. When the trees lose their leaves, the daytime sun streams in, heating the slate floor and the brick wall. We need very little wood stove heat on a sunny day. And the brick & floor radiate the heat in the evening for a very toasty house. We need almost no additional heat from our furnace.