Construction Design-Heated Floors and Floor Coverings

Best Choices and What Should Be Avoided

One of the first jobs I had when I started my career in the construction industry, back when I was working as an electrician, was installing electric resistance heating cables for a basement heating system.  It took two people, one person pulled (me) while a second person pushed (my brother) and guided a small manual plow which installed electric heat cabling just under the surface of a sand layer.  Concrete would then be poured over the sand; the result would be a heated floor.  As it turned out, there was a high percentage of cable failures with this system, which resulted in many people having to install a new heating system.

Hot water radiant heat in a slab on grade home.

A few years later, electric resistance mats replaced the individual cables.  These two-foot by eight-foot drywall mats containing the electric resistant wires were connected in such a way that if one mat failed, it wouldn’t affect any of the other mats.  We would install the mats, then cover them with up to 12 inches of sand.  Concrete then would be placed.  These were storage heating systems, very slow to react to changes in the thermostat setting.  I haven’t seen either of these electric resistance systems used in a new build in several years.

Most people today choose hot water tubing which is cast in the concrete.  Heat tubing can also be placed under a wood sub-floor or installed in specially designed plywood subfloor panels with grooves cut into them to except and hold in place the tubing (Warmboard).  The water can be heated by electricity, gas, biomass, or using a geothermal source.  In-floor heating is a very popular choice in my market and there are many pluses to this type of system, comfort is a big one.  There are some things that installers and homeowners need to be aware of, like there are some floor coverings that can affect heat transfer.

A wood in-floor heating system called Warmboard.
Hot water tubing ready to be cast into a concrete slab.

When a floor heating system is under a concrete floor, the best option is not to install any flooring.  I’ve been in several homes with stamped, stained and painted concrete floors.  There is no resistance to heat flow.  One drawback, concrete can be hard on the feet and knee joints.

Hard surface floor coverings are also good for heat transfer on heated floors.  Tile and natural stone when in direct contact with the concrete can easily transfer heat.  Tiling directly to plywood is not a recommended installation method for tile.  In this case, a cement backer board would be a good choice as the tiling surface.  Other tiling surfaces, such as mats and membranes designed to isolate the subfloor from the tile may slightly limit heat transfer.  These isolation membranes are designed to leave separation between the two surfaces to allow each to move independent of each other.

A tile isolation membrane over a wood subfloor.

There are electric heating systems that are designed for tile warming, Shluter Systems Ditra Heat is one of these systems.  An electric cable is run through an isolation mat similar to the one in the photo above.  These systems are designed for comfort, warming the floor tile, not space heating.  They will supply some heat to the space, but not enough to be used as a heat source in a cold climate.

Thermal image of Ditra Heat.

A finished wood plank floor can also work with in-floor heating systems.  A true wood flooring product may move or shrink more than some of the engineered wood flooring products.  Also, some of the engineered wood floors that are installed as a floating system will have a mat or membrane installed between the subfloor and the wood flooring, which can slightly reduce heat transfer.

Vinyl sheet goods and luxury vinyl tiles (LVT) can also be good choices.  Much like wood floors, LVT systems can be floating and may require a membrane be installed between the subfloor and floor covering, slightly reducing heat transfer.

Carpets are one of the most comfortable surfaces to walk on (and one of the worse for indoor air quality, a discussion for another time).  When installed over in-floor heating systems, carpet can limit heat transfer.  We were involved in an in-floor heating project many years ago where a homeowner complained that his house could not achieve a desired temperature, even when the thermostat was set much higher than needed.  The air temperature of the home could not get above the low 60’s.  We tested and found the concrete slab temp under the carpet to be 120°F.  Turned out the carpet contractor installed the wrong carpet pad, and the result was like installing insulation over the concrete slab.  The heat was being trapped under the pad.  There are carpet pads designed to be used with in-floor heating systems, but even using these pads, there will be some reduction in heat transfer.  The rubber pads have the lowest R-value of all the carpet pads, with values as low as R-.31.  Some of the polyurethane pads can be as high as R-2.15.  Combine the pad with the carpet, and insulation levels go even higher, great on cold floors, but not recommended on heated floors.

Two different carpet pads. The brown pad is used for in-floor heat applications.

The same can be true with area rugs.  The heat radiating from the floor can be trapped under the rug, especially ones that include thick pads.

There are many advantages to using an in-floor heat to heat a home, the radiant heat is very comfortable as long as it’s properly installed.  The systems are typically quiet, most people can’t tell when they are on or off.  Hot water tubing systems can use any fuel source to heat the water, systems can be designed using multiple fuel sources.  Understanding how the choice of floor coverings might affect heat transfer need to be considered during the design.

Leave a Reply

Your email address will not be published. Required fields are marked *