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Rocket stove with mass under mobile home

James Lewis

Joined: Nov 27, 2013
Posts: 2
Location: Kentucky

I am looking to place a very large cob mass under a mobile home with the vent duct from a rocket stove to heat it. Before doing this I would like to know what temp radiant heat could I expect at the furthest end of the mass and if this would suffice as a winter time alternative to the indoor wood stove I currently have. The plan is simple, build a rocket stove at one end of the trailer with the exhaust pipe running the entire length and back to the stove and vertical at the stove. Fill the entire area 12' wide x 65' long x 3' tall with cob material then surround the entire mass with insulation. Before I do this I would like some costing help with the amount of cob material I would need and to know if this would be a viable heat source for this mobile home. Any help would be greatly appreciated.


allen lumley

Joined: Mar 16, 2012
Posts: 3871
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
James Lewis Welcome to and our sister site You will take the prize for the most original, and out-of-the-box thinking, and will be sure
to stretch the mind of your 20,000 fellow members here at permies .

So- have you actually measured the area under your trailer, to make sure that you do not still have any axles and brake drum/rim housings under there? With an 8'' system
you should plan on no more than 40' of pipe in total plus Elbows thats 20' u-p and 20' back, for purposes of this conversation the vertical chimney while 'free' is vital !

This is more than adequate to provide all the heat your trailer should need ! At 20' by 1.8' by 3.33' we have 120 cubic feet divide by p to get cubic yards rounding up you get
14 cubic yards total just for the mass under your trailer as about 20% of this mass will be clay call that 3 cubic yards, You want sharp or masons sand, so you can get a quote
on 11 cubic yards delivered, plan out ahead of time where you re going to stack it and cover it up with a tarp, you could get very lucky and find a local excavation contractor
who has to pay to get local clay trucked away, and will deliver it to you as clean fill free to you, or not If you have to get pay to get local clay trucked in it may be as much as
the the total cost of the sand, plan where you are going to pile it and cover it well with a tarp, a heavy rain could wash it away and start a fight with all your neighbors ! Now
price the 8'' pipe 50'and a storm cap ! (3) 8'' elbows and (2) 8'' 'T's with caps, 120 lite weight 28-30 ounce fire brick, @ $1.50 to $2.00 a brick, get (2) identical 55 gal drums
with removable tops held on withy a clamping band $10 -$40 dollars and you have almost all of the materials, getting some urbanite no thicker than 5 '' as more Clean fill will
allow you a small safety margie on sand and clay and should be free, plan where you are going to pile it !

Now you are ready to gut the underside of your trailer, there is a heavy weight builders felt to cut a way, then most of the insulation, because you don't want the heat of the
Cob thermal mass to be blocked from reaching you by the original insolation !This will expose your wiring and all your plumbing and the original Furnace duct work, you can
rip that out as you need the space of the furnace to set up your Rocket Burner base and barrel !

Now you can try doing a test layout to see exactly where every thing will go before you ever light your first fire, and that one will be out doors for safety !

I hope that you have a poured cement pad in place to receive the Cob Thermal Mass, and I hope that the concrete pad was protected by a plastic waterproof membrane, this is
a safety issue as the first couple of feet of Horizontal pipe could get hot enough to cause unprotected and damp concrete to spall do to the high heat load, building in the Thermal
barriers to keep the heat from radiating into the soil just as much as up into your trailer can be done with Paver bricks and air gaps under a shallow 'bowl of Cob holding your
packed insulation of 3-4 inches of non agricultural perlite price 3-4 bags of that too!

Now its time to start training the team to make the Cob up for you, cement mixers are not strong enough and can only be used to mix the dry sand into the dry clay then that
material will have to be placed on tarps, about as much as a slightly less than full duffle bag and water slowly added and rolled until well mixed, caution, for the unprepared this
gets old fast, and people trying to use brute strength instead of finesse, will fall out with strained back muscles and spasms, One or more people down with strained muscles will
quickly spoil a whole crew ! you can feed your crew but keep them out of the beer until the project is done, fortunately because this project does not have to look nice it should
take no more than 4-5days, as you will be crouched under your trailer nearly the whole time pace yourself ! Without any complications you should be ready to repack new
insulation around your Cob Thermal Mass and Build your Rocket By now you are probably working alone so 2-3 weekends with a lot of cut and fit to both jobs !

Any problems come back here and we will do our best to diagnose and fix them from our remote location ! for the craft! Big AL

Success has a Thousand Fathers , Failure is an Orphan

James Lewis

Joined: Nov 27, 2013
Posts: 2
Location: Kentucky

Thanks for the very detailed explanation and for welcoming me to this fantastic site. It would appear to me that a dual rocket stove system may be more beneficial as the 20' limitation may lead to cold spots which will affect the overall heat of the structure I think. Perhaps if I were to place the two stoves in the middle and run the exhaust for one left and the other right, I could build the fires at the same time and at the same location, this would spread the heat further into the overall mass and lessen the cold spots in the mobile home. Luckily this mobile home already has the duct work removed from a previous owner when they replaced the flooring so that's one challenge out of the way. I will definately under the mass with bricks and a moisture barrier which will also lessen the need for the amount of cob material to pack under it. I have never built this type of stove before but I see it's benefit and I will definately post how this build goes and how well it works after it is complete. I will have to wait until spring to start on the project but doing the homework now will make it easier. I think for others with a mobile home this would solve a few issues such as the ones I already have. My mobile home is an older 1970 model which is unable to be insured as a mobile home due to it's age, however with a solid foundation underneath, it now becomes a home and not a "mobile home" which makes it insurable. The other issue this will help with is that I can use the cob material as the underpinning which will prevent those cold air leaks associated with mobile homes. Not to mention it will prevent the critters that seem to make their way easily into mobile homes. I wonder if it would be possilbe to run a coil from the cold water inlet to the hot water tank into the cob material below and then back to the top of the hot water heater with back flow preventer, this could potentially preheat to a certain temp the water prior to entering the water heater. OK I'm rambling but again thank you so much for the information and I will do my best to post here how the build goes.


allen lumley

Joined: Mar 16, 2012
Posts: 3871
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
James Little : I promise the size Thermal mass I gave you a written description of is more than enough to heat your entire place- IF insulated from the concrete pad and then
sealed to the bottom of your trailer,and insulated again! Think of it like a stationary engine, its nice to build in extra horsepower , but how much are you ever going to need
setting in the driveway ! Designing to your needs plus 10% should be all the safety margin you will ever need !

It probably would be a lot better to actually build on a combination shed roof Entrance, Mud room and Wood Storage Area, at the finished grade level of your concrete slab and
then bring your exhaust back to that point for your mandatory vertical chimney, It will be simpler to do a good chimney at that location than to do one up thru your Trailer roof!

Also, the insurance and other Powers that be are a lot less nosey if the wood stove seems to be in an addition and not in the Trailer !

Alternatively you could have a 40' straight run and increase the length of your core ! (and lose a couple of elbows and 'T's and have a remote Vertical chimney !

Your next task is to learn a little more about the parts of, sizes and ratios of the parts that make up a Rocket Mass Heater RMH, goto> and order the
downloaded PDF Copy $15 (or the physical book that will come snail mail ) of Ianto Evans' Great Book 'Rocket Mass Heaters'. With ~ 100,000 RMHs ~ built world wide,
most were made guided by 'The Book' and 95% of all the First time Builds (that worked!) followed 'The Book'!!!

This is the only sales pitch you will get- and I do not get a single dime or any commission from the people at or!
Continue to come back here and learn as much as you can between now and next spring, be prepared to make several practice RMHs in your yard before you attempt your First
build !

DO NOT accept anything you see on Facebook, or You-tube that has not been peer reviewed here at, as there is so much Ignorant Crap out the in U-Tube land !
And ALL RMHs burn at temperatures to hot for any steel to survive at its core, even stainless steel! For the good of the Craft ! Big AL!
I agree. Here's the link:
subject: Rocket stove with mass under mobile home