Here's the good news.... Solid hardwood flooring can absolutely be installed over a radiant heating system very successfully.
Having made that bold statement; the key to the success of hardwood over radiant heat lies in everybody understanding the science of heat and moisture as it relates to wood.
OK so let's start with the basics...
Wood is hygroscopic; meaning that when exposed to air, it will lose or gain moisture until it is in equilibrium with the humidity and temperature of the air.
Wood is considered to have reached it's EMC once it no longer gains or loses moisture. If environmental controls can be kept at a constant, no visible changes in the dimensional characteristics of the flooring would occur. Here in essence is the goal of any floor installation. In an ideal world we would prepare an environment for the flooring and then maintain it....... no problem right !!
The reality is that in a new construction, the site conditions can be unpredictable due to seasonal changes. There is also the impact of other trades involved in the project, and how in doing their particular job they will inevitably affect the atmospheric conditions within the house.
It is here that the homeowner, general contractor, HVAC contractor, and flooring contractor must all understand that any compromise in procedure will result in an unsatisfactory wood flooring experience.
Radiant heating systems by proximity exert cellular stresses on wood that can prove irreversible, so a detailed chronological timetable must be agreed and adhered to by all concerned.
Radiant Heat Installs
With radiant heat, the heat source is directly beneath the hardwood flooring. Radiant heat does not heat air directly as do more conventional forms of heating, such as base board convectors or forced air circulation. Radiant heat is omni-directional. Unlike warm air, which tends to rise, radiant energy tends to travel in all directions, a large area of mild surface temperatures, such as a warm floor, is capable of transferring as much heat as a small surface area, such as a steam radiator, at high surface temperatures.
The installation of radiant heat involves placing tubing beneath a subfloor, in gypcrete above a subfloor, or inside a panel fixed on top of a conventional subfloor. There are many systems being sold, but in essence the principal remains the same. Your wood flooring will become part of the mass that heats your house. (See examples below.)
The most important factor for a successful wood flooring installation over radiant heat, is making sure that the subfloor on which the hardwood will lay is dry before the two come in contact. For example if flooring contains 6% moisture and is installed over a subfloor with 16% moisture the flooring will start to absorb water and swell. This could result in cupping or buckling. The National Wood Flooring Association recommend that the differential between a subfloor and the hardwood be no greater than 4% in the case of strip flooring (2 1/4 Inch), and 2% in the case of plank (4 Inch & above) flooring.
Problems will occur when flooring over a gypcrete slab, or subfloor system that is not dry enough. The only way to dry a slab, or subfloor is to turn on the radiant heating system before installing the wood flooring. If this isn't done, any moisture left in the substrate will enter the wood flooring as soon as it is turned on. The result is floors that will expand, contract, shrink, crack, cup and bow excessively. If the heat can't be turned on, then everyone involved down to the homeowner should understand and accept the compromises that will appear down the road.
The amount of time required varies widely depending on the style of system used, but in all cases the goal is the same.
Certain species are known for their inherent dimensional stability such as American Cherry, American Walnut, Mesquite, Teak, Santos Mahogany and others. Other species such as Maple and Brazilian Cherry are considered unstable and are not suitable for installation over a radiant heating system.
Quarter sawn or rift sawn wood flooring, is dimensionally more stable then plain sawn wood flooring, and therefore a better option over radiant heat.
Obviously, narrower boards are easier to manage then wider boards, so if a homeowner is sensitive to the appearance of seasonal gaps, smaller widths may prove desirable.
The wood flooring absolutely MUST be properly acclimated, and please keep in mind that acclimation can only be successful if the house is at normal living conditions. This means that all HVAC systems must be operational, and that interior temperature should be between 60-80 degrees, with relative humidity at between 30-50%. The flooring must be stacked and stickered to allow air to all surfaces of the boards. The suggested period for acclimation may vary for different species, and seasons. Generally 2 weeks is enough time if site conditions are good, but this is only a guideline; Only by using an accurate moisture meter can material be deemed to have reached the correct levels for installation.
Expect some separation! No matter what kind of wood you use you must expect at least some board separation.
The key to maintaining your floor in good condition is keeping both temperature and humidity within the house relatively constant throughout seasonal changes, and throughout it's lifetime.
We have completed dozens of very successful solid wood installations over radiant heat. All of these projects have one thing in common. They were all completed with the cooperation of everybody involved!