radiant floor

Warm Around the World: Radiant Floors in Roman Hypocaust and Korean Ondol


From ancient to modern times, radiant floor heating is an efficient and comfortable way to stay warm.  Learn all about the concept that heated Roman bathhouses, keeps Korean homes toasty and inspired Frank Lloyd Wright.  

This post is part of a series on warming techniques.  Learn more about winter coping strategies in world cultures here in Warm Around the World: Igloos and Kotatsu Space Heating.


The concept is simple, and we have talked about it before.  The idea is to warm up the area where people are … not a general enclosed are.  The standard American forced air system has a clear problem – hot air rises, so blowing hot air into your home tends to heat the ceiling – it doesn’t heat the space for people.  Radiant floors, of all types, introuduce the heat to a space at the floor level and let it rise up through all the occupied areas.  Both of these ancient systems use simple heat sources, fire and its associated smoke, to warm up under-floor areas and generate long lasting, pleasant heat.


The Mediterranean isn’t generally a very cold place, but the Romans were very fond of their bathing rituals and had some specific beliefs and preferences about the benefits of bathing in carefully controlled hot, warm, and cold water.  For fun, use your basic latin guessing to match the Caldarium, Tepidarium and Frigidarium with their associated water temperatures.

Common bath houses were engineering achievements involving complicated heat transfer systems.  One way heat was moved around a bath facility was through under-floor chambers called hypocausts (literally, “below” “hot” areas) which had hot air forced through them.  The image below shows the floor supports which created a 2′ high space under the floor of a Roman bath house room.  Heat from the hypocaust would rise to its “ceiling”, soak into the stone above and create a long lasting and steady warmth that radiated upwards to warm the feet and bodies of Roman bathers.

hypocaust radiant floor

This stunning example is the floor of a terraced house in ancient Ephesus, where I traveled in 2013.  The site, near Efes, Turkey, is an active research and preservation site of the Austrian Archaeological Institute.  Below is one of the wall panel re-assembly areas and an example of the work.

efes archaeology

An area of the site encompassing three tiers of paired row houses set into a mountain slope has been enclosed for study and carefully accessed public viewing.  It doesn’t really have anything to do with radiant floor heat … but it IS a pretty cool place.  So we’ve included a few more pictures.

ephesus terrace house, mosaic


We’ve discussed the Ondol or “warm stone” heating system before, in Emily’s post on what restaurants can learn from traditional heating methods, Ancient Hearth to Modern Design.


We think of the Romans as pretty “ancient” but Korean ondol radiant floor heating is possibly older than the Roman hypocaust – they’ve been found at an archaeological site dated to 1000 BC and is still used in modern South Korean homes, although the heat source has changed from cook fire smoke to hot water in tubes in recent years. South Korea has some serious winter weather to withstand, but unlike its neighbor Japan, it is not known for its un-insulated buildings.  Korean culture calls for warming the whole house … and they do it with radiant floors.

The concept is simple, and very similar to the hypocaust, in that a space runs below the floor and is filled with smoke (or hot air) which warms the floor from below.  The key difference is that an ondol creates along winding flue which channels the hot air to keep it moving towards the upward exit quickly whereas a hypocaust floor is just and open chamber that contains smoke or hot air but doesn’t direct it.  Having a long system of flues makes the cook fire “draw” more effectively and while still heating the whole floor.  Unlike the Roman system, which used a dedicated fire to heat the baths, the Ondol uses heat from the kitchen fire to warm the house, effectively making double use of what needed to happen a


Radiant floor heat is a technique that has been readily adapted to modern use.  People use thermal liquids (water or sometimes glycol) set in thin flexible tubing which can be laid under a wood floor or poured right into a concrete one.  You can control it with a thermostat and the heat source can be a standard gas fired boiler or a high-tech solar thermal panel system.  While this type of system can be difficult to add in a remodel project it is a great heat supply method for any new construction.  It should come as no surprise that we love radiant floor heat.  We use it here in the office to keep our feet (and selves) warm all winter.

Do you have any in-floor heat in your life?  If so, do you love it as much as we do?