Combining rocket mass heater and stone walls

My life dream always was to build a stone house. Trouble is, in my country (Slovakia), we have no tradition of building stone houses. No company will build such house, because they have no experience building with stone, so I'm just begging for an advice. I was thinking about building a stone house with double stone walls (I want to see the stone from outside as well as inside) connected with lime mortar or something that breathes. I've read that concrete is not good when building with stone because it prevents the natural evaporation or the natural process of wall "breathing". The outer wall would be made of granite and for the inner wall I was thinking sandstone. The outer wall would be stronger than the inner wall (I'm thinking 25 cm vs 15 cm). Between the walls I want to leave an empty space of some 8-10 cm. The thing is that I want to have a rocket mass heater or similar heat source near one of the walls. This heat source would be connected to a system of pipes. The main pipe outlet would serve only to get the fire started, than it would be closed and the heat would proceed through the pipes laid on the ground in the empty space between the walls, exiting the house on the other side, thus warming the house faster. This is just an idea and I would really appreciate your thoughts on this. If you have experience with stone houses or stone walls. Would this be something that would work or is it a bad idea and if so, why? Or how would you change this if you'd do it yourself. What to use? Mortar or something else? Thank you for any advice.

I am no authority on the subject, and I don't have much experience outside of a few prototype rocket stoves I have built, but I do have a few ideas on this subject that I have thought about quite a bit. If you have an open space between the walls with exhaust going in then the heat will rise up to the top of the wall. You would still get some benefit by limiting thermal bridging between the inner and outer walls. If the floor is not insulated from the earth then the floor would likely always be much cooler than the ceiling.

Using the rocket mass heater idea could be a good solution, but I would stick closer to existing plans that keep the exhaust inside of a pipe. You could also do multiple exhaust pipes or a larger system with a long run that takes multiple trips back and forth within the wall, although this can make it more complicated to build without having draft and smoke-back issues. The pipe doesn't have to be steel, but could be clay or some other material sourced locally to save money. I would try to keep it well buried in a mass so that the exhaust couldn't easily leak inside the house. Monitoring CO and CO2 levels could be life-saving in any construction method. Having a 'breathing' wall with exhaust in it could be dangerous.

One thing that I have given a lot of thought to in particular is insulation and thermal mass. For a very efficient system it seems like going all mass or all insulation is less than ideal. If you have excess wood to burn then maybe this won't matter so much, particularly if it helps you avoid going long distances for building material. I personally like the idea of a large interior thermal mass that is completely insulated from the outside world on all sides. Essentially having the thickness of walls in your diagram reversed, with a thinner exterior, thicker interior, and some sort of gap that insulates the interior mass. Adding some sort of insulation layer between the mass and the ground, as well as insulating the roof would also be beneficial. My idea is kind of like a picnic cooler where there is insulation all around, ice as a thermal mass, and then food and drinks having their temperature regulated by the thermal mass. The main differences being retaining heat instead of keeping it out, and some sort of covering protecting the insulation which could easily be a stone or brick wall.

I could go on for days, but hopefully that gives you some helpful ideas. Thermodynamics is one of the toughest fields in science. Many things that have been learned about it over the past few decades were only possible with the help of computers. Some things seem to be counterintuitive. I would search around a lot and try to look at what others have done for inspiration. There may be mistakes others have made that you can learn from and avoid.

Thank you, you really got me thinking about a few things. Meanwhile I've came across someone who really builds stone houses here in Slovakia. He told me they use a system where they build an exterior wall of stone, interior wall made of bricks and they filled the space between the walls with rockwool. They measured the heating dissipation with a thermal camera and they couldn't find any. He also told me they did a house with both the exterior and interior walls made of stone with this system, so I will try to get some more information from them. They also used sandstone because of its excellent heat and humidity absorbing qualities.

I'm glad you found someone with stone building experience in your area. The double wall with rockwool sounds like a good plan - an air gap would have very little insulating value.

The idea of running the RMH ducting in the cavity would be a huge waste of heat, as half or more of the heat would be going directly to the outside. If you run the duct around the inside of the inner layer of the wall, with good insulation between that and the outer layer, you would be able to use the wall as part of your mass. It would be even more effective to use an interior loadbearing (stone) wall as part of your mass, as that could radiate to the rooms on both sides and even out the comfort.

I would say, modern type of stone house. Stones walls on the inside, for inertia. Outside insulation,  with stone facing.  Most of the stone facing in Europe comes from italy.

This way, you would have stone, and proper insulation.

Glenn Herbert wrote: It would be even more effective to use an interior load bearing (stone) wall as part of your mass, as that could radiate to the rooms on both sides and even out the comfort.

The idea I had was to have a double stone wall dividing  the living room bed/bath sides of the house as a mass for the rocket mass heater.
Take your diagram and move the heater back 3/4 of the way toward the door with the pipe running between 2 stone walls and filled with cob.  This puts the rocket stove between the kitchen and dining rooms where it can be used for cooking. If you make an L shape around the bath room then it would be the warmest room.

Have a look at this method of using stone,timber & cob.  Might give you some different ideas. https://permies.com/t/47813/easy-stone-construction-Dhajji-Dewari

Hallo there,
As far as stone and heat retention is concerned I can tell you from my own experience that stone takes a HUGE amount of fire and heating before it will retain any heat.  I have built a double walled stone fireplace with a wood stove in it and the stone after a full days burning is still ice cold.

May I suggest that you rather build your interior wall in adobe or red clay bricks which retain the heat from your room much better. Then you put in a thick (at least 20-30cm) of styrofoam under your floor to retain heat, plus proper insulation (the rock wool would work for this too).  You still need to put in dampcourse under the styrofoam layer to stop it absorbing cold moisture from the soil.

My house is adobe with an interior wall of baked claybrick/cement mortar.  I have a 10cm air space between the walls with heat barriers of dampcourse (black) plastic (used under concrete floors).  3 layers of this in the cavity = floor then under the windows and above the windows.  This stops any heat trapped from the sun via the adobe outer wall and stays in the wall for longer. (Next time I do this I would also put in a styofoam barrier at the top).

Friends of mine have a rocket mass heater system with pipes running between the styrofoam and adobe floor space which keeps the floor hot and effectively also the room.  Personally I would not build in stone.  It is not a cheap system.  Rather look at adobe or strawbale which retains huge heat.  Here in South Africa we do not have very cold winters like you do but it gets very hot and the way I have done it there is a difference of 10 degC between outside and inside the house.  So if it works for heat it will also work for cold.
Good luck.
rgds
Angora

Hi Lanis
What type of stone is found in your area ? I used to have a sandstone house it was great but then I lived close to where it was quarried. This has a huge impact on the price. Sandstone and lime morter goes well . At the moment my current house is a mixture of Slate sandstone and granite all mined locally  also local bricks round the windows and doors . As for facing I would not both the styone its self will be fine .

Hello,

I think this is not an good idea for the following reasons;
If you heat the empty space between the walls you also heat the outer wall = waist,
A RMH with 2 outlets (left and right)as on your drawing does not work very well / not enough draft
The amount of the  surfacy you want to heat is far to much , i think one wall is about the max.
It is hard to clean out the waists in your design.

what I would do;
The starting chiney is an good idea but;
Only heat one wall
Put /locate the RMH in one corner
Build another thin third wall (in-between space about 5 cm.)in the room and let the hot air rise  zig zag through it
You can make a thin wall by putting the bricks on their side

You can make  a zig zag pattern in between these  walls using thin tiles or bricks wich stick out and reach the other (second)wall when you are building it
It does  not matter if they stick out in the room  becouse it is extra surface /heat exchange
Make max. 4 zig zags . make sure the top of the wall is closed and let the hot air /smoke out only above the RMH using the same outlet as the starting chimney
Think also of a way for cleaning out the system when youre building it
Locate the exit chimney in the corner above the RMH


Rob Grip
Netherlands

For insulation under the mass I'd look at aircrete.  http://www.domegaia.com I'm afraid of all that weight compressing Styrofoam or polystyrene.

The still air between double walls are supposed to be your insulation (though Rockwoll or perlite in space between the walls is more efficient), so adding your heat-source there will make the air move, and heat up the outer wall, which means that half the heat is lost.

If I were to build my house over again I think I might put the pipes from a RMH in the floors... I am actually thinking of doing that anyway in my husbands office (he will not think it a good idea I'm guessing though - I doubt he thinks that ripping out the floor is worth it...).  

What about in-floor heating, heated with a RMH? I've seen modified RMH's that'll heat water, you'd need an accumulator tank or four, and a system to regulate the temperature that goes out from them. I've seen systems like this in Sweden, but using a stove called Calmarpannan instead of an RMH. They heat two houses, five yurts and a jacuzzi with the hot water from the tanks.

In floor heating gives a nice, even temperature and you need less of it to feel warm, as it rises through the room around you.

I'm heating a building with underfloor water. I love the heat but I have a Kentucky wood gobbler furnace and it takes huge amounts of wood. I didn't learn about RMH's till after I had installed this system or I would have gone that route. The warnings that go with using a rocket heater to heat water are pretty drastic. I have modified the system I have to get more efficiency. And, I put a insulated riser in a drum, like on a RMH, as a bell on an old fisher stove and boosted it's efficiency to where I use it as my primary heater. There is a discussion of that here  https://permies.com/t/32812/cantenary-arch  I was worried that the heat would burn out the interior stovepipe that the riser was built around and deteriorate the insulation, but I checked it this fall and it was in great shape. I used drums that I had and now I wish I had gone to 55 gal barrels. With a modest fire the chimney stays cool which shows that all of my heat is staying in the building. As I crank it up from there I'm still getting a lot of heat radiating off the chimney. If I had a shorter chimney I'd put another bell in before the chimney. I might anyway. What I've done has given proof of concept. I'd like to replace the Fisher with a rocket stove to boost the heat and efficiency then, after the riser/drum, put it through a vertical masonry mass. If winters continue to be as mild as this one there won't be much pressure for that though.

I'm not finding much information on that Calmarpannan, and what I am finding is in Russian. Can you link me to more information

If you are going to heat water, particularly if it is under pressure can I suggest you read this thread http://donkey32.proboards.com/thread/1096/hot-water as a starter.

Water heated in sealed containers can explode and you don't want that!

This is not natural building - conventional, modern practices from the USA - but it's a very useful basic primer for anyone who is starting to build for themselves (or others).
https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall

Humanity has been building with stone, mud, lime, and mortar for tens of thousands of years.  this means there are a lot of known problems and known solutions.  
Traditional or conventional trades (skilled masons, families that have built in the area for multiple generations) tend to know exponentially more tricks than a novice - a master builder knows ten times what his journeymen do, and a 3rd generation stonemason may know two or three times what a first-generation master builder knows.  You can learn enough in 3 years to be deeply embarrassed about your early work, or have to rip out and do it over.  So if possible, go train under someone else and make your mistakes on their projects.    After that, build your barn first, and live in it to test the method while you build your next building.  Or build a chicken coop, or oven, or any small project that will give you relevant experience.

For areas without good local traditions in that style, you can look at analogous climates or geology elsewhere in the world, and sometimes get interesting ideas or find a good trainer.

Finding a good köppen-geiger climate map is one way to do this.  There are others, it's worth looking at soil and rock types too, for how people build in areas with similar geology.
(My part of inland North America turns out to be similar to parts of Armenia, Mongolia, and Kazakhstan, where a few hours away the coastal climate is more like Belgium.)

-Erica

So what I take away from the article Erica cited you want the outer rock wall to protect the house from rain, wind and sun. Inside the outer wall is an air channel to remove moisture and keep the insulation dry. Then insulation  and a vapor barrier. The inside rock wall is for thermal inertia. That is it stores radiant heat from the mass of your rocket stove and radiates it back into the room. During the summer it absorbs radiant heat making the room cooler. You do not need to heat the internal wall directly but use it to collect and store radiant heat.

Now if you build an underground stone house you could use the floor wall on all 6 sides. This saves on what is often the most expensive material for the outer wall and roof.  If you don't have trees then build a stone wofoti.