Passive Cooling Techniques for Apartment Buildings

Okay, so I'm building a world with an empire originally based out of an arid/semi-arid (Sahel type) ecoregion. As the empire expands, it will eventually shift its capital to a more temperate climate, but for now its capital is in a dry savanna. Since the empire is prosperous, the capital is thriving with a high population density. There are zones in the city, according to social class. The rich live in their villas, the merchants/middle-class live in two storey buildings, and the servants/workers live in apartment blocks, 3-5 storeys high. (Think Roman apartments for plebians)

And this is where I run into a problem: namely, the sort of ventilation that would be required to comfortably house people in multi-storey apartment blocks (8 apartments per floor), in a dry tropical region, with ancient tech. My research has taken me to Persian windtowers, but I'm not certain whether a single windtower would be able to redirect sufficient wind to 5 floors of people. I've thought about building plans with windtowers dedicated to each apartment, but that just creates a clutter of windtowers around every building. I've also looked into earth air tunnels and passive downdraft evaporative cooling, but again I'm left uncertain if they would be enough for a multi-storey building.

Any suggestions, people? Is the idea of cooling a 3+ storey apartment building completely unfeasible with ancient tech? If so, I'll scrap the idea, but given how it's such an effective way to pack a lot of people into a small area, I'd rather stick with it. Would highly appreciate your help in this!

• The quirk of comparing things to the Romans is that the Romans had concrete while most people were building with mud. You'll have to give us a technology level to work with here. – Separatrix Apr 13 at 9:00
• Hmm, Roman-Sassanid level tech of technology should be about what I'm going for. – Baron_vonCernogratz Apr 13 at 9:05
• 2 rows of 4 apartments per floor will give you 8 apartments, which means those in mid will get 1 window, and those on corners will get 2 windows. This should be enough for a slum. – V.Aggarwal Apr 13 at 9:47
• Consider the simple addition of making your roof brilliant white, to reflect the sun away, but your side walls dark, to radiate internal heat more easily. It is a 100% passive way of cooling , even if just a couple of degrees. – PcMan Apr 13 at 9:53
• You pick a very difficult climate. Most arid regions have a wide swing between hot days and cold nights. You can then tune the thickness of the walls to cool the room during the day and heat it at night. The Sahel is just always hot. – UrQuan3 Apr 13 at 20:08

It seems you've already done the research into the vernacular architecture of the sort of region you're describing, but there's one critical factor you've overlooked and it's hard to emphasise how important this is, especially to someone used to a basic level of modern western luxury.

What you're describing are slums

Slums don't get fancy expensive cooling systems. Slums are places you put the largest amount of people while extracting the highest rent you possibly can. They won't be at home during the heat of the day, they'll be working. If they're not working they're looking for work. If they don't have work they don't eat.

Don't put luxuries on slum buildings.

People with money will live in low rise structures with courtyards and open space, wind towers and all.

Don't back away from the high density housing because the conditions are going to be horrific, that's what slum living is like. Pack them in and make them utterly miserable while charging them most of what they earn in rent. Be true to the reality of slum living.

• Apparently plebians were often not slum poor. – DKNguyen Apr 13 at 20:18

Shibam style.

https://www.nationalgeographic.com/travel/article/shibam-mud-skyscraper-yemen

Shibam is often called "the oldest skyscraper city in the world".[5] It is one of the oldest and best examples of urban planning based on the principle of vertical construction.[16] The city has some of the tallest mud buildings in the world, with some of them over 30 m (98 feet) high,[17] thus being early high-rise apartment buildings. In order to protect the buildings from rain and erosion, the walls must be routinely maintained by applying fresh layers of mud. The city is surrounded by a fortified wall,[16] giving it the name "the walled city of Shibam".

Shibam is pretty awesome. Mud multilevel buildings in the desert in Yemen. It gets really hot. How do they keep cool?

I found a scholarly paper. It is worth reading. My takeway: mud is good for cooling. The streets are very narrow and laid out in a purposeful zigzag manner which slows windspeed at ground level and causes more air mixing. The buildings are really close together and shade each other. Maybe the city wall is the first shade level.

We are used to think of cooling via air movement and spaciousness. This is a different route to cool. I did not find a picture of streeth level Shibam but I imagine it to be pretty claustrophobic and dark - towering mud buildings on either side, narrow streets and you can't see more than a block in any direction.

http://www.uia2017seoul.org/P/papers/Full_paper/Paper/Poster/P-0430.pdf

. Thermodynamic analysis of Shibam

The old city of Shibam owns a unique form, as mentioned above, high density high rise housing, narrow streets and spacious public spaces these features are unique worldwide. In this complicated climate environment, how does Shibam manages to maintain a good thermodynamic comfort environment worth researching. Public squares are very significant in the old city of Shibam Hadharmawt together with the special roads in Shibam they decided the general pattern of the whole city. The form of old city of Shibam Hadhrmawt is a synthesis of its unique urban layout, building volumes, special open public spaces and high thermodynamic efficient adobe walls.

• That's some hardcore minecraft dirt building right there. My goodness, this place is awesome. – T. Sar Apr 13 at 18:14
• You can see the wall in the photo you provided, along the bottom. It's not nearly as tall as the buildings and is so far away that the shadow it does cast doesn't land on them. That said, if OP wants to make the wall tall enough to act as a "first shadow", more power to them. – No Name Apr 15 at 23:39

Apart from what Separatrix answered, you can go for the "poor man" solution which was widely adopted in rural buildings in the Mediterranean area: high ceiling with a high window facing North.

The high ceiling (I am talking about 4-6 meters) would ensure that the hot air would go up, well above the heads of the occupants. The window located North and high would take care of letting that hot air exit the room, replaced by fresher air taken from the doors/windows open at ground level.

It's not really cooling but just ventilation, but for poor people that was it.

Another option, but difficult to implement for high raise buildings, is to have very thick walls. In the countryside of South-East Italy is common to see houses built with just stones, no concrete, with very thick walls, around 6 meters thick.

It was done because stones were widely available and had to be removed to clean the soil for farming, and having been in one of those I can ensure you that they protect you from the scorching heat of the outside pretty well. Even more if the inside room has also the access to the underground rain water collection room.

• Termite mounds. – DKNguyen Apr 14 at 20:47

Techniques:

1. Paint everything white, especially the south wall and the roof.

2. Build of earth -- mud/stone/brick. Build thick. This acts as thermal mass and the inside is the average of the day and night temps outside.

3. Insulate your roof. (If your climate is dry enough a cookie sheet 4 mm deep in water, and placed on a hay bale will freeze on a still night, from radiation into space, even if the air temperature is much higher.)

4. Build an air well down the middle. Shade the top of it to the point where sun doesn't hit the inside of it at all. Cooler air falls down the well, and into rooms that open onto the well.

5. Suggested by John Quanats can be used both for moving water and for chilling air. I suspect this is very high end, as building underground tunnels is difficult.

6. Build large openings on walls that don't get the sun. Closable openings on sunny walls made of reflective material. (Think bamboo screens painted white.)

7. Create thin walled chimneys on all 4 sides. These extend up above the roof as far as is practical. They are painted black. They get hot, heat the air inside, and the hot air rises. Connect these to openings in the top of each room. This evacuates the hottest air from the room.

8. If water is really cheap, put evaporation mats in the well. Evaporation cools the air, and puts more flow into the apartments.

9. If water isn't cheap enough to throw away on this scale, use water features. Fountains, pools.

10. Cook outside on balconies.

• you can also add underground air intakes to get a lot more cooling, qanats are an ancient and effective cooling method. – John Apr 14 at 18:40

Working in the Sahel region, about 2 weeks ago my air-conditioning unit failed, and I was without it for about 6 days. I stay in a 3 story building, which two sides open, and 22 rooms per floor. will try and show what it looks like below, sorry for the bad layout.

[Room]   --walkway between two room rows

[][][][][][][][][][][]
open side----------------------open side + stairway up
[][][][][][][][][][][]


What I did was open the two windows, one to the walkway side, and one to the outside. there is a lot of wind that gets channeled down the open walkway, much like a valley, and opening the windows gets the wind to flow through the room. It was not pleasant, as the air is not cooled, and still like 40 degrees Celsius, but was definitely better than without the breeze when the room would get to almost 60.

For your bottom of the pile, dirty slum dwellers, it would be more than sufficient, why do they need all the fancy wind towers, insulated walls etc. Next they will want clean water and a wage increase.

Move them underground.

Have your empire gets its early wealth from the salt trade. The capital was built near one of the best deposits and as it has been mined the resulting caverns get occupied. (See the Wieliczka Salt Mine, near Kracow Poland)

Or alternately the land is easy to dig and caverns can be made without collapsing like in Coober Pedy, South Australia.

Or there could be a natural cave system that the plebs simply move into and occupy.

• underground living was not practical pre-industrial era due to the difficulting of lighting a room with no window access. – Nosajimiki Apr 14 at 18:35
• check out from 8th–7th centuries BCE en.wikipedia.org/wiki/Derinkuyu_underground_city – Allan Apr 14 at 18:41
• Derinkuyu is believed to be a hiding place during times of war, not a permanent settlement. I'm not saying living underground is impossible without electric lighting, just not as pleasant as you may imagine. – Nosajimiki Apr 14 at 19:28

Evaporative cooling

Sudan is a dry tropical climate, and it runs on swamp coolers:

(image source bigworldsmallpockets.com)

The square boxes on balconies are a metal box filled with grass. A fan blows through air and a pump sprays water onto the grass. The grass provides surface area for the water to evaporate and the fan blows cool humid air into the room. In Khartoum the river Nile provides the water.

This reduces the problem into one of making motion for the fan and the pump. That you could do in numerous ways, but simply a team of animals spinning a shaft is one that's been done for thousands of years.

The same also applies to cooling drinking water:

(image source khartoumgateway.com)

The water evaporates through the porous pottery and cools the contents of the jar.

Obviously you'd need the humidity to be sufficiently low for evaporation to be sufficient.

Qanats and water stairs

this is an ancient and very effective cooling method. 5 stories is no problem for such a system, it just effects how you design the room to room ventilation.

A wind tower has opening on the downwind side, this creates suction pulling air up a central shaft (or just though the building) and air intakes are mostly through undergrounds piping or tunnels (qanats), so you are using the ground as thermal mass. Cool air is drawn in during the day warmer air at night. (the ground is always about 55 degrees). Wind catchers (funnels pointed in to the wind) at the intakes can help increase the air turnover especially if you need lots of natural light, which means lots of windows and thus have little benefit from a draw tower. Both of these also mean the air stays fresh which has the benefit of making a city smell better.

Adding water makes the system even more effective. Water can either be passive such as undergrounds cisterns or tunnnels (which often double as wells or irrigation tunnels) or as salasabil (AKA water stairs) which may or may not require pumping depending on the local water pressure. Generally water stairs are on the lowest floor and often doubled as drinking water sources similiar to roman fountains. As a side benefit water stairs aerate the water which makes it taste better.

Sherwood already covers several of the supporting techniques thick stone, brick, or earth walls which act as additional thermal mass and insulation, reflective coloration to reflect instead of absorb, ect. But you can also add self shading design, either by building double roofs or building tall tightly packed buildings.

All of these and more are known historic techniques, and some are still used in buildings today.

https://www.jstor.org/stable/24955643?seq=1

There are even a few cities where the underground complex was huge and acted as places with steady fortifications, storage, and places to live and work with nice temperatures. Like Derinkuyu and Capadocia in Turkey

The Romans Already Solved this Problem

Think Roman apartments for plebians

Plebeians rarely lived in buildings more than 2-3 stories tall for very practical reasons. Roman Plebeians were not lower class at all, they were full fledged citizens who just did not have any sort of title of nobility; instead, they were a kind of free man who owned whatever business they did for a living. The apartments the OP references is a kind of housing called Insulas which were designed to be both a place to live and a place to do business; so, the first floor of an Insula was typically designed to be the workshop/store-front area where the Plebeian would make his living, and then the next 1-2 floors were where they lived. Going taller than 3 stories means that you could no longer place all of your businesses at ground level and still cater to a Plebian demographic. This is because a storefront that you can not see as a passer-by is doomed to fail in an economy without search engines or phone books. Since Plebeians were free to buy/rent their own Insulas, and street access was so vital to survival, there was no economic benefit to building more than 3 stories high for a Plebeian to live in.

That said, Insulas that were more than 3 stories tall were not uncommon in the most densely populated parts of the empire. The Plebeians would live and work in the bottom 2-3 stories, and any higher housing could be used by the true lower class (Peregrini, Nexia, etc.)

Since the upper stories would be were people of true lower social status live, there is no real expectation of air conditioning for them as Separatrix's answer explains. The upper story "slums" would be exposed to the full heat of the sun, and incidentally, this would actually help insulate the lower floors keeping them cooler during the day and warmer at night. So, by just building poor folks homes on top of your Plebeian housing you are actually creating a form of passive air conditioning for your Plebeians to enjoy. If you want to go one step further, you can also use a geothermal cooling system for your lower floors. The Ancient Egyptians had it as Allan was nice enough to provide a link for in comments. I lived in a desert 3 story complex once that used passive geothermal cooling so even if it may be inadequate for > 3 stories, it will be sufficient for keeping your plebeians on the bottom floors cool and comfortable.

• @Allan, I retract my question, Nosajimiki did a better job explaining what they meant by geothermal, before with just the word and no explanation it was not clear. – John Apr 15 at 15:33

What your city needs are Solar Chimneys? They're passive and ideal for a hot climate:

A solar chimney – often referred to as a thermal chimney – is a way of improving the natural ventilation of buildings by using convection of air heated by passive solar energy. A simple description of a solar chimney is that of a vertical shaft utilizing solar energy to enhance the natural stack ventilation through a building.

The solar chimney has been in use for centuries, particularly in the Middle East and Near East by the Persians, as well as in Europe by the Romans.

• Hi Christopher welcome to worldbuilding. Could you please describe it in a way that is easily understood, Thank you. – the questioner Apr 14 at 17:13

https://en.wikipedia.org/wiki/Yakhch%C4%81l The cone shape of this building draws cool air from the base of the building and forces hot air out of the top of the building. How about building an aqueduct that funnels water to these buildings? And shape all of your residential buildings in the same way? Except fit multiple floors inside the same cone.