Rocket stove in a static caravan

Hey everyone. I've been feeding on the forum's priceless source of inspiration and information for a long time. Today I'm happy to be able to add something interesting to it. last year I bought a piece of land in order to start a permaculture project (the goal is creating 1 hectare of food forest) and although I'm not living on the land for the moment, I bought an old caravan to have some shelter when needed. So a couple months ago I wanted to get ready for winter and started to think how I would heat the small space (4 x 2 meters). I really liked the idea of a rocket mass heater. They are so beautiful, simple, energy efficient, economic, free to run if you have wood, etc... So I was about to start building a RSMH in my caravan, but then started to realize how heavy the bench could be and the danger of running out of oxygen in the caravan or having a back-draft of carbon monoxide... I was about to turn to other alternatives such as a small woodstove or going electric with solar panels, but came out with something.









The how to...

Starting point








































Water heating surface



As you can see some there are two barrels. The black one is there just to make it better looking and to hold the rockwool in place. (the combustion chamber barrel is covered with rockwool to get all the heat go inside the caravan.

It's been about two months since I got this up and running, so much more testing must be done in order to give a good review about functionality, but until now it works really great. the draft is very good, and when I use dry wood there is no smoke at all (either out of the feed tube or the exaust pipe). It heats the caravan in a very nice way, not too hot, but definitely making it warm.

The main advantages are that no matter what happens on the flow, no smoke or gases will go into the caravan (to my understanding). It is not space consuming, in fact I gained quite some space and a bookshelf.

lighting the candles at night transforms the caravan in a magic cave.


The main disadvantage is having to go outside to feed (maybe 6 steps...) but the caravan is sheltered with roof so I don't get wet if it's raining. Also, I don't really hear the roaring or see the fire lights from inside the caravan, but well...

This is an experiment, I tried taking all precautions and doing things as good as possible (with my liittle DIY experience), I hope it will last a long time, but from the beginning I've been conscious about the risk of ruining the caravan... We'll see!

Beauty final-layer cob-work on the interior man. Looks real nice. Its not really "in" the caravan, like the title says, but technicalities aside, I really like the final result.

I'd give you an apple or a cookie or a high five, but I'm new to all this forum stuff and I don't know how to do it lol. Make sure thos square holes are "sealed-up" on the exterior, but this looks like good stuff to me. Especially the smooth artistic cob interior.

Beautiful work! I'll be interested to hear more about how it works over the winter.

Thanks Glenn and John for your nice comments, I'm happy you like the result. You are right John, the title can be confusing...

Two important things that I forgot to mention.

When I first gave the final plaster, some white mold appeared while it was drying (lack of ventilation inside the caravan, before buying the small fan) . Once it dried, y sprayed white vinegar diluted in water, let it dry and then gave another layer of plaster, but this time I added borax and casein. No mold has reappeared for the moment.

Second, and this is the thing that worries me the most, the cob being inside the caravan (with the two holes of exit and enter pipe) won't breath as a cob wall would, so I guess condensation could create in the caravan wall against the cob. In order to try to equilibrate the breathing of the cob structure, I gave two coats of linseed oil after applying the plaster layer. Plus one layer of casein mixed with borax. To my understanding, the borax and casein layer won't affect the breathability, but the two linseed oil layers should make the cob less permeable, hopefully not making it completely impermeable and thus letting it breathe the moisture that can transfer from the exterior through the two holes. Any thoughts about this?

I never thought about the cob drying or humidity problems when I saw a fan in your one photo. Perhaps a de-humidifier until the cob is better dried? Mould is bad for health, and so are the chemicals to clean it up. Try to keep dry heat in there but I've never worked with cob before. I have only a small internal concrete/fire mortar mass in mine, so I'm completelyt unfamiliar with cob, but I'm guessing it takes at least a week of low humidity and warm temps to cure.

Thanks for your answer John. The cob is now competely dry, and the white vinegar plus de layers with borax have solved the mold problem, at least for the moment. My concern is that condensation could create between the cob and the caravan wall, making the straw rot and thus debilitating the cob structure in time. Time will tell...

Jorge, I and I'm sure the rest of us are interested to know how your system held up over the winter. Can you give an update on its performance and current condition?

Also, all of the pictures and videos have disappeared. Can you upload any of them directly to this thread so future readers will be able to see them?

Hi Glenn, thanks for your interest, I'm sorry I've taken so much time to answer. I'm not sure what happened to the pictures, and I felt so lazy about loading them up gain...!

The stove is doing great. To be honest it hasn't been used intensively since I don't live in the caravan, but the 10 to 20 uses have been very satisfying. It heats up to 20 degrees celsius on cold nights. The main structure hasn't suffered any decay, there's just one joint (the one with the red arrow in the photo) which has cracked and the piece of wood has moved about 5 mm, the problem being that I didn't make the joint strong enough, knowing that a caravan wall moves a little each time you open or close the door...

All in all I'm really happy with the stove, it adds a unique warm magic cave feeling in the night with the candle lights!

Thanks for the updates!  This is a fun project to see; lots of folks are interested in caravan heaters, and this is a new version.

I'm glad you are getting good results.  This is the first time I've seen this particular method for insulating the barrel and sending heat indoors.  I hope you have cut back the plastic and other burnable materials far enough around the hot pipes, if there are exposed edges outside maybe you can check for any signs of heat damage like, brown, black, small cracks, or melting/slumping on the nearest burnable materials.  Hopefully it is already far enough, and you may be fine.

Jorge Mar wrote:Thanks for your answer John. The cob is now competely dry, and the white vinegar plus de layers with borax have solved the mold problem, at least for the moment. My concern is that condensation could create between the cob and the caravan wall, making the straw rot and thus debilitating the cob structure in time. Time will tell...

Regarding condensation on the wall: Typically, the worst condensation happens where warm indoor air strikes a cold interior surface (or warm outdoor air striking a cold wall, in air-conditioned buildings).  If you do not have moisture leaks from the outside, and you do not have warm air from the inside reaching the cold wall behind the cob, then the condensation should not have much chance to start.

However, in case of any moisture, it's important that there is effective drying.  Any wall between indoors and outdoors - or any wall at all - should have no more than ONE vapor-seal layer.  If you create two waterproof layers that block drying, then any moisture from an accident, a spill, etc. can get trapped in between these layers, and that is when you have bad structural or mold problems.

You already have a water-impermeable layer in the original trailer body, so the cob should NOT be made waterproof at the surface (at least, don't do any more than you have already done).  If you do spill something, or there is some condensation from water in the wood exhaust, you want it to dry out in both directions.  Outward on the outside of the wall, inward on the inside.  

If others are considering a project like this, it would be a good idea to have a slope on the exhaust pipes so any moisture in the smoke would drain outside the building, not inside the trapped space.  

Mold on cob is most likely in the first week or two after installation (when the cob is most wet).  Get it dry as fast as you can, don't finish it smooth or "waterproof" it before it is dry, and any problem should go away.  
If you have mold more than one month after the cob is installed, or if it happens again when it rains, there is probably a water leak somewhere, like the roof, or walls whose clay touches the damp ground.

I recently found a building blog (conventional building) that explains walls and shelter very well, and where to put the vapor-proof layer to avoid condensation.  Basically if the only vapor barrier is on the warm side of the insulation, and the wall at that point stays above the dew point, it should be good.
Here is "The Perfect Wall" - at least, according to someone using a lot of steel and plastic in his projects.  You can use the same ideas with natural materials - basically, if you already have a vapor barrier, don't add a second one.  EVER.  
If you don't have a vapor barrier, and you need one, it needs to go on the warm side of the wall.  https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall#Foot_01


Thanks for re-posting the pictures, it is lovely to see the whole thing!  
Please do let us know if you see any signs of heat damage, or if anything gets too hot to touch (near combustibles) when running the stove.  These would be important safety information for anyone who might want to do the same/ similar projects.

Yours,
Erica W