A contractor's guide to selecting floor coverings for radiant heating.

While consumers are warming up to radiant floor heating systems, it's surprising how little information is available about the performance of floor coverings used in conjunction with this type of heat.

How can you, as a contractor, protect your customers from potential damage to floor coverings and an ineffective heating system resulting from improper choice or application of a floor covering? The answer is to arm yourself with knowledge. Knowing the right questions to ask the flooring manufacturer or retailer and who to involve in the flooring selection process will prove much more beneficial than laying down a laminate and hoping it stays put.

Flooring Factors

One of the most overlooked, but important, factors in every radiant floor heating installation is the properties of the floor covering being installed. Temperature is one of the greatest forces affecting the stability of the floor covering.

Every material has a "maximum exposure temperature." Maximum exposure temperature is defined as the highest temperature a material can withstand just prior to a loss of physical properties and/or a reduction in product performance.

Don't confuse maximum exposure temperature with R-value, which is the heat conductivity of the flooring material itself. The higher the R-value of the floor covering, the less heat will rise through the floor. A lower R-value indicates a more conductive type of flooring. The maximum exposure temperature that the floor covering can withstand directly results in a surface temperature that determines the amount of heat available from the floor. The conductivity of the floor affects the efficiency of the flooring system or the ability of the heat to be transmitted from the source (bottom of the floor covering) to the surface.

First, establish the heating requirement for the room by performing a standard heat loss calculation. As an example, take a room that is 12 feet in length by 12 feet in width, for a total area of 144 square feet, with a heating requirement of 4,059 Btu (the heating requirement is determined by use of a heat loss calculation method). Floor heating is best understood by looking at 1 square foot at a time, so divide the total heating requirement for the space by the floor area available for heating. For this example room we will have all the floor space available for heating, so the calculation is 4,059 Btus divided by 144 square feet, yielding 27 Btu per square foot. Based on the chart on the next page, the floor surface temperature required to maintain a room temperature of 68 degrees F is approximately 83 degrees F.

Btu/sq. ft. - Surface Temperature In Degrees F

    5 - 70

    10 - 73

    15 - 75

    20 - 78

    25 - 80

    30 - 83

    35 - 85

Another factor affecting exposure temperature is the composition of the material between the radiant heat system and the floor covering, whether that material is a subflooring, insulator, adhesive or even air.

Any material that comes between the radiant heat system and the surface of the floor needs to be evaluated within the specific context that it is being applied. A subfloor or insulating material applied directly over the radiant system will have its own maximum exposure temperature, while the presence of both materials will affect the heat transmission to the floor covering itself.

Each material applied must be evaluated according to its level of heat exposure and how its R-value affects the transmission of heat to the next layer in the floor's construction.

Another important installation consideration is ensuring that pipes are spaced evenly so that there is no "striping" or cold spots between the pipes. In addition to feeling uncomfortable, varying temperatures can cause uneven reactions and damage to the floor's surface.

Now that you're equipped with calculations and installation considerations, you'll want to apply them to the type of floor covering desired to enhance the aesthetics of the setting. Selection should be a team effort, with all parties - interior designer, builder, contractor, flooring vendor and customer - participating in the final selection to ensure the customer gets the look he wants, as well as the long-term performance he deserves.

Wood Floors

While many people would expect a hardwood floor to be a poor choice because of the direct heat produced by radiant pipes, this is a common misconception. In reality, many hardwoods are well suited for radiant heat applications with the proper level of caution and care.

"Wood flooring and radiant heating can perform very well together," says Warren Spradlin of the National Oak Flooring Manufacturer's Association (NOFMA).

Spradlin says that the No. 1 question NOFMA receives from installers is "What can I do to prevent too much drying?" The association recommends installing and running the heating system before the wood flooring is delivered in order to dry the system. Most contractors report a minimum of 72 hours; however, a week is recommended, as lightweight concrete, gypcrete and gypsum slurrys tend to dry slowly. Since heating is not typically required year round, the contractor should allow for expansion during the nonheating season.

Other considerations for hardwood flooring:

  • Use strips or planks less than 4 inches wide. Edge grain and quartered products tend to be more stable.

  • Use a moisture meter to check the average moisture content of the flooring with 20 or more readings.

  • Expect some extra cracks between the strips during the heating season. Cracks, however, should not be significantly greater than a nonheated floor when proper installation guidelines and occupied jobsite conditions are met.

  • Provide an outside thermostat to call for heat during rapid outside temperature changes. Preheating helps even the demand load, whereas setback thermostats can shock the flooring and cause performance problems.

  • Avoid asphalt felt (or rosin paper) under flooring, as odors may develop when heated.

Carpet & Padding

When installing radiant heat under carpeting, heating performance issues are right up there with the more common concerns about wear and tear and how to clean spaghetti stains.

According to the Carpet & Rug Institute, most floor manufacturers suggest a total maximum R-value for carpet and cushion not to exceed 40. The institute recommends lower pile carpet and the absence of cushion, or cushion no thicker than 3/8 of an inch. Both the Carpet & Rug Institute and the Carpet Cushion Council are working with the Radiant Panel Association to provide a system that will allow floor-heating designers to properly compensate for various floor coverings. Until a rating system is developed, R-values for various types of carpets are the best indicators available.

However, two other factors appear to have more bearing on carpet/radiant heating compatibility than the thickness of the carpet itself. According to a study conducted at the University of Illinois, the thickness of carpet actually has nothing to do with its conductivity; instead, conductivity is dependent on the amount of heat generated from the material below it.

The weight of the carpet pad, or cushion, which directly affects the life of the carpet, can also inhibit heat conductivity. The Carpet Cushion Council provides a table of typical R-values for the most common carpet cushions. This value, according to the RPA, must be added to the R-value of the carpet or other flooring covering laid over the cushion. The combined R-value is used in floor heating calculations to determine the subfloor temperatures.

Vinyl Flooring & Laminates

I recently called several major floor covering retailers and asked the same question: "How do laminates hold up when used with radiant floor heating systems?"

Half admitted to experiencing problems with expansion, contraction, and warping, while the other half claimed any type of flooring, if installed properly, will hold up.

Who do you believe? The manufacturer. When in doubt, always consult with the flooring manufacturer. Armstrong's Web site, for example, has specific installation guidelines for its vinyl floor coverings complete with an "Ask the Expert" section that allows you to ask specific questions via e-mail.

Here's what the site has to say about vinyl flooring, temperature and radiant heat: "Just as proper conditioning of the installation site and flooring products is important before and during installation, it is equally important to maintain room temperature at a minimum of 65 degrees F for 48 hours after completion of the installation. Maintain a minimum of 55 degrees F thereafter. Proper room and material conditioning not only affects flooring products and adhesives during the actual installation but also assures proper curing, setting and bonding of these products. Sudden temperature changes and temperature extremes can adversely affect the installation and performance. The surface of radiant-heated floors should not exceed 85 degrees F."

Tile & Marble

"We're often asked if ceramic tile can be safely installed over radiant heating systems," says Duncan English of the Tile Council of America. "The answer is yes, but the installation must be carefully worked out in the early design of the structure."

The council stresses the need for careful measurements of the material's thickness over the pipe, as well as proper treatment and installation. Installation methods, such as a thin-set or conventional mortar method, will protect the tile from direct heat or cracking concrete. Under no circumstances, warns the council, should organic adhesives be used over radiant heating systems.

Larry Drake, president of RPA, says contractors should always control cracking in the concrete over which a marble or tile flooring is laid.

"Any time a crack appears in a floor laid over a radiant heating system, the client tends to blame the radiant system," he explains.

Cracks in the tile or marble can be prevented by making soft cuts into the concrete that is laid over the heating pipes, so if the concrete does crack, it cracks along the soft edge, rather than at sharp edges where cracks form naturally.

When laying the floor covering, apply an epoxy-type grout between the floor covering on that crack or a Bostec elastomeric bonding agent. This will break the bond between the floor covering and the actual slab so if a surface crack occurs on the slab, it doesn't penetrate to the floor covering.

Since many people are involved in the design process, it's easy to overlook a basic issue like floor covering selection. When dealing with radiant floor heat, don't make the mistake of assuming someone else will ask the right questions.

As the contractor, you can protect your clients by asking the right questions to the right person. A thorough knowledge of exposure temperatures, heating calculations, R-values and installation methods will help you help your client choose the right floor covering solution and leave everyone standing on solid ground.