Josh Warfield

I built a kiln roughly following this design. Mine is maybe 1.5-2 times the size, and I used rebar for the grate because I'm not as much of a purist as the guy in the video.

Turn CC on, he puts descriptions of what he's doing there instead of talking.
https://www.youtube.com/watch?v=wbc443KVWfI&t=91s

The hardest thing about it is not accidentally firing too high or too quickly, which can cause the pots to crack and/or melt. I have found that it's a bit easier to control if I limit both the air intake and output (I just put a metal trash can lid partially over the top of the chimney, and another scrap piece of metal in front of the intake). I have a thermocouple stuck through the door and I watch it constantly throughout the firing, trying to stick to my intended schedule of target temperature. Fiddling with the airflow can have counter-intuitive effects on temperature sometimes, but eventually I worked out a rhythm for it, and I can keep the temperature slowly rising without too huge of swings when adding fuel.

Low temperature glazes are tough to make from raw ingredients, but they are available commercially. There are a bunch of technical reasons that higher temp glazes are much more common. Also there are plenty of practical uses for un-glazed low-fired pots.
https://digitalfire.com/glossary/low+temperature+glaze
https://digitalfire.com/glossary/earthenware
https://digitalfire.com/glossary/terra+cotta

humans have been living in caves for centuries without ventilation

This is more or less my attitude toward most things that are considered "unsafe" in the modern day, but nevertheless have always been done. Just about everything will give you some sort of cancer, it seems, if you pay attention to every study that gets published that shows a 0.1% increase in risk. On the other hand, people also ran all their drinking water through lead pipes for centuries, but there's no good reason to do that now that we know better.

For me, the radon issue is in the second category, mostly because of my location.

Maybe I'm searching with the wrong terms, but I can't seem to find that type of heat exchanger for sale anywhere, except for on manufacturer sites that say to call for a price (which I know means it's gonna be expensive). But I did find some DIY instructions that don't look too hard: https://makezine.com/projects/heat-exchanger/

You know I just realized I hadn't actually checked the price on getting my own radon measurement device. I just assumed that requires expensive professional equipment. But it looks like there are household radon detectors and they're not that expensive, maybe test-as-I-go isn't as impractical as I thought. https://www.homedepot.com/p/Airthings-Battery-Operated-Digital-Radon-Detector-2350/313741641

Patrik Schumann wrote:There's no escaping some actual homework, scratch estimation, trial & error, in getting there.  And you can always test & adjust after the fact.

Oh for sure, I'm not expecting some engineer to come in here with numbers, when I haven't even given a square footage or anything to start that estimation with. From the bit that I've read about radon testing though, it's not necessarily a quick and easy thing. Measurements will vary day to day, so borrowing or renting a device for a short period might not give you an accurate answer. And pre-construction measurements are not even that reliable for predicting as-built conditions, either, both because of that natural variability and also because the accumulation of a gas in an indoor space depends wildly on how that space is constructed.

I'm just hoping someone can point me in the direction of something I can use in my planning, not asking for easy answers straight away.

Patrik Schumann wrote:you can design for & set up natural ventilation flows that favour desirable air temperature, humidity, quality/ health levels.

Right... but I'm not stoked on signing up for the second highest risk factor for lung cancer. Radon comes from the decay chain of Uranium, and I live just a few miles south of the area formerly known as the U.S. Strategic Uranium Reserve. The specific layer of sandstone they were planning on mining was the Salt Wash Member of the Morrison Formation, which is the exact layer that forms the ceiling of my tunnel. Un-refined Uranium itself is not particularly dangerous if you're not eating it, but it generates Radon, which is a gas that you can breathe, and a proven carcinogen.

If you're wood firing then the challenge with a hot tub is more on the insulation side than the fuel side. Rocket stoves excel at using fuel efficiently, but that's an entirely separate thing from keeping the hot thing hot. As others have noted, it is very easy using wood fuel to get your water up to hot tub temps, and even well beyond that into unsafe territory. No rocket stove required, just burn a campfire under a tub for a few hours. But without insulation, when the fire goes out the water will pretty quickly go back to uninspiring bathtub temperatures, assuming you have a reasonable volume of water for the typical hot tub. To hold temperature for a few days like you're hoping for, you'll need very thick insulation, but really that has nothing to do with a rocket or non-rocket stove. And as others alluded to, there is no need to worry about the "mass" part of "rocket mass" here -- the water itself has significantly more thermal mass per unit volume than any building material you're likely to have heard of. If the specific goal is to increase thermal mass, then instead of building thicker walls you'd be better off increasing the volume of water. But the temperature difference here is just too much for thermal mass alone to be enough; you need insulation for this to be practical.

Benjamin Dinkel wrote:Could you get the radon levels tested and take it from there?

The EPA's info sheet says it's probably not even cost effective to do the testing ahead of time; basically if there's any question about it you should just install the mitigation measures. https://www.epa.gov/sites/default/files/2014-08/documents/buildradonout.pdf#page=22.09

Benjamin Dinkel wrote:Too much air exchange will be super uncomfortable, bring in moisture and/or dry everything out. Also it will heat the cave up in summer (and cool it down in winter). So you need just enough air!

Yeah, construction worksite levels of fans running 24/7 would definitely make this place even drier than it is already. Whatever ventilation system I end up with, there would have to be a control knob somewhere to turn it down if needed. I just hope that doesn't come with the trade-off of increased lung cancer risk over time...

Trace Oswald wrote:I would like to do this on my land, but I am worried about cave-ins, and I don't know how to protect against that.  I would think not having wide flat expanses of ceiling would help, but anyone have any thoughts about this?

I looked into this just a bit, figured it's good to know just in case, and I found two sorts of information. One sort requires a civil engineering degree to fully understand, and assumes a professional construction context where you have heavy equipment, dozens of crew, a budget at least in the hundreds of thousands, and access to a geologist or someone who can professionally assess your specific site. The other sort is complete amateurs just posting their opinion, with some "common sense" explanation that may or may not hold up.

I'm certainly in the amateur category, but from the reading I've done on earth and stone based construction, support from a wall or pillar sort of extends up at an angle, naturally leading to the kind of arched shapes you normally see with these materials. So if you dug a tunnel with a flat ceiling, it would be liable to drop chunks of material out from the center until the roof became an arch shape. So you should just dig some sort of arched shape to begin with, unless you're planning on building wooden or concrete supports the whole way.

This guy talks like he drinks a 12 pack of red bulls a day, and he's definitely optimizing for the youtube algorithm, but tons of people recommend him for info on one-person tunneling techniques: https://www.youtube.com/@Askjeffwilliams

I'm currently in the process of digging a big hole in the side of a hill. Sometimes, in order to make the project seem more approachable, I tell myself it's just gonna be a root cellar. But really, in this summer heat, all I'm thinking about is how nice it would be to go hang out down there during the middle of the day (without the dust mask and safety glasses and earplugs and whatnot, that is). I haven't started on house construction at all yet (I've got a house on wheels already, so it's not my top priority), so I'm starting to seriously look into what it would take to turn this hole in the ground into my primary dwelling.

There are some major advantages of this approach, compared to any method of above-ground construction, given my situation and priorities. I'm digging through a material that's somewhere between sandstone and compacted clay, directly underneath a thick solid sandstone layer. So I'm not terribly worried about the risk of collapse for modestly sized tunnels or rooms, even without any shoring. That means my budget for wall, floor, and roof materials is potentially $0. But there's precious little information I can find on this method of "building" a house. There's a few pages about it in the Permaculture Design Manual, and that's good for inspiration but not nearly enough info to be taken as a how-to guide. I can find a bunch of articles online about Coober Pedy, but nothing much more detailed than "wow look at these people living underground."

Currently, my one big question mark is ventilation. I live in an area with high radon risk, so I definitely don't want to cheap out on this. But when it comes to actually sizing the system, I've seen recommendations ranging from <1  to 30+ air changes per hour. Regulations for mining (which seem like they should be somewhat more applicable than home building guidelines) dictate a minimum CFM/person that's way higher than that (of course they're doing stuff like running diesel engines underground, so maybe that's not exactly a great reference point either). So on the low end, we're talking about some 4 inch dryer ducts and a $50 fan, but on the high end just the fan alone will cost over a thousand dollars, and the larger ducting starts to add up too.

Can anyone share their experience with larger-than-a-root-cellar underground construction? Or point me to any relevant resources? Any big pitfalls I'm missing, besides ventilation? Building permits aren't a thing in this jurisdiction, so I'm just asking about actual safety or practical concerns, not red tape.

Timothy Norton wrote:I was just talking about pickles with some family and had the epiphany that my favorite pickles always have grape leaves in them! They seem to help retain the ‘crunch’ of the cucumber through the pickling process.

I've heard this about grape leaves, but also I've found black tea to be effective. According to the reading I've done, tannins are the active ingredient here. If that's the case, then acorns would be another good source of tannins which are much easier to come by in most climates in North America.