If you tried to insulate your underwater home, making it water tight... wouldn't you suffocate yourself? Water does get stale, and run low on oxygen, creating a dead-zone, so wouldn't you want some flow of water to keep fresh water coming in?

Worse is that water has about 5% the oxygen content of air per volume (not counting H2O itself), and fish are only about 80% efficient at absorbing it.

The issue there, is you can't really insulate a home which has fresh water cycling through? That's going to keep it at the same temperature as the sea outside.

How could merfolk, or other underwater people, insulate their homes without suffocating themselves?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Jan 6, 2021 at 4:13

7 Answers 7


Ventilation with heat recovery:
You just stumbled upon the basic problem home builders are having in the real world. To keep the energy inside the building, buildings are more or less made airtight. Windows are no longer leaking significantly (c.f. blower door test). However, people need to get fresh air in from time to time and the bad air (low O2, humidity) out. Opening the windows to exchange air kind of defeats the purpose of building "airtight" in the first place.

Therefore all new buildings (at least here in Germany) are required to have some kind of heat recovery ventilation that exchanges air while at the same time keeping most of the energy inside the building. The principles of those systems are applicable to water as well as air.

  • $\begingroup$ Thank, Manziel. Great to have a similar RL example to the problem. I'll look into it. $\endgroup$
    – Johnny
    Commented Jan 4, 2021 at 21:23
  • $\begingroup$ I was thinking, should I just answer "Heat exchangers"? But your answer is a better way to explain that idea. $\endgroup$
    – Joffan
    Commented Jan 5, 2021 at 1:28
  • 2
    $\begingroup$ @Joffan That's what I did (see below), only doing it as an ASCII-art drawing took so long Manziel beat me by five minutes. :-)) $\endgroup$
    – Karl
    Commented Jan 5, 2021 at 21:38

Vents in the floor

I'm assuming you're wanting to keep the interior warm from the cold sea. Not the other way around.

Warm water rises, cold water sinks. Oxygenated water rises. deoxygenated water sinks. (Warm water holds slightly less oxygen than cold water - so you don't want it too warm!)

Your mermaids insulated houses have grates in the floor. "Dead" water sinks down the grates. If the house gets lower oxygen levels than outside, that water will rise up, warm, and expand, oxygenating the warm water inside.

One option is to build the house on stilts:

enter image description here

A multistorey house is probably fine to just connect to the floor below.

If you have big apartment buildings or just a really big city, the oxygen requirements might exceed what the sea can provide in a small space, even if everyone has a garden. You may need oxygen generation to be a city utility, pumping oxygen out to bubblers in every home and workplace.

  • $\begingroup$ Great idea, and I love the diagram. I think this combined with Dutch's answer would work wonders, if you can have the warmed stale air replace the less-cold oxygenated air. This does raise a confusing question, as to how much oxygenated water rises compared to warmer water.... They could probably work out the details, living with the stuff all the time. $\endgroup$
    – Johnny
    Commented Jan 3, 2021 at 19:16

Photosynthesis/chemosynthesis can help you converting CO2 into O2.

Seaweed and other sea vegetation can supply the needed oxygen to the environment. Even blue algae can do the same.

Basically nothing that different from having home plants for keeping the air fresh on land. There are even sealed bottles where you have a mini biome kept alive by the plants inside it.

Alternatively, you could set up a system where outside water is let in and inside water is pushed out with crossing flow, so that the inbound water is warmed up by the outgoing one. Again, nothing different than a land based re-circulation system with heat recovery.

  • $\begingroup$ The sealed bottle is interesting. Algae will actually absorb oxygen when not photosynthesizing, so I presume these bottles are kept in day-lit areas? Cross flow is also a good idea. Thanks for the answer. $\endgroup$
    – Johnny
    Commented Jan 3, 2021 at 19:11
  • $\begingroup$ Photosynthesis or chemosynthesis? There's little sunlight at the bottom of the ocean $\endgroup$
    – mcalex
    Commented Jan 4, 2021 at 8:26
  • $\begingroup$ @mcalex can be both, depending on the depth. $\endgroup$
    – L.Dutch
    Commented Jan 4, 2021 at 8:48
  • 1
    $\begingroup$ You would need a very large bottle to support a sentient life form, open ocean already imposes limits on animals via oxgyen. How large do you envisage the bottle/container being to achieve this? $\endgroup$ Commented Jan 4, 2021 at 16:47

Different degrees of of isolation

It is important to know "what from" mermaids would isolate their home. Houses aren't really airtight either, otherwise the water in the air would cause mold growth pretty much instantly.

  • If you don't want stone/dirt to accumulate to much an house with open windows for water exchange would be enough.

  • If you don't want fish to enter, use fish nets or finer fabrics to keep them out (similar to mosquito nets).

  • If there are additional constraints like keeping "warm water" it in the house having removable seals (think windows in a modern house but transparency doesn't matter since you can't see far anyway) might work, although airing cools thing down (or heats up). Although since water has pretty much the same temperature over the day (ocean temperature varies a lot less than air temperature). I don't see a good reason why merfolks would need to isolated themselves from water temperature. One possibility is that they migrated to place they didn't develop of that the climate changed in an unfavorable way. However this temperature sensitivity might place strong limits on what depths of water they can swim at. The temperature change over 1km beats the difference between tropical and subpolar regions. Temperature change vs depth in different regions of earth

  • If industrial age technology is available the analog of a heat exchanging HVAC would allow a constant flow of water. Similar to the low tech solution proposed by L. Dutch.

If you can build green houses fresh air above sea level is only meters away.

I am not a fan of algae based solution as the houses would have to be really close to the surface. Algae would probably also be a thing you want to keep out of the house as accumulations of algae can also turn water stale.

It might be more sensible to compress air and release it where you want it. That can happen via bucket chains or tubing.

Implications for architecture of houses

Since water contains less breathable than oxygen houses might be bigger with more and larger windows (statics is less of a concern) to have larger and faster exchanging buffers while you are sleeping.

However water circling mechanism might be necessary if the density of more oxygenated water is unfavorable to the sleeping place (on the other hand you could sleep at any height from the floor by floating).

Under water there is little reasons for floors other than to put things on. So houses might look like giant libraries with shelfs filled with tools compared to human houses. Sleeping/resting places are probably communal as it might be hard to keep many small rooms oxygenated. Isolated sleeping places would be a much bigger status symbol then above ground. Just for fun there is nothing preventing builders to have door in the ceiling and that would probably useful to transport things in and out of the house.

  • $\begingroup$ Thanks for another great answer, WSH. Manually bringing down air might be tricky. Enough air for a human for one day, by my figuring, is 135^3 feet of volume, which would have about 4 tons of up-thrust. They might manage on a lot less than this, if they regularly "air out" their homes in the morning, and only try to warm them before sleeping. Then they'd only need about 10-12 hours of air, and a large house might hold quite a bit in its water. As an aside, a mermaid would theoretically need 337ft^3 of water, per day, if they need as much as a human. $\endgroup$
    – Johnny
    Commented Jan 3, 2021 at 20:13

Electrolysis of water.

electrolysis of water


Your mermaids can accomplish 2 goals with the same process. Electrolysis of water splits it into hydrogen and oxygen, and the oxygen released can be used to re-oxygenate the water. Also, electrolysis of water produces heat and so if the insulation is to retain warmth the process will warm up the dwelling.

Electrolysis was discovered in 1789 and so doing this depends on the tech mermaids have available.


  • $\begingroup$ Thanks, Will. Love the article you linked, and I like your idea of getting the most out of a technology. $\endgroup$
    – Johnny
    Commented Jan 3, 2021 at 22:07
  • $\begingroup$ There will be a problem with chlorine if it is salt water (as in seawater). $\endgroup$ Commented Jan 5, 2021 at 0:23
        /  \
       /    \
      /      \
     /        \
    /          \
   /            \
  |              |
  |      ________|_______
  |    __|______________|_____
  | <--_______________________  <--
  |      |  __________  |  
  |      |  |    |   |  |
  |     --> |    |   | -->
  |              |

One word: "heat exchanger"

  • $\begingroup$ That looks similar to the system Manzeil pointed out. I really appreciate people are going to the trouble of making diagrams for the question. Thanks, Karl. $\endgroup$
    – Johnny
    Commented Jan 4, 2021 at 21:24
  • $\begingroup$ Words are easy; BTUs are hard. If a passive HVAC system could actually do any appreciable work I wouldn't have a job. $\endgroup$
    – Mazura
    Commented Jan 5, 2021 at 5:32
  • $\begingroup$ @Mazura It's not supposed to do any work. In lieu of a pressure difference, it even needs some work, aka some sort of pump to keep the water flowing. Hence the arrows. ;) $\endgroup$
    – Karl
    Commented Jan 5, 2021 at 7:33

Heated Pumps

Reindeer have enormously complicated plumbing in their noses, all of it wrapped in capillary blood-vessels (this is the origin of Rudolph's famous red nose)

Essentially, by the time the sub-zero air they breathe in reaches their lungs, it's already up to their core body temperature. They have a similar system for reducing the temperature of the air as they breathe it out again.

This protects them from the extreme cold without greatly impeding their breathing.

Your mermaid civilisation could use a similar approach, forcing fresh sea-water through a heated set of pipes (take your pick of the ways to heat the pipes, maybe geothermal vents) to provide fresh oxygenated water to the house. Your old water can then be allowed out of the building under positive-pressure.

  • $\begingroup$ This would be the best system for an industrial setting. Thank you, Raudhan. I really love the story about the reindeer and Rudolph. $\endgroup$
    – Johnny
    Commented Jan 4, 2021 at 21:25

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