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Cost Estimating for Rocket Mass Heaters

Posts: 1289
Location: Okanogan Highlands, Washington
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I just got an email wanting a ballpark estimate for the cost of materials to build the Annex 6" rocket mass heater.

There are step-by-step building pictures up at, so you can see where different parts are used.

Ballpark estimates are tricky given that "purchasing everything" means different things to different people.

The foundation cost depends on what's already there; if you don't have an existing slab, this may be the most expensive part. We had an existing slab. Code calls for non-combustible supports from below frost on upward; Cob Cottage does this with 18" of compacted gravel, but your cost may vary.

The chimney cost depends on your house, and may cost more than the rest of the heater put together.
We did double-wall though the room, insulated Class A chimney (6" ID) from the ceiling through the roof eave (one story=about 6 feet of insulated chimney), and re-used some existing stovepipe that was on site for the last 6 feet or so. I think it came to about $1200 all told, and that could be double if you have a taller house or a colder climate (insulated stovepipe all the way up to the proper height, if bought new, would have added about $500).

The pipes, brick, and perlite for the bench probably would come to another $800 or so, at a very rough guess. Depends on local prices. We used reclaimed and recycled materials here, and paid less than half price for it all together.

Buying new, you'll need about 120 brick for the fire box - call that $250. (Fire brick varies from $1.25 to almost $3 per brick, depending where you get them. Red clay brick if you can find it may be $.60 to $1.25 per brick. I've seen used brick at gravel yards for as little as $.25 per brick, or free for the hauling at construction demolition sites.)
$60 to $100 for 25 square feet of refractory insulation blanket, or $50 plus time for a perlite-and-scrap-metal insulation column as described in the plans.

Barrels - I've seen anywhere from free (some paint and gick removal required) to $10 or $30 for used barrels (still requires paint removal) to $200 or $300 for a new, non-painted barrel. You can probably get a custom-fabricated 1/8" rolled steel cylinder with a steel-plate lid for about the same as the unpainted barrels, if you're going to spend that kind of money. You'll need some woodstove door gasket (flat ribbon, 1/2" wide, as thin as they offer) if you have a removable lid.

The thermal mass is the real wild card. I calculated it out at about 60 cubic feet total for this particular bench, and of course some of that is the brick and other materials buried in the bench.

We used mineral soil clay from the backyard, and a $100 load of sand, for our thermal mass. That plus about 6 people's labor for 2 days made the bench, and another 4 people helped us plaster it.
$100 plus 16 FTE days ($1600 value? Rates vary by skill level.)

The price for a different thermal mass could be almost anything.

For example you could use rocks and dirt from the backyard and pay nothing but time. I've built a 50-cf bench with natural backyard granite, and I probably put about 8 or 10 days' easy labor into it over the different weekends I worked on it. (I work slow, which is not always a bad thing when it comes to the open-ended 3-D puzzles of natural stonework.)

You might need to buy bagged clay at anywhere from $5 to $40 per 50-lb bag, and depending on the quality of the clay you might need anywhere from 5 to 20 bags.
I'd guesstimate 8 bags, and order 10 in case I'm wrong; after the first weekend, I'll know roughly where we stand.
(Our stickiest clay so far took only 1 part clay to 10 parts sand; our least-effective clay was about 1:2. The sand volume tells you roughly the final volume of the bench, so that's anywhere from 6 to 30 cubic feet of clay. Test batches are crucial here, especially if you're going to have to order clay from far away. Although in that case, I might suggest a non-cob facade that you can backfill with local mineral soil, instead of shipping clay long distances.)

So that's anywhere from $0 (site-sourced) to about $800 for clay.

If I really wanted to estimate the mass cost accurately for a project, I'd need to contact local suppliers and get cost estimates for delivered materials.
Take the the 60 cubic feet (over 2 cubic yards, so call it 3 yards for bulk deliveries), and call your local building supply or sand-and-gravel place.

If you want a non-cob facade (stone, brick, pavers, or a tile-board outer box that you can tile to match your kitchen backsplash), call the building supply.
The facade would come to about 30 square feet of facing material for the front vertical sides, another 30 to 35 square feet for the back sides (the ones against the wall in our original project), and about 40 square feet for the top.
I prefer stacking cob, brick, or rocks to make a self-supporting wall about 4" thick, and filling behind with cob, mortared rubble, mineral subsoil, or what-have-you.
Get at least 10% extra for breakage and cutting.

If you want to glue flat rocks or heavy tiles to a vertical surface, you'd also need some Portland cement mortar and some hardee-backer/tile board or a self-supporting masonry wall behind it to take the weight. We don't glue big rocks to cob; the technologies are incompatible, though some light tile or mosaic work tends to turn out OK. Anchor the tile board well back into the bench mass, taking care to avoid the pipes.

If you want cob, first check out your local fill dirt options. Good garden soil is bad building dirt, and vice-versa.

If you have "adobe soils" (poor drainage, tends to dry brick-hard and not crack much), you're in luck, you may not need to add anything.
If you have mostly silt, do you also have rocks? because fieldstone may be a better option than cob. See above for a non-cob facade, silt is not good for cob but may be included in non-structural backfill.

If you have mostly sand, and it's suitable for building (tends to compact down reasonably well, rough like sandpaper not velvety), you are buying clay or scrounging from local ceramic art studios. (Check with colleges and other places that teach pottery; they often have mixed bins of student "recycle" clay that are such a jumble of different firing grades and junk that they'd gladly let you scoop out any amount for a good cause, if you bring your own bins.)

If you have clay soils available but no sand, look for a sand and gravel place that can sell you either of the following:
- crushed rock fines (aka rock dust, quarry fines, 3/4"-minus road grade, pit run, or whatever fell through the screen when they pulled out the 1" washed rock),
- a masonry sand with a nice blend of different grain sizes and some sharp tooth to it.
In most areas, the delivery charge will be more than the cost of the material itself. Get more than you think you'll need - at least 3 yards. (There's always some waste.)

You can also include a couple bags of fine sand for mortars (under $20).
You can throw in various rubble and gravel and so on if they are already on site, but I probably would not order them.

A bale of straw is about $2 to $10 depending where you are; one bale is usually enough for the casing if it's a full, tight, 2-string bale (about 80 lbs).
I have never yet had to pay for horse manure (fine fiber for plasters), but if you decide you want to weed-whack and sift the straw instead of letting a munching horse do that job for you, allow another bale of straw and day or so of manual labor for that process.

What's the upper limit?
There really isn't one.

You might decide to build most of the bench out of premium soapstone bricks, a dense and lovely heat-storing material, and one of the few masonry materials available in greens and blues if that's an important aesthetic consideration. At 2012 Sacramento prices of $15 per facing brick (9" x 4.5" x 1 to 1.5"), that would come to about $700 per cubic foot, or about $7000 for the front facade (built 4.5" thick) and top surface only. (You'd still need 50 cubic feet of infill for the rest of the bench.) If you got really silly and decided to slice up facing bricks for infill, you could spend $42,000 on 60 cubic feet of soapstone thermal mass. And your bench would be maybe 4% to 20% more efficient as thermal mass than a box of free dirt (well-packed clay dirt, anyway). Clay brick and soapstone are about equal in heat-capacity per pound, and depending on how tight you pack it, clay-bound earthen masonry can be as dense as brick, or a bit lighter.

There are also decorative tiles made for kachelofen that run about $60 per tile. Each tile is a little bigger than 2 soapstone bricks in surface area, except the corner tiles. So that's maybe $6400, plus or minus, for a tiled bench skin.
(Given that the surface tiles aren't handling flue gases, you don't really need kachelofen tile, though it would be pretty. You can also tile mosaic the surface with just about any non-combustible tile that suits your fancy - estimate about 40 square feet for the top surface, 40 linear feet of corner tiles for the edges, and 60 square feet for the vertical surfaces front and back. I found stone floor tiles from $3 to $10 per square foot on a quick online search, so that would be something like $1,400 for all-new flat tile, not counting the backer board, anchors, and grouts.)

So finish materials could be anything from
- site-sourced earthen plasters, maybe spending $75 for pigments, binders, pizza and beer for your plastering friends, and a booby prize for whoever brings the horse poop;
- $200 for a final coat of lime-plaster with a tile detail;
- $500 for a brick facade;
- $1500+ for tile (up to $8000 for fancy tile).

So what does that all come to?

As I recall, we put about $1500 into the original bench, for new and recycled components. We collected much of the material at scrap prices through repeated trips to the ReBuilding Center and ReStores. Old red brick, clay soil, and a few pieces of stovepipe were already on site. We purchased new chimney parts, a dump-load of masonry sand, and odds and ends like perlite, pigments, straw, foil tape, screws, etc. I would guess that the chimney itself cost about $1200 of this, and the rest of the bench came to about $300 or $400 altogether. If I'm forgetting something, I would still guess it was under $2000.

I think you could easily spend $4000 to 5000 if you purchased all-new everything, including all-new masonry materials without shopping around much, instead of using site-sourced or local fill dirt.
Double that if you wanted to get fancy with premium facades and finishes.

Some of the more expensive materials save on labor costs (for example brick and mortar involves a lot less mixing time than cob), so if you are paying for labor that's a consideration.
Expert help costs more per hour than unskilled labor, but unskilled labor wastes more time. Tools and tool rental also cost; skilled contractors generally bring their own.
One client who hired most of the labor (including hiring us to run a couple days' work-party for volunteers) spent about $9000 on their project for both materials and labor.
Another couple spent about $5000 on parts and labor, and did most of their own finish work.

The original inventor proudly claims he built one of these heaters for under $50. Please understand that this is not an estimate - it's a boast for a personal best, coming from someone with a lifelong dedication to spending time instead of money.
He teaches all visitors to rummage through dumpsters, scrapyards, and roadside resources on their way over. He has a salvaged-materials yard that's almost as big as his garden, sorted by type, with sheds for the delicate stuff.
I know he occasionally shells out money for perlite, and sand deliveries since his site is almost all clay. Whether the sand comes from a yard or a river bank, transporting a truckload has got to cost something.

He does use rose-tinted glasses for cost-estimating. If the dumpster is on his way home from somewhere, the gas doesn't count. His time doesn't count. His students' time doesn't count - or may even be a source of income. And he builds low, non-combustible buildings with membrane-lined roofs, so his typical exhaust chimney involves about 3 pieces of rusted-out single-wall stovepipe. Etc.
Working on their projects, we have had to fabricate T's from old straight sections and paint cans, or hand-sculpt an elbow from broken bricks. I have seen innumerable "mysterious failures" of his style of heater in a taller or leakier building, that just as mysteriously cleared up once a proper chimney was installed. So I would not consider the inventor's lowball personal record as a realistic estimate for someone who wants to buy new parts and get the job done quick.

However, if you're thinking about building a heater someday, it's worth starting a collection of parts now. Get to know your local scrap metal haulers, post wanted ads on Craigslist, stop in and leave a will-haul note with cell number on the piles of brick or rubble that you'd want from any local demolition projects you happen to see. If you start a few months ahead, even a year ahead, you will probably be able to offset your costs substantially with minimal effort. In fact, start sourcing free firewood at about the same time, get it into a dry woodshed, and it will be ready to burn by the time you get around to building the heater.

You can see why we don't publish any particular number.

The folks who want to salvage a bunch of scrap and build it cheap would be daunted by the higher numbers.
If you have an existing chimney you can connect to that's in good condition, and minimal foundation work (e.g. an existing slab), you probably won't build it for $50, but you could easily come in below $500 if you are a reasonably good scavenger.

The folks who buy all-new parts, use off-site masonry, and especially those who hire expert help, would feel cheated by a lowball estimate that didn't pan out.
Depending on your house, the type of foundation and chimney work needed, and how fast you want the project completed, you could easily spend $10,000 without being frivolous.

For comparison, I've heard of people getting a masonry heater installed by a certified masonry heater builder for between $14,000 to $20,000 for simple brick-finished models, and upwards of $100,000 for larger and fancier works of art.

I'd love to hear from other people who have built these DIY style, especially if you're the sort of person who likes to track costs accurately.
I'd also like to see the lowest (safe) installation cost for a reproducible heater.

Erica W
Erica Wisner
Posts: 1289
Location: Okanogan Highlands, Washington
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Oh, and, um, the reason they asked about this heater specifically is this is one of the plans we sell. Can you believe I completely forgot to put any link to it in the post?
You can also see some sample pages including a cutaway diagram, without buying anything, so you might find the link interesting for purely non-commercial reasons too.

-Erica W
Posts: 227
Location: US, East Tennessee, north of Knoxville
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Excellent info. Erica, and well written too. Have you considered adding this to RMH Builder's Guide? Perhaps in the appendix?
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excellent post! thank you for the time and information.
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It's wonderful having you here Erica...thanks for such an indepth post!!!

Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Bump !!! - and a Tip of the Hat to Erica W. Big AL
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