Preparing to put in a rocket mass heater, I want to add thermal mass to about 4' of nearby interior wall (around 8' away from the heater barrel) to help improve the rocket mass heater performance in the space.

There are a few 12/2 romex wires running through the wall but no boxes in the area I want to fill.

I have wet clay and was planning to mix this with sand and gravel and pack it in to the stud cavities.  Now I am uncertain whether the sheetrock will survive the process.

What's a good way to turn a 2x4 stud wall w/ sheetrock into a thermal mass wall?

Thanks.

What about tile?

I think a great way to add a thermal mass body is to cover the desired walls in stonework. Maybe some of us here have been around a stone fireplace and noticed the heat radiating from the stone walls and hearth long after the fire has gone out.

Stone and tile are both excellent ideas!
Not quite as permie, but great thermal mass that adds easily is the cement board used under tile. It could go under tile or stone and add more mass to the wall. If you are strapped for money, consider doing the cement board a piece or two, as you can, replacing out the sheetrock, when it's up, then tile or stone over it, as you can. Latex paint makes it look decent, and will scrape off fast when you are ready to tile or stone over it.

I'm not good enough with wires to know whether claying in those wires is a good idea or not, I hope someone wiser than me answers that part of the question, I'd wonder about a few things with that. Codes, what if you ever have to replace the wires, and heat dispersal are all questions that come to mind, but I don't know the answers.

IMHO-

I would NOT put anything directly on the wall. Especially if there's a fireplace right next to it. Wall should be 5/8" sheet rock (not critical imho) and then put a spacer of some kind on the wall, metal studs come to mind, and then put cement board on the studs and then put stonework or tile on the cement board. Make sure you leave a air gap at the bottom to keep the actual wall cool.

Double benefit- you have twice the air flowing over the same mass of brickwork to keep the heat in the house.

Thanks for all the ideas.  Some further details and clarification.  I'd really rather put something in the wall than on the wall.  This is for aesthetic and practical reasons.  I'm not modifying the full length of the wall, just a ~6' length of it.  And the wall is around 8' (at the nearest point) from where the firebox and barrel of the rocket mass heater will eventually go.

When I put my wood burning stove in the shop I used Durock concrete board as the outside layer.     It's considered noncombustible.  

I don't think the drywall will stand up to the weight of the fill. The weight pushing down would blow out the drywall at the bottom. I'm not sure about the wire in the wall, but there may be a problem if it's encased in clay/sand. I think another problem might be that the infill could never dry.

The best suggestions have been to add a thermal mass to the exterior of the wall, but I can understand if you not want to do that. If you do, the thicker the better. I don't think tile has enough mass to be very effective.

But specifically to answer you original question, I don't think the drywall would hold up to an infill of wet clay, sand, and gravel.

-Tom

Tom Gauthier wrote:I don't think the drywall will stand up to the weight of the fill. The weight pushing down would blow out the drywall at the bottom. I'm not sure about the wire in the wall, but there may be a problem if it's encased in clay/sand. I think another problem might be that the infill could never dry.

The best suggestions have been to add a thermal mass to the exterior of the wall, but I can understand if you not want to do that. If you do, the thicker the better. I don't think tile has enough mass to be very effective.

But specifically to answer you original question, I don't think the drywall would hold up to an infill of wet clay, sand, and gravel.

-Tom

Thanks.

Drywall (sheetrock, not sure if there's a practical difference here) blowing out at the bottom is something I've read about elsewhere as well.  That would be a bummer.

Another random idea was to mix in a little portland cement - just enough to suck up all the water and perhaps provide a _little_ bit of compressional strength.

And, actually, the original idea was to take the sheetrock off one side of the wall and pack the cavities a little bit at a time, letting the clay mixture dry before working up to the next level.  This still leaves the question of the sheetrock on the back side getting wet, though.  I thought about stapling plastic sheets (random otherwise-landfill stuff we have lying around) to that sheetrock to protect it.  I guess this reduces the drying rate of the clay but it seems sound to me otherwise...?

The plan above sounds best to me.  My 3/8" drywall in the ceiling (why they used that I'll never know) bowed under the weight of 8" of blown in cellulose insulation.  5/8" in the wall would very likely bow out if the material didn't hold its shape.

Plus you can form the clay mix around the wires so that they can be pulled out in the future if needed.

Another simple way might be to get empty tin cans and fill them with sand and stack them in the wall.

Is this an exterior wall?

William Bronson wrote:Is this an exterior wall?

It is an interior wall.

Disclosure: This topic is not (yet) well established in my wheelhouse. I'm learning.  

Would perlite or vermiculite (poured into the wall) fit the bill? I know it can be insulative, but I'm not sure if the thermal mass...

Perlite will not hold the heat.
I suggest pulling the plaster off, moving the cables up and around the area and fitting fired bricks into the space between the studs.
Then use an earth plaster over the bricks. I lay the bricks with a sloppy earth mortar I make.

Thanks all.

I went with a concrete/sand mix, 1 part cement to between 5 and 9 parts sand, with as little water as I could manage without leaving any dry cement.  This was partly because sand and cement are very readily available, affordable, and easily mixed.  It was also partly to not worry about whether the fill was going to soak the drywall and bust through, either immediately or a few weeks down the road.  I figure the cement would both quickly suck up the water and provide stability to the fill so the weight is all down to a solid support instead of out against the drywall.  This also cut the total height of the fill to around 4' (reducing the weight, the materials required, and the labor to mix and install it).

I installed a 2x4 cross pieces above the wiring and filled up from that point, avoiding concerns about whether packing fill around the wiring is safe or not while also not having to move existing wires running horizontally through a stud wall.  The space below the wires would be of marginal heat storage value anyway as it will not be directly exposed to radiant heat from the rocket mass heater.  I filled the cavities in two sessions about 24 hours apart, filling approximately the bottom half of each cavity the first day and the top half the next day, based on the idea that a day of curing for the cement would stop it from pushing out against the drywall any more when the 2nd day's mass was added on top.  I also added more screws to the drywall, including going in to the new 2x4 cross pieces.

I observed a small amount of wetting of the paper on the drywall after packing the cavities - with water moving perhaps a quarter to a half inch beyond where the paper was in direct contact with the fill material.  It seems to have dried quickly.  The drywall has bowed out just the smallest bit, perhaps around an eighth of an inch at the center of the center cavity.  It's also possible this was already the case as I didn't check how flat it was before starting.