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ANSWER:

No matter what radiant heating system or style of wood flooring you choose, four basic guidelines will help to assure a successful installation: Acclimate, Low Temperature, Even Heat and Wood Selection.

Acclimate

Make sure the sub-floor and the wood flooring are normalized or acclimated to the finished room before the wood is installed. The sub-floor or slab must be dry, and moisture barriers are always recommended. Unregulated temperatures can harm the wood, affecting its core moisture content. As the temperature goes up, the moisture content generally goes down. Excessive heating of the wood will cause it to shrink, producing gaps between the boards. Once the temperature is lowered, the moisture usually returns and the gaps close up.

In most climates, winter air is dryer than summer air. This can cause “seasonal gapping” between boards and will occur regardless of whether or not there is an in-floor heating system installed. If an indoor humidity control is not present, occupants should expect some seasonal gapping on any wood floor with or without radiant heating. “Seasonal” board gapping the size of a dime or quarter is considered normal in the wood flooring industry but can be managed through indoor humidity controls.

Temperature

While normal in-floor heating temperatures will not harm the wood, variances in moisture content and humidity (air moisture) will cause the wood to move in numerous ways. When installing soft or hard wood floors in any situation, moisture content is a key factor to successful floor performance. Adding heat to the floor makes attention to moisture even more critical, as a dry-out or overheating situation can occur. Before installation, the wood floor installer and the heating contractor should consult for the the special considerations required when combining in-floor heating and wood floors. For example, the radiant surface temperatures should not exceed 85˚, and an outdoor reset/thermostat can help with gradual heat control, reducing the likelihood of shock to the flooring.

It is important to note that almost all movement in wood floors is due to moisture imbalance in wood, and ideally, MC, or moisture content, is 6-9. Therefore, the more constantly the wood MC is maintained, the less likely there will be any problems with the floor. Gaps between the boards and checks (cracks in the board) are a result of the moisture content being too low, causing shrinking. Warping or cupping is the result of the moisture content being too high or uneven, causing the wood to expand. In either case, it is usually a result of not allowing the wood to adjust to its environment before it is installed, or maintained after its installation. While cupping of boards can be a result of improper wood floor installation, it is almost always job site or storage related. Excess under-floor moisture can result in edge cupping upwards. Dry cupping is common with engineered wood when the environment is over dry, or materials have been improperly stored in a too hot environment.

Even Heat

Spreading the heat in the sub-floor as evenly as possible is important to avoid hot spots. A moisture barrier in between the wood and the sub-floor is needed, and is a must over concrete. This barrier helps maintain an even heat with the floor and helps to protect it from moisture or condensation.

Wood Selection

Not all species of wood are good selections for this application. They are listed below in the common order of stability, the first being the most dimensionally stable, least likely to be affected by moisture or temperature fluctuations.

Laminated flooring. These products are not fixed to the sub-floor and are designed to look like real wood. These have a very hard laminated surface heat bonded to pressed wood or an (MDF) Medium Density Fiber Core.

Engineered wood flooring. This floor consists of some 3-6 plyboards of wood, each layer bonded together at biased diagonals. Depending on the style, they can be floated or fixed to the sub-floor. The top layer can be a domestic or an exotic hardwood; most can be re-sanded twice.

Solid wood flooring. There is a wide variety of wood species available, each reacting differently to moisture or humidity. If using a solid hardwood over radiant heat, the key is to select stability. Width and cut of the board also determines how stable the solid wood floor will be.

a. Most stable of woods are Mesquite, Australian Cypress, Teaks, Heart Pine, Bamboo, Walnut and Cherry

c. Softwoods as Yellow Pine are generally less stable, but can be mechanically held in place easier than hard woods.

d. Quarter or rift-sawn wood is more stable than plain sawn wood flooring. When exposed to moisture, Quarter-sawn will generally expand upwards rather than side-to-side.

e. Narrow boards (2 1/4" or less) are recommended for hardwood floors. The facts are, the wider the board used, the wider the gaps can be, narrow boards result in smaller gaps.

Thank you for your question, I hope this helps.

Expert's Bio

Robert D. Middleton Jr. has enjoyed over 30 years in the residential and commercial flooring trades. He is proficient and familiar with hardwood installations, both exotic and domestic, border and medallion work, board replacements and repairs, ceramic tiles (mortar and epoxy grout mixes), laminates, carpets, VCT (vinyl composition tiles) and 6’ commercial vinyl floors. Robert is the Technical & Installation Manager at LUMBER LIQUIDATORS, assisting customers, corporate and outside store staff with installation recommendations, specifications and product warranty details via phones and internet. Look for Robert as the In-House Installer at LUMBER LIQUIDATOR’S Flooring 101 site.