A modern architect specialized in low-energy houses suddenly finds himself in the medieval times, say around the year 1000. Thinking about how to make a living and noticing how badly heat isolated the houses, even the castles, are at the time, he manages to convince a king that he can build him a much better, and especially much warmer castle.

Unfortunately only after the deal was closed, it occurs to him that he doesn't have the modern materials and technologies available. But he now cannot retract, so he has to just do his best with the means available. On the positive side, he has virtually unlimited spending budget for building the castle. Also, the king of course doesn't know modern standards, so he will be satisfied as long as the new castle is significantly better than his old one.

Now my question is: What is the best the architect can do with his modern knowledge, but medieval technologies? The castle must also be built so that it can be maintained by medieval people without too much training.

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    $\begingroup$ Side Note: (Assuming we're dealing in English...) If he's limited to modern knowledge, he's going to have a hard time simply communicating with people before the Great Vowel Shift. $\endgroup$ – Frostfyre Oct 14 '15 at 19:58
  • $\begingroup$ @Frostfyre: Looking at the old pronunciation of the vowels as given in the linked article, I conclude that knowing modern German would be very helpful in figuring it out (the pronunciation is not exactly the same, but it is quite close). $\endgroup$ – celtschk Oct 14 '15 at 20:12
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    $\begingroup$ A modern-day architect tasked with building a medieval (European) castle will fail big time without prior experience with smaller projects. There is a lot of expertise that goes into building such large and complex structures which is neither taught today nor is everything obvious. The architect will also have to try to remember how to do all the math and drawings by hand like in their first years of college but never again. $\endgroup$ – Crissov Oct 18 '15 at 23:27
  • $\begingroup$ @Crissov: Actually a medieval castle is being built in France with what historians know about medieval materials and techniques: Guédelon $\endgroup$ – mouviciel Dec 9 '15 at 13:37
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    $\begingroup$ I think everyone is kind of missing a major point here, which is that castles are supposed to keep your enemies out. That's why they have those thick stone walls, arrow slits instead of windows, and so on. You put up with some discomfort in exchange for security, so any of your modern architect's innovations must not detract from that security. $\endgroup$ – jamesqf Jan 11 '17 at 19:24


Packed earth and even packed hay are incredible insulators. This, along with upper-ceiling, open/close air shafts allow for some pretty good moderation of the temperatures.


Better use of glass, much like a church's stained glass allows for safe viewing and natural light. Double-pane them for insulation, and allow them to swing open for ventilation.


It requires no electrical devices to build a toilet for more comfort than using the gold chamber bowl. Water can easily be brought to above the castle ceiling in order to have flowing water on demand. Wastewater can flow downhill as it does.

Rainwater Catchment

Water in 'the olden days' was often mixed with alcohol, because it was from a river, tasted bad, and could give you diarrhea or worse. By using and storing rainwater, you have a pretty good tasting and more sanitary water. You can still mix it with wine, but you have a lot less chance of getting ill.

Heated Floors

Build small passageways or 'pipes' that allow a 'downstairs' heat source to channel up directly under his wooden floor, and out the castle. During cold winters, this technique is used, so the king has nice, warm floors and warm chambers.

Hot Water Heater

Everyone likes a warm bath, but the King wants it on demand, and doesn't want to wait for the chamber maids to haul hot water up the stairs in buckets. A continuously heated chamber of (captured rain) water sits above the bathroom, and when he wants it on demand, the chamber maid turns a faucet allowing hot (and cold) water to his liking, to flow in. Then she goes and refills the water upstairs.

Recreation Facilities

Everybody likes bowling. Have your architect build the facilities and instruct the king how to play a number of modern sports that are easy to learn.

Bonus: Bacon

A pipe or vent between the interior walls goes from the basement kitchen up to the king's chambers. In the mornings, the cook makes a lot of bacon, so he wakes up to the smell of bacon each day.

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    $\begingroup$ Points for bacon $\endgroup$ – TrEs-2b Oct 14 '15 at 20:47
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    $\begingroup$ Don't forget wool. That's used for insulation even today. $\endgroup$ – Monty Wild Oct 15 '15 at 22:10
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    $\begingroup$ Hay bales are stupid good at being the walls of a house. Stupid good. $\endgroup$ – Draco18s no longer trusts SE Dec 9 '15 at 15:36
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    $\begingroup$ Hay as an insulator has the drawback that it will attract enormous rodent populations. The consequent destruction of food stores and the overwhelming smell of mouse urine may be considered a drawback. $\endgroup$ – WhatRoughBeast Dec 9 '15 at 18:44
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    $\begingroup$ @WhatRoughBeast .. kitties. $\endgroup$ – Mikey Dec 9 '15 at 21:19

Just building a nearly airtight castle heated by steam will go a long way and you don't need modern materials though that helps.

Native Insulation

Even if the materials themselves don't insulate terribly well, if the structure can made as airtight as possible, the energy required to maintain a given temperature will go way down. Sealing cracks with pitch or tar should help a lot.

Stone and wood are naturally decent insulators, not as good as fiberglass insulation but plenty good.

Steam heating

Older homes before central heating and air conditioning heated themselves with steam. Water was heated in a boiler which was then fed into a system of pipes and radiators. Middle Ages blacksmiths certainly had access to cast iron technology so making the boiler, pipes and radiators shouldn't take too much experimentation.

Inventing threading will be a bit tricky but if backed by the King, it shouldn't take too long to describe to a competent blacksmith how to do threads. (Edit: Okay, threads are crazy crazy tricky with high precision and metallurgy requirements. Perhaps all you could do is get them started.) The architect may have to invent measurements to standardize on but that's not too hard.

Safety testing will be essential because it won't do to have a radiator explode in his majesty's privy.

Design of the castle

The architect will have taken an architecture history course so he need only examine the king's army to decide how to build a maximally defensive castle. Or, if the architect is really lucky, the king will have him design a castle without all the required defensive measures. The design of defensive structures changed with advent of firearms and cannon so the architect will need to design appropriately.

Anything, absolutely anything that the architect comes up with will blow the king's mind. It will be a structure hundreds of years before its time. A good architect will get to know his client's sense of style and make something astounding and approachable because it will be very easy to design something that offends the king's sense of style. However, if the architect does well, everyone in the known world will want one of his castles.


Glass is the middle ages was super duper expensive so making double walled windows will be exorbitant by any standard. It's up to the king to decide on that one. It's a big bonus if he can pull it off. Every piece of glass was cut from a giant clear wheel of glass. Very time consuming to make.

If the architect is familiar with the float glass method of glass making then he could suggest it to the blacksmiths/artisans of the king to get large quantities of high quality and perfectly smooth glass for windows, mirrors, whatever.

Pest Control

Removing ways for mice or rats to invade will be huge. Disease goes down, food spoilage goes down.

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    $\begingroup$ The architect will know how float glass is made. A big vat of molten tin in an iron container is within reach. Medievals never thought of it. $\endgroup$ – nigel222 Oct 15 '15 at 20:41
  • $\begingroup$ @nigel222 thanks for the pointer. I've added it to my answer. $\endgroup$ – Green Oct 15 '15 at 20:44
  • $\begingroup$ @nigel222 Actually float glass is pretty hard to make. You need the right mix at the right speed, you need a layer of nitrogen or similar to prevent the tin oxidising, etc. $\endgroup$ – Tim B Dec 9 '15 at 13:11
  • $\begingroup$ "it shouldn't take too long to describe to a competent blacksmith how to do threads. " OMG. There speaks someone who has no idea of the precision required, or the difficulty of producing that precision by hand. $\endgroup$ – WhatRoughBeast Dec 9 '15 at 18:46
  • $\begingroup$ @WhatRoughBeast Fair point. It's changed. $\endgroup$ – Green Dec 9 '15 at 19:59

Double walled structures with bales of hay between them are simple to make, and are the basis of some "live off the land" type structures today. These are probably easy to build with the technology of the Middle Ages (the main sticking point will be sealing the gaps between the boards, since the sort of technology used to rapidly make large, flat, standardized boards that we are used to using does not exist), but for most practical purposes the house or shelter will be far superior to what is available.

The downside is the cost. Even today most people do not care to pay extra for energy saving houses, and in the Middle Ages, the amount of available wealth is so much lower that few, if any, people would have the surplus money to gather materials or the extra time needed to build such a building (they already need to spend virtually all their time just to stay ahead of starvation). Other issues would include air quality (a sealed building would rapidly trap the smoke from fireplaces, especially since things like hearths with proper chimneys and dampers were uncommon until much later), and even the fire hazard of building with such a large load of combustable material (double the amount of wood with bales of hay in between). Finally, such a building would not last anywhere near as long as a modern one, since the wooden structure is not particularly waterproof, the hay insulation would become wet and ineffective, and eventually rot away.

If it was not for the steep initial expense of the building (all that hay alone would be the fodder for several cattle or sheep), this would be a fairly practical way to build low energy cost housing in the Middle Ages.

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Air conditioning: This one is very labor intensive to construct.

You need a long enough buried tunnel and a means of moving air through it. Buried pipes work well but would be very hard to construct with the technology of the era. Tunnels carved through the ground and protected with arched brickwork are more labor intensive but less technological. (Note that there are examples of such tunnels being carved for reasons of bringing in water, thus they certainly can be done.)

In a perfect situation there's a nearby mountain that can be used, dig a deep enough trench up the side of the mountain, build your tunnel in it and then fill it in. This will set up a natural air flow and not need any pumping mechanism, otherwise you'll need servants or perhaps draft animals to move the air. I doubt the king will want to live next to high enough terrain to do this, though.

The key here is that deep enough underground (and that doesn't need to be all that deep) the temperature is basically constant year-round--it will be the average surface temperature. There are almost no climates in which this temperature is warmer than you would like your castle to be. You need to make the tunnel long enough that the surrounding ground has enough thermal mass to average out the temperature over the year. (And note the flip side of this--it also provides heating. Unfortunately you can't set up a system where one tunnel provides both passive heat and passive cooling, either you use two tunnels or you use forced air for one of the two modes.)

Think of the benefits to the King--not just the comfort but what will his enemies think when their diplomats pay a visit and find at least the throne room cool when it's unpleasantly hot outside.

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  • $\begingroup$ The problem with this theory is air density. That cool air doesn't want to rise, which means you have to expend energy to push it up. A lot of it. Most historical examples of ancient A/C were far more costly in terms of energy and resources than they were worth. The ancient Romans had a solid idea though, run water from the aqueducts through the walls to carry away heat. Or do what Emperor Elagabalus did and haul wagons of snow down from a nearby mountain via donkey trains and surround your keep with it. $\endgroup$ – b1nary.atr0phy Oct 2 '19 at 17:24
  • $\begingroup$ @b1nary.atr0phy Note the slope. If you have a tunnel buried deeply enough and going up a mountain you will get the needed air flow. If you want heating and cooling you need tunnels going both up and down, block off the one you do not currently want. $\endgroup$ – Loren Pechtel Oct 3 '19 at 5:04

Heated water piping through the floors would be my first thought. The water pipes could be heated using solar energy on the roof as well as lining the chimney with valves to direct the flow. A lot could be done with maximizing insulation as well and with the placement and management of windows.

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Franklin Stoves, or sealed box cast iron stoves would be the easiest ways to get started. Food tastes better without cinders and ash :) You could follow that up with Rocket Stoves. All are well within the ability of Medieval Blacksmiths to create. Pegs or pins to hold it together and clay to seal it up. The seals don't have to be perfect, and no need for threading. They could use existing chimney structures. build on those techniques to build boilers for hot water.

If you can master a sealed, watertight system, you could have the makings of a solar powered water heating system for both passive heating and hot water on demand.

To work within the existing castle, sacks of wool affixed to the wall and hidden by tapestry could make decent room insulation without too much renovation.

For new construction: Earthbags! Grain sacking for fabric maybe treated with tree sap. Lay the earth filled sacks out in courses like bricks and then coat the wall with stucco like mortar. It yields a thermally efficient and amazingly strong and fireproof structure.

Use Catalan vaulting for roofs and such. Very strong and relatively low cost and material.

The earthbag and Catalan vault aren't difficult, just labor intensive.

I yield to the other answers on water management, which is very important.

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