Underground dwellings and ventilation

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.

That sounds super cool and exiting.

Here in Andalucia there's tons of cave houses, most without ventilation.
But then there's no radon (that we know of here).

Could you get the radon levels tested and take it from there?

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!

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?

Sorry I don't have any information that will help about your ventilation issue.

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

In liberating an old thermal mass house with crawl space from evaporative cooler & floor furnace, I ended up closely managing ventilation all seasons whether between rooms or inside-outside.  No longer needing all that fresh cold air underfoot for ill-placed thermostat heater, just a little combustion replacement for the modest central woodstove, in winter to greater comfort I closed off all but one vent below.  Eventually local authorities & media promoted a free radon testing kit program, & I discovered it did just reach threshold levels only below if not actively vented.  I figured my winter-long daily wood-burning kept enough dilution going.  There are ancient traditions of underground buildings in China, Turkey, Tunisia, & I did a my first study  in architecture grad school of their typology on passive & low- energy criteria.  With different spaces, surface-to-volume ratios, passageways, outdoor connections, seasonal & daily orientations & exposures, you can design for & set up natural ventilation flows that favour desirable air temperature, humidity, quality/ health levels.

I've never stayed in one for more than an hour or so, but when I lived in the Outback I visited a couple of underground houses in Australia in a similar opal mining area to Coober Pedy.  They didn't seem to worry too much about ventilation in the small houses. There were no internal doors and the main shaft into the house was enough. Bigger houses with deep inside rooms incorporated some air vents to the surface. No air pumps or anything I saw, everything was simply left open for natural ventilation. Maybe a simple fan.

But the situation there is very different. It doesn't get that cold in winter, and there's no radon issue in that area, as far as I knew. So their methods are probably not relevant to your land and your underground rooms.

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.

You're going to be doing some kind of ventilation, so might as well have as much natural passive free as possible & supplement with artificial as necessary & desirable.  Nobody's got your answer just like that.  There's no escaping some actual homework, scratch estimation, trial & error, in getting there.  And you can always test & adjust after the fact.

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.

Radon poisoning is a cumulative exposure moderated by productive surface area to enclosed volume ratio to nr of air changes.  I got two free test kits from the state of New Mexico: left one enclosed in crawlspace & other in winter livingspace for the week or two specified.  You could likely find or bore suitable placements.  They would give you an early ballpark proportional indication before risking going any further.  Also, talk to old miners & check any mineshafts in same strata.  

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

One thing that helps a lot with both passive and active ventilation systems is a heat exchanger that gets the incoming and outgoing air as close as possible to the same temperature without mixing. This can help with humidity control as well.

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/

Erv energy recovery ventilator.

This reminds me that humans have been living in caves for centuries without ventilation ...

Josh Warfield wrote: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/

I think that would be more than adequate for your situation. A couple of small CPU fans could run 24/7 off a small solar panel with a battery and simple charge controller. Might even build one of these myself....

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.

Josh - what an awesome project! I hope it all works out for you. I would be inclined to build a bit and then think about the ventilation once you see what happens naturally.

If you haven't seen this video about Baldassare Forestiere's underground homestead in Fresno California it is well worth a look:



He built almost an underground village in what appears to be a bit similar rock. The courtyards and ceiling holes give natural light and ventilation. You'd have to think about your climate differences and what would be appropriate though.