Under-Floor Radiant Systems: Feeling the Heat, Starting With the Feet

(By Susan Biddle -- The Washington Post)

Q I've been hearing a lot about under-floor radiant heat. How do these systems work, and are they cost-effective?

AIn-floor radiant heat is a delight to live with. Instead of having warm air blast at you from vents, you get gentle, even heat, without noise, nuisance or dust. And you avoid the too-hot and too-cold zones typical in houses with standard radiators or wood stoves.

The traditional type of in-floor radiant system usually needs to be installed when a house is under construction. Builders embed metal or plastic tubes or electrical heating cables in a concrete subfloor as they pour it. Later, warm water or electrical current flows through to heat the floor. The floor warms objects it touches, including people, which in turn radiate some of that heat and eventually warm the surrounding air.

It is just the opposite of what occurs with forced-air heat. Because radiant heat warms more directly, many people find that they are as comfortable with the thermostat set at 65 degrees as they are with other systems set at 68. This is the main reason traditional radiant systems are touted as being more energy-efficient than other heating options. It also helps that radiant heating doesn't cause drafts or suck cold outside air into the house, as poorly balanced forced-air systems and standard wood-burning stoves often do.

The new, under-floor radiant systems work similarly to systems embedded in concrete and have some of the same benefits. But they are typically installed between the subfloor and the finished flooring, which makes them suitable for existing houses as well as new homes and additions. These new-generation systems are also much more suitable for installing over wood subfloors.

Some of these systems stick with tubes as a delivery system for hot water, but they skip the need for a thick layer of concrete. One product, for example, comes in 4-by-8-foot sheets that double as standard subfloor material. The panels have grooves that installers stuff with half-inch PEX tubing, the same type of plastic usually embedded in concrete, and they have an aluminum coating that helps disseminate heat from the tubes. Other under-floor systems incorporate thin electrical heating cables into mats that resemble electric blankets or window screens. Installers roll them out into a mortar bed for use under tile or smooth them underneath carpet pad and carpeting. Some mats add just an eighth of an inch to the thickness of the floor, which is ideal for remodeling projects.

Because these systems aren't embedded in concrete, they perform very differently in some respects compared with traditional radiant systems, which take advantage of concrete's ability to absorb and store a considerable quantity of heat. If you heat with electricity and live where electric rates are lower in off-peak hours, a system embedded in concrete is more cost-effective because you can heat up the concrete with cheap power at night, then tap that stored heat during the day and early evening, when rates are higher. The under-floor radiant systems don't offer this opportunity because their thermal mass is negligible.

However, the trade-off is that the new systems respond far more quickly. Because a thick slab of concrete takes so long to heat up, traditional systems usually run at a constant but relatively low temperature all season. That's great if you're home all day. But if you come home mostly to sleep and shower, or if you want to take the chill off in the kitchen while you sip a morning cup of coffee, spot heat when you need it makes more sense. The new systems give you that flexibility. One caveat, though: Because these new systems are designed for fast response, they warm the floor more unevenly than standard radiant systems do. If you install wood flooring over a fast-response system, it's more likely to crack or buckle. Stick with manufactured wood flooring, which is built up in layers, or verify with the manufacturer that the wood you want to use is suitable. Avoid maple even though it works wonderfully for flooring in other situations; it expands and contracts more than many other types of wood.

Also be wary of marrying the new systems to thick carpet, even though some manufacturers brag that this will work. The thicker the carpet and its pad, the more the carpeting acts as an insulation blanket. Although heat will eventually break through, it's not a particularly efficient way to heat a room. Thin carpet works better.

Figuring out which heating system is most cost-effective is difficult, especially because the equation includes both start-up costs and operating costs, which vary tremendously depending on how individual families live. Electrical radiant systems are clearly most efficient for small, spot jobs, such as warming up a bathroom or a kitchen. Not only do these systems cost less than hot-water systems to buy and install, but the cost correlates well with the size of the installation. There is no big upfront cost, as there is when you need to buy a boiler for a hot-water system.

Systems that use hot water or are embedded in concrete may be a better option if you want to heat an entire house, particularly if you live in a cold climate. One bonus: If you have a system that distributes warm water and rates soar, you can always buy a new furnace and switch to gas, electricity, oil or even solar.