A warm bathroom floor is a something to look forward to on a chilly winter morning, and C.L. is poking around for ideas on the best way of accomplishing that.
One option is installing a grid of electric cables beneath the finish floor in tandem with a polyethylene underlayment manufactured by Schluter Systems called Ditra. These installations are often topped with ceramic tile, which is impervious to water damage and readily transmits heat from the buried cables.
But C.L. has another idea.
“In regards to finish flooring over the heated floor in the non-shower part of the bath, cork looks like an interesting product,” C.L. says in a recent Q&A post. “Although cork is sometimes discussed as an insulator, the [manufacturers] of solid cork flooring propose it as an ideal finish floor for a heated floor; supposedly it heats fast.
“Does this sound reasonable, or is this just marketing hype?”
That’s the topic for this Q&A Spotlight.
First, how efficient is electric heat?
C.L. begins his post with a question on whether there’s a recognized metric for measuring the efficiency of electrically heated bathroom floors. Actually, there is, points out GBA Editor Martin Holladay.
“Yes,” he writes. “The metric is called ‘efficiency.’ It describes the conversion of electrical energy into heat energy. All electric-resistance floors have the same efficiency, namely 100%.”
This is technically true of all electric resistance heat, whether it’s in the floor or not. But Jon R suggests that definition doesn’t go far enough.
“A reasonable definition of efficiency would involve useful work,” he says, “so I’d exclude any heat lost to the underside of the floor. This efficiency will be less than 100% and cork will lower it (as compared to something more thermally conductive like tile).”
Would it make any difference if the radiant heat in the floor were provided by a hot water loop installed beneath the subfloor, C.L. asks. One advantage of this option, he adds, would be the opportunity to replace the finish floor in the bathroom at a later date without affecting the heat distribution system.
“I think this would be less efficient as the heat would need to transmit through the subfloor,” C.L. adds. “The subfloor material probably has an impact — temperaturei.e. vs. OSB vs. plywood. Is there any simple way to calculate the efficiency hit of going through that additional layer?”
There’s no need to make it that complicated, Holladay says.
“As long as your home’s thermal envelope has adequate insulation, an electric-resistance heating pad or PEX tubing installed as part of a radiant-floor heating system aren’t less ‘efficient’ if there is a thick subfloor or inappropriate flooring. The heat remains indoors, so it isn’t ‘lost.’ The problem is that a floor assembly with a thick subfloor or inappropriate flooring is less responsive, and takes longer to heat up, than a floor assembly with well-chosen materials. Moreover, the heat may end up in a different room than intended (the room below the floor assembly).”
Understanding heat delivery
It’s not a hydronic system C.L. system has in mind, but an electric under-floor mat made by . How effective would that be in delivering heat to the floor?
Dana Dorsett suggests C.L. look for a chart provided by the manufacturer of the electric heat that describes the amount of heat it can deliver through different types of subflooring and flooring.
“If this system is not temperature-controlled but has a watts per square foot spec or watts per length spec, the R-value of the subfloor + floor don’t matter as much as the ratio of the R-value of the floor materials to the R-value of the insulation below,” Dorsett says “If it [is] R-1 of floor materials to R-20 of insulation, about 95% of the heat will be coming through the floor. If it’s R-2 flooring to R-11, something like 85% will be coming though the floor. To convert watts to BTU/hr, multiply by 3.412.”
He uses as an example the system C.L. has referenced, which delivers 72 watts for a 6-foot-long section or 12 watts (41 Btu/hour) per running foot.
“If it’s between joists 16 inches (1.33 feet) on center it can deliver 41/1.33 = 39 BTU/hr per square foot of floor going into the system, but only part if it is going up,” Dorsett says. “Some is going down through the insulation. If it’s only delivering 85% of it up, the rest [is] going through the insulation and joists. It’s about 33 BTU/hr per square foot coming through the floor, and the surface temperature of the floor will be about 16 F° warmer than the room temperature. In a 75°F bathroom that would be a bit north of 90°F, which is warmer than most people like on bare feet, but not super uncomfortable.”
At 50 Btu/hour, the surface temperature would be about 25 F° warmer than the room temperature. “On a tile or stone floor in a 75°F room that can be pushing the limits for barefoot comfort,” he adds.
Is cork an appropriate floor finish?
The Schluter Ditra system is typically used with tile, not cork, says Holladay. “I’ve never heard of anyone installing cork flooring above this type of heating mat,” he says, “and I’m skeptical as to whether it’s a good idea.”
Steven Knapp has similar concerns, adding that the cork products he’s researched are not recommended for bathrooms because they can swell and buckle when they get wet.
C.L., however, says that a manufacturer of cork flooring actually recommends the Schluter Ditra system topped with cementitious layer made by .
“This provides a waterproof membrane (Ditra) and a cementitious ‘subfloor’ (Ardex),” C.L. says. “Then install their cork tiles with contact adhesive onto the Ardex. This also allows for a future finish floor replacement without destroying the heated floor — you scrape the cork tiles off the Ardex.”
C.L. also makes a distinction between solid cork flooring and engineered cork flooring in which a top layer of cork has been applied on a backing made from a different materials.
“In regards to cork being unsuitable for wet areas due to swelling and buckling, wouldn’t that only apply to engineered cork products on a backing that would swell?” he asks “The [manufacturer] does warn that their engineered floating product is not suitable for wet area installation. They have no such warning on their solid cork product. Solid cork tiles have air pockets; would that preclude or reduce swelling?”
If the manufacturer warranties a glue-down, solid cork tile, there’s no need to worry about it, Knapp replies, although it would be smart to check whether the backer plus the tile will create an “awkward elevation change” between the bathroom floor and any adjacent flooring.
How much floor should be covered?
Whether C.L. uses cork or tile as a finish floor, there’s still the question of exactly what parts of the bathroom should be covered. Should the shower floor, for example, be included?
“We have Ditra heat under our bathroom tile floor,” says Stephen Sheehy. “It works very well. We generally turn it on 30 minutes or so before a shower. I suspect tiling your entire floor, with heat underneath, won’t be more costly than doing part with tile and part with something else. You wouldn’t need to manage the transition between the two floor types.”
In Sheehy’s bathroom, the shower is included. The shower is open to the rest of the room, with the floor in the shower sloped toward a linear drain near the wall. “There’s no shower door or partition,” he says. “The whole floor is heated. When I shower, I usually don’t bother with turning on the floor heat, but my wife likes it.”
One caution about the extent of under-floor heat comes from Dorsett: Do not put radiant floor under a toilet. The heat could melt the wax seal connecting the toilet with the drain line.
And then Peter Engle made this suggestion: “You do want to run the radiant at least under the toekick if you have standard vanities, or about 6 inches past the front if you have furniture-style vanities (open bottoms with furniture feet),” he says “Otherwise, your toes are touching cold tile when you brush your teeth. Voice of experience, here.”
Our expert’s opinion
GBA Technical Director Peter Yost added this:
Here are a few thoughts on this thread, particularly about thermal comfort:
Thermal comfort versus energy efficiency: For C. L., with a central HVAC system covering loads in the bathroom, the radiant floor heat is all about thermal comfort.
Thermal comfort of feet: ASHRAE Standard 55 gives this range for thermal comfort of feet in shoes as between 66.2° and 84.2° F. That’s not terribly helpful for a bathroom floor around a walk-in shower. provides more information (see Table 5). It’s interesting that the “comfortable” temperature range for a concrete floor is narrower and higher (78.8° to 83.3°F) than for a cork floor (73.4° to 82.4°F) and even narrower and higher for a marble floor (perhaps the most like ceramic tile (82.4° to 85.1°F).
Flooring contact coefficient and foot thermal comfort: The most interesting paragraph to me in this paper came under the start of “Discussion” section:
“It is apparent from the series of experiments performed with 16 persons that it is not possible to find a floor temperature where all persons are satisfied. Neither is it possible to achieve less than 2% dissatisfied for short periods of occupancy (1 min) nor less than 11% dissatisfied for longer periods (10 min). These values can be attained when the temperature of the floor is optimal, i.e., that temperature which causes a group of persons occupying the floor on an average to evaluate foot comfort as neutral (voting = 0). If the floor temperature deviates from the optimal, the increase in the number of dissatisfied persons will depend on the flooring material. For floors with a small contact coefficient (e.g., cork, wood) the increase in the number of dissatisfied-persons will be moderate compared to floors with a large contact coefficient (e.g., concrete, stone).”
In additional experiments covered in this paper, with occupants standing and seated and with “light” clothing on, the change in clothing had little or no significant impact on comfort results.
Service life and performance of grouts: I have done quite a bit of ceramic tile work over the years and used both epoxy and polyurethane grouts. I have found both superior in terms of watertightness and stain/discoloration resistance but have found the polyurethane grouts a bit easier to work with.