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There’s a reason why in Korea and other Asian countries people stay so close to the ground, literally sitting on pillows or mats set on the floor. And have you ever noticed when you see ancient Romans portrayed in movies they always seem to be lounging on low beds or on the floor, reclining on lots of luxurious pillows? You’d think with massive stone interiors so common in ancient times those people would be freezing. But in fact, they weren’t cold because since Roman times, different cultures have figured out how to heat their floors and walls from below.
Romans called their under-floor heating systems “hypocausts,” and even heated indoor pools in posh bathhouses. In Korea, the under-floor heating system was called ondol, literally meaning “warm stone.” The Korean and Roman systems were similar: Heat from stone fireplaces and stoves below the living-area floor circulated through vents and ducts under the floor and behind walls, warming them and radiating into the room. Hot air or steam from the fires eventually vented through chimneys at the corners of the rooms.
The idea of heated floors is relatively new in America, and was first introduced here by Frank Lloyd Wright, who discovered the Korean ondol concept while in Japan designing the Tokyo Imperial Hotel in the 1920s. Wright brought Asian-inspired under-floor heating to America, but with a twist, by inventing hydronic radiant flooring. He designed a system to run hot water heated from a natural-gas boiler through metal pipes that were embedded in concrete floors.
Radiant floors are finally catching on in the U.S., as improvements since Wright have made the method much more reliable. Now the heated water is not run through copper or other metal pipes, which are more likely to leak and harder to install, but through PEX, a sturdy but flexible plastic that some plumbers now use to plumb an entire house.
PEX was introduced in the 1970s in Sweden by one of the world’s oldest plumbing and piping companies, Wirsbo, founded in 1620. The material is now used in the majority of radiant systems, and is the reason why radiant flooring is increasing in popularity. PEX tubes are inexpensive, easier to install than metal, and last a long time without the need for any maintenance.
What you Need to Know
Many different flooring options are possible with radiant heat. There’s a lot of misconception about what floors work best over radiant heat; the bottom line is many floor types work perfectly fine if they are installed correctly and the system design takes them into account. Some flooring materials are more efficient in radiating and holding the heat than others, but you do not have to feel limited. If a contractor tries to tell you that you cannot use radiant heat in a material you might prefer—wood instead of poured concrete, for instance—then find a contractor who is able to work with your preferred flooring, because chances are it can be done. Floor choices that can be coupled with radiant floors include marmoleum, tile, wood, bamboo, cement, and stone.
Radiant tubing can be put over the sub floor, under the sub floor, or be incorporated into the sub floor itself. Most new homes have the radiant tubes installed over the sub floor, or within the sub floor as part of panelized radiant sub-floor panels. With a renovation of an older home, it might not be possible to easily replace the floor and sub floor, or put the tubes above a sub floor, so often the tubes can be installed underneath the sub floor from the basement or room below.
Choose the right power source to heat the radiant floors. Just as without radiant heating, there are many options on how to heat the water that is used in the home. The heat source could be a conventional water heater that uses propane, natural gas, or electricity; or even better, you could get a geothermal heat pump or solar water heater to do the job.
Here are a few ideas on how to get the hot water you need:
- The simplest option is to get a super-high-efficiency boiler such as a condensing hot water boiler. These are more than 90 percent efficient and well worth the upgrade.
- If you want to go even greener, you can install a solar hot-water heater to run hot water through the tubes under the floor. You can also use some of that hot water you get from the solar roof panels for your hot-water heating in general. You’ll still need a back-up hot-water heater, but it’s pretty affordable and saves energy.
- An even more ecologically responsible option is a geothermal heat pump to generate the energy you need for hot water. Better yet is to add a solar PV system to offset the electric use of the geothermal heat pump. With this kind of combined system you can get radiant heated floors and a home that might reach zero net-energy use.
