RMH Floor Insulations

Hi! I've been reading about RMH heaters for a while after catching one of Paul's videos.

I'm planning to give one a shot in the basement of my home, alongside the wood stove. I'm curious to hear about anyone's experiences with concrete floors, and either insulating (or not) between the RMH and the floor. I've seen comments saying that all your heat will get pulled out to the earth if you don't insulate. The idea of having a heated floor is actually pretty appealing since the basement is part of our regular living space, but not as important as heating the house. The foundation is poured and UNinsulated as well, so I figure even radiant heat in the basement will get drawn out by it, making insulation between the heater and floor irrelevant anyway?

I have about 35' of 8" stovepipe and a pile of cinder blocks. The plan for now is to box them in and fill with pea gravel. Chimney for the existing wood stove is only 6" but is really tall (15'+) so I still anticipate good movement for exhaust gas.

Nick, that's a great experiment. I'm looking forward to your reports on the pea gravel RMH.

Hi Nick, It is always worth experimenting but  If you rest the core on a concrete floor it will act as a heat sink and stop the fire box and tunnel getting to high temperature.

If you lite a fire inside a concrete  box, the heat will be sucked into the concrete and eventually radiate from the outside at a lower temperature but, if you insulate the box the heat will stay inside the box making the inside much hotter so full combustion can take place.

So if you build a rocket stove using a fire brick box, you would need to insulate the back side of the bricks, the fire will first have to heat the bricks but the insulation will hold the heat back and stop it radiating into the air.
Now if you have a concrete floor  the fire would have to saturate the whole floor before it could start heating the box to maximum potential.

Pea gravel, like sand will not be a good dense mass, there will be air trapped between the individual gains and air is a very good insulator.
Unfortunately concrete blocks are not very good either for the same reason, some blocks may be more  porous than others but as a rule concrete blocks are full of air.

The best mass to use is solid material that has no trapped air like rock or vibrated concrete.
So on its own, pea gravel  wont be able to absorb  heat or store heat very efficiently  but, if you add sand and cement to the gravel it would become a solid dense mass.

In reality most rocket stoves only hold a small fire in a small box but if they are constructed in such a way to get the very most heat possible, then the rocket stove works very well.
However if we move away from known specifications the stove is just a small fire inside a small box!

Fox James wrote:If you rest the core on a concrete floor it will act as a heat sink and stop the fire box and tunnel getting to high temperature.

If you lite a fire inside a concrete  box, the heat will be sucked into the concrete and eventually radiate from the outside at a lower temperature but, if you insulate the box the heat will stay inside the box making the inside much hotter so full combustion can take place.

So if you build a rocket stove using a fire brick box, you would need to insulate the back side of the bricks, the fire will first have to heat the bricks but the insulation will hold the heat back and stop it radiating into the air.

Now if you have a concrete floor  the fire would have to saturate the whole floor before it could start heating the box to maximum potential.

So on its own, pea gravel wont be able to absorb  heat or store heat very efficiently  but, if you add sand and cement to the gravel it would become a solid dense mass.

Thanks for the input!

I’m planning to construct the burner from ceramic fiberboard and firebrick with a 5min riser. The cinderblocks are for holding up the stovepipe, the pea gravel will go over top of the pipe and filter down around the whole setup. I’ll def add sand over the gravel to increase density and will probably end up trying to create a cob outer layer to retain the mass instead of using a wooden box.

I think with my stove design I’ll be ok on getting temps up, my big concern is how direct contact between the 35’ mass and the floor will impact heat retention and distribution as a joined mass.

You can create a heater base by laying full size bricks on rowlock to create a grid and fill the spaces of the grid with broken glass . This technique was traditionally used when building bread baking ovens for bakeries.

Fox James wrote:Pea gravel, like sand will not be a good dense mass, there will be air trapped between the individual gains and air is a very good insulator.

So on its own, pea gravel  wont be able to absorb  heat or store heat very efficiently  but, if you add sand and cement to the gravel it would become a solid dense mass.

Adding "sand and cement to the gravel" sounds very confusing to me, what are you suggesting? The use of 'urban crete' or whatever?

Sand and pea gravel may both be granular with air in between, but have notably different characteristics when used as the mass in an rmh. The fisher price RMH when filled with sand was judged too insulative by Ernie, Erica, and Paul, but the pea gravel alone works very well.

I keep hearing mention of filing gaps in pea gravel with sand but can't figure out how that would be significantly different from just sand...

Ok sorry, that was a little tongue  in cheek …. Gravel, sand and cement = concrete.

Well yes it does seem like  pebbles, sand or rocks filled with clay are all commonly used but a solid mass will be more effective at holding heat.
That is not to say some folk might be happy with the performance of pebbles or gravel.

There could be other factors at play like how long it takes to heat a box of solid mass compared to how long it takes to heat a box of pebbles, perhaps a small house with lots of insulation does not need the very best heat storage?

I build and install pizza ovens, this type of oven relies on effectively and economically  storing heat for as long as possible.
Wood ovens have been around for thousands of years and have been extensively tested, so we know how to get the best from them.

Rocket stoves are still in development, new ideas and info is appearing all the time, there are very few set standards, even if a stove has a set design it wont have been tested with every conceivable  amount or type of mass or have the same chimney or the same amount of house insulation as the next one.
My point is a box of pebbles might be quite adequate but not necessarily the most efficient.

Yes, when discussing gravel fill people who have worked with it talk about what percentage of efficiency for storing heat they think it is compared to cob. They would have used cob in the FPH if the floor could support the weight.

But my point is the sand didn't make the mass work like a dense material. While pebbles don't store the heat as well as many materials, they do allow more to escape from the ducts and convect into the room, where the sand traps and pushes the heat out the exhaust. I think filling a box of gravel with sand will create more tiny insulative pockets of air and block any convective release of heat into the room almost as much as pure sand will.

The subject of sand as thermal mass for the RMH is a weird one.  The popular consensus is that sand makes a poor thermal battery, yet researchers are spending loads of money on thermal sand batteries for the purpose of heating water and whatever else.  Maybe we're just not using the sand in the right way.  At any rate, something is definitely off here and worthy of further research.

You peeked my interest Thomas and so I looked up
thermal sand battery
Indeed, an interesting subject due to its simplicity and therefore potential in using it as a mass material for an RMH. Perhaps the pipes they flow through the sand to transfer the heat to it are close together so the insulating aspect of sand is mitigated? Need to research more.

Gerry,
Thank you for taking the initiative and sharing that link.  I ran across this tech some years ago and had no idea it had matured to this point already.  So, they are using hot air to heat sand.  Sure doesn't seem too far off from what we do here.  It would be great if we could collect anecdotal evidence from everyone and anyone who has actually tried using sand as a thermal mass so they could share their experiences and maybe we could come to a conclusion about why it is we think it doesn't work in the RMH world.  Maybe it really is as simple a matter of mass density v.s. the enormous size of something like what Polar Night Energy is constructing?    I think I remember Ernie Wisner saying sand wasn't any good for mass due to the tiny air pockets.  Hopefully someone with experience will chime in.

One analogy would be sand on a sunny beach….. at day break the beach sand is cool, no more than abeyant temperature, by mid day, on a hot sunny day, the same sand can be very hot on the surface, almost unbearable to walk on with bare feet because the sun has been beating down on it for 5 hours but…  all you have to do is push your hand down a few inches and the sand will feel cool.

Or on the same beach there are  pebbles  made from very hard and dense stone, they are so hot under the tropical sun that you can only just hold one in your hand like a hot potato but, dig down in the sand just a few inches and find another pebble , it will be much cooler.

Or … I catch a bus first thing in the morning, the bus stop is alongside a large south facing granite wall, even though the sun has not touched the dense granite for 12 hours the granite is still warm to the touch from yesterdays sun.  

If we get the same sand and pebbles and mix them with cement, then vibrate all the air out of the mix, once dry and hard we will  get a very dense mass that will hold heat for a long time even though we are using the same material… just without the trapped air.

However… perhaps, if we have a box of pebbles that allows rising heat to flow through the box, the individual pebbles will get warm really quickly and still allow hot air to warm the room but….  the individual stones wont  get very hot because the rising air is just going around them and being dispensed into the room rather than being stored.

Two points about sand:
1) the Wisner RMH book has a list of a dozen common materials in order from best heat storage to best insulative. Sand is the next one after dirt/Pebble fill. The differences in our usages are based on a relative comparison. Overall they both fall near the middle of that scale.

2) sand itself has varying qualities. Beach sand has been buffed into perfect little spheres by an onslaught of endless waves. Other sand maintains more of a platelet type structure that can give it qualities closer to clay. My area has been known for a special type of sand that is good for frak mining and corporations pay to dig it up and ship it to mine sites. I wonder if less common types of sand have characteristics that would move them past other substances in either direction in that list...

What weights more, a ton of sand or a ton of concrete?

Which one stores the most heat?

Which one releases heat faster?

Which one takes the longest to heat up?

I'll be using a sand heat sink with my Rocket Stove.

Sand or gravel usually cost money.
If an enclosure is being built anyway and there isn't a problem with weight, why not use earth?
Cob takes some skill and prep, tamped earth, not so much.


To keep the floor from sucking up the heat, build your bench on a platform.
Cement board layed across bricks or just cement blocks laid on their sides should do the trick.

William Bronson - It seems your response is the most direct toward the question and I have the same question.  I'm getting ready to do my first RMH build and want to put the RMH in my basement.  I'm in SE Michigan and have an endless supply of very fine clay on my property.  My concrete floor is on that same clay and could soak up a lot of heat, making it tough to harvest the heat for my bench.

I've thought about using the pink foam insulation board between my basement floor slab and some concrete board for the foundation of my RMH.  I'll probably need to use a layer of brick/ block on top of the concrete board for the base layer and then start building my J-Box out of fire brick.  The pink foam insulation board has a compressive strength of 30 psi so it should be able to bear the weight of the stove.  I am concerned about making the base of the RMH too high above the floor to where my bench would need to be uncomfortably tall.  

Back to the original question - what are your opinions of insulating the RMH from the concrete floor?  Best methods?  Materials to avoid?  

Additional question - when ducting the bench, what are the "rules" for ducting at/ below the elevation of the horizontal burn tunnel?

Dan Henn wrote:
Additional question - when ducting the bench, what are the "rules" for ducting at/ below the elevation of the horizontal burn tunnel?

I've been trying to Crack this nut and haven't found a hard and fast rule. But it seems there are two areas of concern.

First, can the exit from the manifold be at any level? The most common design takes the gasses exiting the riser and falling around the radient chamber down close to the burn tunnel level where the manifold transitions them to the ducts. I have yet to see a manifold below the tunnel, but haven't seen anyone advise against it either.

Second is the elevation of the ducting run. Erica Wisner prefers to make sure the entire run gradually rises. Not sure if this is a matter of avoiding hot spots or helping draft or what.
But at one point the tipi RMH was redone to reroute the ducting so that it dropped down at the end, ran under the walking surface, and surfaced next to the radiant barrel where the vertical exhaust ran up and out the top. Despite adding length, dropping down at the end, and running through the ground, it was reported to draw better this way. I plan to stay there and try it out next time I visit.