Carpet and Pad with Radiant Floors

Most buildings will include carpet and pad, often the majority of the floor coverings will be carpet. This presents a challenge for the designer of a radiant floor heating (or cooling) system. Carpet and pad is, by design, a thermal insulator. It would probably be wise to state right up front that any thought of using carpeting over a floor cooling system should probably be abandoned. While it is possible, the use of carpeting dramatically reduces the cooling capability of the floor. Radiant floor heating, on the other hand, is quite tolerant of carpeting although there are limitations. The designer and installer of radiant systems should be aware of these limitations in order to direct the building owner into making wise floor covering decisions.

Carpet is generally accompanied by carpet cushion. The insulation value (expressed in R-value) is often greater for the carpet cushion than for the carpet itself. 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.

There are three major types of carpet cushion that most homeowners recognize: urethane, fiber and rubber. There are different types and grades within each grouping. Thickness and weight are the yardsticks to differentiate one from another. This differentiation is divided into two groups. Group One uses density per cubic foot while Group Two uses ounces per square yard. This leads to confusion when someone attempts a simple direct weight comparison. 'Density per cubic foot' is the total weight of cushion material in one cubic foot, this is 12"x 12"x 12". 'Ounces per square yard' is the total weight per 9 square feet regardless of thickness. All carpet cushion can be stated in both weights 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 or "bonded" Urethane are the only ones we need to know. Prime Urethane is carpet cushion made from virgin polyurethane foam and is usually uniform in color. Rebonded Urethane cushion 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.

Group Two materials include Sponge Rubber, Synthetic Fiber, Hair and Jute. Cushions found in Group Two are more dense than Group One and have lower R-Values, therefore these are more suitable for radiant floor installations. Fortunately, a dense cushion is also recommended by most carpet manufacturers. Dupont says, "Quality carpeting needs quality padding (or "cushion") for maximum durability, comfort, and beauty retention. That's because padding helps absorb foot traffic pressure. Its primary purpose is to provide a firm support for the carpet. That's why a dense, resilient pad is usually recommended; it performs better than a thick, soft pad." The less dense urethane cushions in Group One are most commonly used in residential applications because of price and ease of installation due to their light weight and workability.

 Note: All R-values listed are approximate. Check with product supplier for actual values.

Density determines the minimum cushion thickness recommended by the carpet manufacturer. The minimum recommended Prime Urethane cushion is 3/8" while the minimum Rubber cushion is 1/4". The difference in R-Value between these two materials and minimums is dramatic. The 3/8" Prime Urethane has an R-Value of 1.62 while the 1/4" 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°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 start-up. The impact on radiant floor design is obvious. A radiant floor design which may be perfectly adequate using a rubber cushion could end up unable to meet the heating requirements when a urethane cushion is substituted.

Not all rubber cushions are the same. There are two types; slab and ribbed or "waffle". The slab rubber is a flat foamed rubber where the waffle is formed in a corrugated pattern similar in appearance to a waffle. The quality of rubber used is important. The cheaper products use clay fillers and have an oily substance which can leach out over time, particularly when heat is applied. The result is a cushion which 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.