I am trying to develop a nomad population of human beings, living in a steppe like environment without modern technology, which for plot reasons don't use fire for keeping their refuges warm.

I was thinking that they could use a bed of mowed grass with suitable thickness, possibly adding new grass when needed, which they would lay on the ground of their refuge and that would develop heat via fermentation.

Through the researches I have made I have found that:

  • fermenting non dried hay might result in self-combustion of the said hay
  • fermenting dried hay can reach up to 60 C of temperature
  • fermenting hay develops asphyxiating gases, like CO2 and methane

Considering that during the cold season drying the grass would be difficult, and that the first and last points are kind of unwanted for a closed environment, is there a way to make this approach feasible?

  • 3
    $\begingroup$ It does not look good. In cold temperatures (how cold is it in your environment?) grass fermentation is reduced and different bacteria will be present. The grass won't get as warm as it would in a warm summer. Also consider that the fermented grass will have higher pH. May not be appealing to the herd anymore. Will also produce molds if not properly aired. I would not worry about the produced gases. Was never a problem for anyone who slept on fresh cut grass (because it was anyway important to provide ventilation). $\endgroup$ Feb 9, 2021 at 18:46
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    $\begingroup$ The nomads would do better closing in well insulated low shelters, together with their animals and warm each other by contact and breath. $\endgroup$ Feb 9, 2021 at 18:50
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    $\begingroup$ Decomposing grass isn't a good heating system for nomads as it's too slow and isn't very transportable. Burning dried dung would be easier. Composing as a heat source would suit a non nomadic population better. $\endgroup$
    – Thorne
    Feb 9, 2021 at 22:33
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    $\begingroup$ your nomads will need fire anyway to cook their food. $\endgroup$
    – John
    Feb 9, 2021 at 23:14
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    $\begingroup$ @Thorne depending on the particular people, but most nomads from history don't move each and every day. Would typically pitch tents for a few days or weeks or even a season, then move on to new pasture. $\endgroup$
    – frIT
    Feb 10, 2021 at 11:09

7 Answers 7


Source: growing up on a farm. (Fine, have a link or two.)

That's an innovative idea. Many a barn have burned down because farmers didn't tedd their hay and let it cure properly before baling and storing it. Things to note, though:

  • Hay goes through heating and cooling cycles when it's fresh because respiration is still occurring, and colonies of bacteria are feeding off the by-products. These bacteria thrive in wet environments. You probably don't want damp conditions or large amounts of bacteria inside your shelter.
  • Hay has to be relatively fresh to produce real heat- six to eight weeks old max, and even if this method were otherwise problem-free, you can't grow fresh hay in winter.
  • Hay heats up enough to cause fires when there's a metric crapton of it in the same place, because there has to be enough insulation around the hottest core to prevent the heat from simply dissipating. A single bale won't get nearly so hot.
  • Heat release is rather slow. Putting a bale or two in a room won't make a difference in winter.
  • Harmful gas release is directly proportional to heat produced. Venting it means loosing your heated air, but in any case a bale or two of hay won't produce enough to matter.

So, bottom line is that using straw as a heater won't work. However! As mentioned in bullet 3, straw is a great insulator- good enough to be used as modern housing insulation. Don't use it as a heater; build walls with it! And though decomposing hay doesn't release heat fast enough to benefit, humans produce 100 watts of energy at rest. So when your people hunker down for the winter, keep your spaces small and insulate them very well with that densely-packed hay. That will make them function like an igloo, which can be 50 C warmer than the outside air because of body heat alone.

  • 1
    $\begingroup$ I like the idea of straw bale igloos (requiring at least wagon transport though), just want to add that air is about 3.5 times a better thermal insulator than compressed straw (engineeringtoolbox.com/thermal-conductivity-d_429.html). Loose straw (or tents, clothes, etc.) that trap air pockets may thus be an even better practical solution. Straw bale houses are great if you don't need to move them and have enough space and cheap otherwise waste material straw bales to add until you have the desired thickness. $\endgroup$
    – frIT
    Feb 10, 2021 at 11:19
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    $\begingroup$ @fr13d that's for still air, which is never achievable in bulk insulation due to convection. That's why we add things like straw in between walls, to stop the convection and reduce heat transfer. $\endgroup$
    – Rick
    Feb 10, 2021 at 14:27
  • $\begingroup$ Air > straw: From a strict physical properties standpoint, perhaps, but you do bump up against implementation. Air between two air-tight layers of wall would insulate better, sure, but I assume the nomads could figure out an acceptable density. While I don't know what type of nomads OP has in mind, whether or not they winter in the same area long enough to collect an adequate amount of straw is a detail for him/her to decide. I certainly wouldn't try to make buildings entirely of straw, much less move them around, but lining temporary dwellings every season may be feasible $\endgroup$
    – automaton
    Feb 10, 2021 at 14:48
  • $\begingroup$ @automaton Although straw doesn't seem quite as versatile as reed, you'd be surprised at what people are capable of: en.wikipedia.org/wiki/Mudhif#/media/… $\endgroup$
    – user79901
    Feb 12, 2021 at 13:12
  • $\begingroup$ @Servaes That's really impressive! Though I wasn't saying one shouldn't make the building entirely of straw because it isn't possible, more just the feasibility of the amount nomads would have to harvest every season for little if any benefit vs simply using it as insulation $\endgroup$
    – automaton
    Feb 12, 2021 at 15:57

Ferment grass inside a large friendly animal.

cow nap


Fermenting grass within a large animal is unlikely to catch fire. Animals give off noxious gases too, but usually in controlled amounts unlikely to be lethal. Large animals full of fermenting grass become quite warm. Most large animals are pretty soft and some are friendly too.

Your people have large animals who like to ferment grass inside them. These animals are also furry and friendly, and they come inside with the humans when it is cold. The animals control the speed of fermentation and release of heat with biological mechanisms evolved over millions of years. Everyone wins!

  • $\begingroup$ odd heading, but the idea seems good. +1 $\endgroup$
    – Topcode
    Feb 10, 2021 at 0:35
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    $\begingroup$ +1 for "Animals give off noxious gases too, but usually in controlled amounts unlikely to be lethal." $\endgroup$
    – Daniel B
    Feb 10, 2021 at 6:12
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    $\begingroup$ And you can use their dried poocakes for campfire, when needed. $\endgroup$
    – PTwr
    Feb 10, 2021 at 10:40
  • $\begingroup$ Oooh, the old Jerusalem sty solution :) Made famous by the guy who wrote the new testament about two thousand years ago :) $\endgroup$
    – mishan
    Feb 10, 2021 at 16:25
  • 2
    $\begingroup$ en.wikipedia.org/wiki/Housebarn $\endgroup$ Feb 10, 2021 at 19:31

An interesting question!

I think another source of information on the process would be to look at composting. The composting process breaks down organic material through the action of bacteria, fungi, and other microorganisms, releasing heat in the process.

Optimal heat is a desired outcome as it sterilizes seeds (including of weeds) and pathogens. Sub-optimal piles also convert the material into compost, but take longer, without much of the sterilizing effect.

The heating effect of compost has been employed to extend the growing season (providing heat in early spring and late fall, keeping plants clear of the first light snowfalls). People in snowy regions also report that compost piles keep clear of snow well into winter, so your idea may not be too far-fetched.

In very broad (rule-of-thumb) terms, an efficient compost pile looks as follows:

  1. Dimensions in the order of 1-1.2 meters (1' 3" to 4'). Dimensions meaning a cube with all sides of those sizes, or an upright cylinder with height and diameter of those sizes.

    Rationale: It has been found through various people's experience that a pile smaller than that produces less heat (sub-optimal composting as mentioned above), while larger piles sometimes spontaneously combust, creating a fire hazard.

  2. Material composition is a ratio of 1:1 up to 2:1 of "brown" material:"green" material. "Brown" material is higher in carbon content and is usually dry grass or straw, dry leaves, wood shavings or sawdust; but can also be shredded paper products like newspaper, corrugated cardboard, egg cartons, etc. "Green" material has a (relatively) high nitrogen content in addition to the carbon. This is typically freshly cut green plant material, but can also be kitchen scraps/peelings, various manures, urine, and organic fertilizers like bone meal, blood, fish emulsion, etc. Raw and cooked meat, egg, and dairy, including dead bodies, also provide nitrogen-rich organic material, but are often avoided due to smell and pest attraction potential. (However, others use such materials but just bury them deep into the pile, and claim no adverse effects.)

    Rationale: the microorganisms require the right amount of nutrients to live and multiply. Carbon (carbohydrates, to be more precise) are necessary for fuel, so a pile with just green material as you propose will not provide optimal fermentation. An overabundance of "green" material favors putrefying organisms, which in practice cause a slimey stinkey goo and a lot of outgassing of ammonia, methane and other noxious gasses - note that these are typically nitrogen-containing volatile compounds, so this is a way for nature to bring the carbon:nitrogen ratio back to more acceptable levels by dumping nitrogen compounds into the atmosphere. Not pleasant for oxygen-breathing bystanders, though.

  3. Speaking of oxygen: a well-aerated compost pile is usually the way to go, by "turning" the material every week or two: basically shoveling everything onto a new pile where new air pockets will be included (no compacting). Other methods exist e.g. perforated pipes actively or passively carrying in air. Note that some processes are anaerobic (e.g. Bokashi, silage, animal digestive tract, biogas digesters) but are considered less effective (especially for composting purposes).

    Rationale: experience has found that a freshly-built compost pile reaches around 60-70°C (140-160°F) within half a day to around 3 days. After temperature peaks, it falls off again. It is then required to "turn" the pile, after which temperature may peak again, but taking longer (a week perhaps) and the temperature may not be as high as the previous time. A heap may need 3 or 4 such "turns" before the material has been composted. With heat buildup and nutrient and oxygen depletion the microorganism population changes in makeup and numbers, so the turning brings in new oxygen, evenly distributes remaining nutrients and sort of "resets" the process (within limits).

  4. The composting process also requires water. Again not too much (which will displace oxygen) and not too little: about like a damp sponge when squeezed, will just release 1 drop of water.

    Rationale: microorganisms don't have legs, they need to "swim" where they want to be, and nutrients need to be in soluble form.

(If you need more theoretical knowledge regarding composting, I recommend the Humanure Handbook.)

On to the practical side:

I was taught in primary school, many many years ago, that the San people of southern Africa would use a similar method than what you propose, by digging a person-sized trench, adding organic material and water, and covering with sand, as protection against the cold desert nights. Not knowing any San, I have no clue how true that is... I've also read an account of a military special operative's pursuits in roughly the same region, who in survival situations would sleep in a similar trench but with hot coals covered by soil. I guess both methods would make cold temperatures bearable rather than comfortable, and wouldn't be suitable much past one night's use...

Other posters have observed the requirement for a fairly good amount of plant material, which I'm worried about too. Then again, as shown above a variety of sources can work (and is even recommended), and one would assume that a nomadic people of animal herders would go where the animals could find sufficient sustenance (and provide at least some of the raw materials). Yeah, nomadic steppe dwellers are not unknown on Earth, so it might be worthwhile to research how they solve the heat problem.

  • $\begingroup$ I think traditional steppe people on Earth usually just use fires if they need extra heating. en.wikipedia.org/wiki/Dry_dung_fuel The problem is the OP has plot reasons for not using fire. $\endgroup$ Feb 10, 2021 at 19:28
  • $\begingroup$ There are reptiles which build self-heating nests from compostable materials. $\endgroup$
    – arp
    Feb 12, 2021 at 19:27

Nomads don't stay long enough anywhere to make hay

Nomads live off the land and what is already there. They are not farmers and they don't have the time or equipment to make hay.

If they start harvesting and building haystacks, they aren't nomads any more, they are farmers. They may as well settle down, build houses and breed cattle.

Nomads? Obviously they cuddle up to their sheep and cattle. Or each other. Or build fires. Or travel with the seasons so they are always in a reasonable climate.

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    $\begingroup$ Small note: I gave the example of hay, but I wrote that the nomads use generic grass $\endgroup$
    – L.Dutch
    Feb 10, 2021 at 4:47

I think that both problems may be solvable at once. Fermenting hay gets VERY hot - hot enough that it wouldn't be comfortable. To slow the fermentation (which will cool it down and slow the production of CO2 and CH4), you can have the nomads occasionally turn/stir/disturb the hay pile. This is the same thing that folks who are serious about composting will do with their compost piles, and with practice (which will be very much part of the culture) it is possible to fine-tune the amount of heat produced. You can also have the structures have vents that can be opened when the pile is stirred, as this will allow the CH4 and CO2 to escape.

  • $\begingroup$ Are you sure hay will ferment to high temperatures if gathered and stored in cold weather? $\endgroup$ Feb 9, 2021 at 18:58
  • $\begingroup$ It will ferment at high temprature but it takes a long time and won't be suitable for animals afterwards. also nomad are not going to be able to do it. $\endgroup$
    – John
    Feb 9, 2021 at 23:12

The only way you and your tribe will get warm without fire is with a lot of clothing. Using it in layers, that's what people in really cold places do. Why I said only with clothing? Because you said "a nomad tribe". If you settle in a place for a long time until you consume all its resources, you are not a nomad. A nomad respects the land and takes care or it, moving with the seasons or food supply and doesn't destroy the area where they settle, so that nature can reclaim it in a short time.


Through the researches I have made I have found that:

  • fermenting non dried hay might result in self-combustion of the said
  • hay fermenting dried hay can reach up to 60 C of temperature
  • fermenting hay develops asphyxiating gases, like CO2 and methane

I'm not so sure that these are as big of a problem as you think. Starting a fire is bad but people know how to put out a fire. Getting to 60 C is bad if one is sleeping on it but not so much if used like a central campfire heat source in the "straw igloos" that others here proposed. CO2 building up is bad but humans evolved to know when this happens and with experience have learned how to deal with it.

A bit of extra CO2 isn't an "instant kill" and will wake people up. Assuming a reasonably healthy and able person is among the group in the "igloo" to watch the "fire" then they can all sleep in relative safety and comfort. Or as comfortably one might sleep in a building made of straw. If there's too much CO2 then open up some ventilation. If it gets too hot then stir it up to cool it down.

Are other means to get heat out of the question? Is the time and place on Earth within the last few million years? I ask because this can open up alternatives.

Every so often a fire from oil soaked rags makes the news, so everyone reading this should know that we can get heat from oil and plant material in the proper ratio. There's heat from this that is much like your proposed compost pile heated bed, and carries many of the same risks. I suspect that with experience people should be able to figure out a relatively safe means to get heat from oil and plant material without setting themselves on fire.

The oil for this heating can be animal, vegetable, or mineral. The "rags" can be most any plant material. Coal and charcoal can produce this same self heating effect as they are in many ways not all that different from oil soaked rags. Heating their homes from fire was ruled out but not fire for cooking. The same fuel used for cooking can be used as fuel for heating even if not burned. Oils produced for food, or collected from cooking meat, can be used for nighttime heating of beds without flames.

I ask if this is a setting on Earth in the last few million years because, geologically speaking, it was not that long ago when Earth was more radioactive than today. Geothermal vents and hot springs today are powered by radioactive decay deep in the core of Earth, on a younger Earth these vents and springs would be more common and more powerful than today. A nomadic tribe living where heat basically bubbles from the ground should be able to find ways to exploit it.

With naturally occurring uranium and water we cannot produce a self sustaining fission reaction. A billion or two years ago this happened naturally because the ratio of U-235 to U-238 was different then. https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

People on a "young Earth" where fuel for fission is far easier to obtain could build a pool type reactor without any real understanding of how or why it works. At first we'd expect them to merely exploit the natural reactors as they occur, then with time, and trial and error, learn to build and maintain their own. https://en.wikipedia.org/wiki/Swimming_pool_reactor

Because water is such a great radiation shield the risks of radiation from even a very primitive pool type reactor would be minimal. The people should learn not to swim in the pool or drink from it as that would result in bad outcomes quickly enough to equate contact with the radioactive material with the harm it does. I suspect the water would taste bad enough from the brew of fission products that people would not want to touch it, therefore preventing people from bathing in this radioactive water.

This kind of reactor left unattended would likely see the water needed to sustain the reaction boil and evaporate fairly quickly. Isotopes that "poison" the mix in a modern reactor, like xenon and iodine, would escape into the air and allow the fuel to last far longer in such a primitive reactor than in a modern reactor. To restart the reactor they could divert water into the reactor pool from some river or stream.

A nomadic tribe should be able to build a sod house or log cabin over a pool type reactor to contain the heat. This would take a lot of time and effort to build and so such a tribe would maintain these structures for a long time, coming back to them winter after winter for shelter. The tribe leaving the reactor dry and not operating gives time for the decay of more "fission poisons" that would prevent the reactor from sustaining fission, as well as giving time for most everything that could poison the tribe to decay away. The fuel will have a half-life in the millions or billions of years but the fission products will not, most of them will decay away in months.

Even without building a fission reactor there's heat that can be obtained from naturally occurring radioactive decay. Finding rocks and sands that produce enough decay heat to help hold back the cold winter can be done by looking for places where frost isn't forming when the air temperature is just below freezing. People could collect these rocks and use them to build a bed to sleep on.

Naturally occurring radioactive rocks will have isotopes with a half-life on the order of a hundred million years or a billion years. The rocks richest with these isotopes will be warmer to the touch than other rocks around them. These kinds of rocks won't be a fire hazard, or even a radiation hazard. Long lived isotopes decay predominately with alpha and beta radiation. Alpha radiation will not penetrate the skin. Beta radiation will not penetrate more than a few millimeters of rock. Sleeping on a bed of uranium or thorium sand is not going to be much of a health risk. Not a risk that a primitive nomadic tribe would likely notice as it would take years to develop a small chance of developing some effect. People have lived with naturally occurring radiation for as long as humans have been human, this means people developed means to defend against long term low levels of radiation.

The idea of what is basically paper towels soaked in bacon grease is far more feasible than my other ideas. I want to come back to it though so it's not lost among the lengthier descriptions of utilizing hot springs, a primitive nuclear reactor, or naturally occurring "hot rocks". I spent more time on the more complex heating because it's more complex. Oily rag heat is simple. If there's a potential in the setting for oil to bubble from the ground, or get squeezed out of sand or something, then this can be quite easily done.

  • $\begingroup$ Sorry, I don't see how this is answering my question. You just commented on the premises, then went on completely ignoring my requirement of "no fire" and "use fermented grass" $\endgroup$
    – L.Dutch
    Feb 12, 2021 at 15:24
  • $\begingroup$ I took the question as having the requirements of no fire, for primitive nomadic people, in a steppe like environment, with fermented grass as a place to start with ideas. I did answer your concerns about CO2 asphyxiation, people will be awakened from too much CO2 and know to ventilate to the outside to rectify this. Your concern of being too hot from sleeping on fermented gras was answered by not sleeping on it, but instead using the fermented grass for space heat in a primitive structure. I then offered alternate sources for heat other than fermented grass, none of which involved fire. $\endgroup$
    – MacGuffin
    Feb 12, 2021 at 15:40
  • $\begingroup$ It's turned-up in the low-quality post queue, but as it presents somewhat of a frame challenge I'm inclined to leave it open FWIW. (From review). $\endgroup$ Feb 12, 2021 at 17:07

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