RHR 1999: Carpet & Radiant Heat
What do expansion tanks, circulators, auto-air purgers, zone valves, elbows, ball valves and carpet have in common? They are all components of my radiant heating system.
I bet it's hard for many radiant contractors to think of carpet as an integral part of a complex network of pipes, valves and boiler(s) that make up a heating system.
But if the floor is to be the radiator, you are going to have to think about carpet. And it's even harder for consumers to understand the relationship. The floor-covering supplier generally doesn't have a clue either. Take my local carpet store. When I asked a salesmen to show me a carpet pad that had a low R-value, I got a blank stare. Then I asked what carpet pad would work well with radiant heating. The salesman picked up a nice fluffy blue foam pad and said: "This one will do the job. Look, you can blow through it. That means the warm air can get through it."
OK, so carpet salespeople aren't supposed to know about heating systems and radiant contractors aren't supposed to be carpet experts. But who does that leave? Somebody has to have the answers. Well, who do you suppose will get the call when the heating system doesn't keep up on a snowy day? I'll bet you dimes to dollars it won't be the carpet store.
Radiant heating contractors need to know enough about carpet to advise consumers how to choose the proper products.
Carpet & CushionThe carpet and cushion combination is, by design, a thermal insulator. Radiant floor heating, however, is quite tolerant of carpeting but there are limitations.
Although carpet is made from a variety of materials, most are synthetic and have similar R-values. The R-value can be determined by the thickness of the carpet (see Table 1.) On the other hand, the R-value of carpet cushion can vary greatly depending on the materials it is made from. The insulation is often greater for the carpet cushion than for the carpet itself.
There are three major types of carpet cushion to keep in mind:
What's more, there are different types and grades within each group. Thickness and weight are the yardsticks to differentiate one from another. All carpet cushion can be stated in both density per cubic foot or ounces per square yard, but density per cubic foot is the best comparison for our purposes (see Table 2.)
Group one materials include prime urethane, grafted urethane, densified urethane, rebond and mechanically frothed urethane. To most of us, prime urethane and rebond (also known as "bonded" urethane) are the only ones we need to know about. Prime urethane is carpet cushion made from virgin polyurethane foam and is usually uniform in color. Rebond is made from trim generated in the production of urethane foam, which is granulated, then bonded to form padding. It is easily recognized by the small chunks of foam of different shapes and colors seen on the surface.
The less dense urethane cushions in Group one are most commonly used in residential applications because of price and ease ofinstallation due to their light weight and workability.
Density Is KeyGroup two materials include sponge rubber, synthetic fiber, hair and jute. Cushions found in Group two are denser than Group one and have lower R-values; therefore these are more suitable for radiant floor installations. Fortunately, most carpet manufacturers also recommend a dense cushion.
Density determines the minimum cushion thickness recommended by the carpet manufacturer. The minimum recommended prime urethane cushion is 3/8 inch, while the minimum rubber cushion is a 1/4 inch. The difference in R-value between these two materials and minimums is dramatic. The 3/8-inch prime urethane has an R-value of 1.62 while the 1/4-inch slab rubber foam is only R-0.31. That's over five times less restrictive. This means that the floor under the prime urethane cushion must be approximately 40 degrees F warmer than under the rubber cushion to transfer an equal amount of heat to the surface! It will also take longer for the heat to transfer through the prime urethane on initial startup. The impact on radiant floor design is obvious.
Keep in mind, not all rubber cushions are the same. There are two types: slab and ribbed or "waffle." The slab rubber is a flat foamed rubber. The waffle is formed in a corrugated pattern similar in appearance to a waffle. The quality of rubber used is also important. The cheaper products use clay fillers and have an oily substance that can leach out over time, particularly when heat is applied. The result is a cushion that crumbles when it gets old. Good quality rubber products do not have this problem and often carry lifetime guarantees.
Most modern carpet cushions are environmentally friendly and have low levels of TVOCs (total volatile organic compounds). The Carpet and Rug Institute has developed testing and labeling programs to aid in the selection of carpet, adhesives and cushion material. The testing program identifies low-emitting products for consumers by requiring representative product samples to meet scientifically established standards for emissions. Look for products displaying the CRI IAQ Testing Program Logos.
It is not unusual for the radiant floor designer to know more about carpet pads than the carpet salesperson when it comes to radiant floors. It is important that the building owner and the carpet salesperson be capable of making an informed decision when selecting floor covering. Be aware that a last minute substitution of carpet pad could mean the failure of your heating system to perform.
Until the carpet industry is educated on radiant floor applications it is up to the radiant floor designers, sales people, installers and manufacturers to assist consumers in making and sticking to the right choice. Done correctly, there is no heating system that compares in comfort to radiant floor heating - even with wall-to-wall carpeting.