Q: We have a radiant floor in our kitchen that now has a linoleum flooring. Will a bamboo floor work over the radiant heating system?
—Asked by Polly D, Concord, NH

A: The short answer is that radiant heat works well underneath certain types of linoleum, ceramic tile, stone, hardwood, bamboo, cork or carpeting as long as the floor temperature is not too high, the flooring is designed for radiant heat and is acclimated properly before installation.

Most flooring manufacturers require certain conditions prior to, during and after installation of their floors for their warranty to be valid over radiant heat. Now for the long answer.

First, check the warranty. Not all flooring can be used over radiant heat. In most cases engineered or floating floors are warranted, but not always. There are some types of solid wood floors, usually exotic woods, which will work over radiant heat too. But in every case, you and your contractor need to follow the manufacturer’s guidelines very closely.

For a floor to be warranted for radiant heat means it must be capable of expansion and contraction without cracking or delaminating. For this, it must be designed and constructed specifically for radiant heat. That usually means it is engineered flooring, although certain types of solid wood flooring may work too.

Engineered flooring can be designed for extreme conditions of moisture and dryness by using alternating layers of wood layered perpendicularly one on top of the other. Usually the top and bottom layers are made of the same type or density. In addition, very stable quarter-sawn wood or high-density fiberboard cores may be used along with special elastic adhesives and fabrication processes which all contribute to a floor that will accommodate excessive moisture and excessive dryness.

Second, check the floor temperature requirement. Most companies say not to exceed 84-85 degrees F after installation. This is relatively easy to control but takes some experimentation to get it right. If the water temperature in your boiler is, say, 115 degrees F, it may end up being 85 degrees F by the time it gets to the second floor. You have to test throughout the house. Closer to the source of heat will be hotter.

The danger in exceeding 85 degrees F is to dry the wood out completely. If this happens, it can split the wood. All wood is hygroscopic which means it absorbs moisture and then gives it off. This causes the wood to expand and contract which is a natural process. However, when the floor is heated too much, it can stress the wood causing it to warp, twist, buckle etc. beyond its normal capacity.

Third, acclimate the floor properly. Acclimation refers to the adjustment time for the moisture level in the wood to synch up with its new environment. Unlike solid wood products, engineered flooring generally requires very little, and in some cases, no acclimation. However, there may be strict requirements for the sub-floor moisture conditions prior to installation.

The drying process is something that can't be ignored as it will definitely create problems for any type of flooring if the heat is cranked up right from the get go. Most manufacturers who warrant their flooring over radiant heat require a gradual increase in floor temperature over a period of several days in order to make the transition to a lower moisture condition as natural as possible. Speed is the key here. Too fast, forces too much moisture out of the concrete and into the wood which may stress the wood.

Moisture is always the key factor in determining suitability. If the subfloor is damp, it will need to dry out. All new concrete floors are full of moisture when new and need time to dry out. If a concrete floor has been poured 6 months ago, which is typical, it is assumed that most of the moisture has dried up. This can be true if the weather is hot and dry most of the time. But in some states or times of the year when it rains a lot or it’s very cool, concrete can take months to fully dry.

While radiant heat will certainly dry things out more quickly, you don’t want your flooring to be sitting on top of a damp subfloor when this process starts.

Using proper moisture meters or moisture testing procedures (such as Calcium Chloride tests) are vital to this process. These can be rented from rental companies or purchased on line. Most experienced contractors have these too. Each manufacturer will provide you with specific details about the proper moisture level for their floor. If you don’t see it written down, call them and find out.

If you ever have a warranty claim, the first question the manufacturer will ask you is “what was the moisture level of your floor and your subfloor”. If you don’t know because you did not test, then your chances of getting them to accept your claim will be seriously compromised.

Back to your original question: "Can bamboo be used over radiant?" Yes, there certainly are some manufacturers who make products for this purpose and others who don't. The Teragren, brand of bamboo for example, makes several types of engineered floating floors that are all designed for radiant heat. Most can be floated over your linoleum floor. Kahrs hardwood flooring is another favorite brand which invented engineered flooring in Sweden where 80% of the people have radiant heated floors.

There are basically two types of radiant heating methods: electric and hydronic. Some manufacturers will warrant their floors over both and others only over hydronic.

Regardless of how the energy is created to produce the heat, both methods are used inside or on top of concrete slabs or underneath or on top of wooden subfloors.

Wood, bamboo, linoleum, cork, tile and carpet all have different R-values from 1-3 and they typically use an underlayment or pad with an R-value of 1-2. The question naturally arises whether the radiant heat can get through all that. Radiant heat is infra-red which means it heats objects and transfers energy through any medium depending upon the density, humidity and thermal resistance of the object. It is well known that radiant heat can and will penetrate just about anything no matter what the R-value. It may take longer to penetrate, but it will eventually get through.

Ideally, lower the R-value in the floor and underlayment, the better. Some underlayments made from synthetic rubber have extremely low R-values and are ideal for radiant heated floors because they allow the heat to transfer through more quickly SCI DeciBlok.

Radiant heat transfers heat through the floor to your feet and eventually your entire body. Slowly, quietly and naturally the heat moves in all directions unless it is reflected by a “radiant heat barrier.”

Radiant heat barriers are usually recommended to keep the heat from being lost downwards or through the ceilings or walls of your building. These barriers are usually in the form of a foil backed bubble wrap material or in the form of tiny glass beads added to paint and sprayed on the walls or ceilings: Radiosity 3000. Both are highly recommended as a means to keep the heat inside your home. Radiant barriers can also reflect the sun's energy back outward keeping a house cooler too.