Installing Hardwood Flooring -- Over Radiant Heating

Hardwood Floors and Heated Flooring Systems Now a Good Match. November 23, 2001

Hardwood Floors and Heated Flooring Systems Now a Good Match

Reprinted with permission from The Hardwood Council website.

Technology has paved the way for you to provide your clients with the beauty of North American hardwoods and the warmth of radiant heat flooring. Building professionals have become more adept at managing wood's natural expansion and contraction.

At the same time, advances in the heated flooring industry are permitting easier installation of hardwood floors.

Builders, architects and designers now achieve faultless installations of oak, ash, cherry, maple, hickory, walnut and other fine hardwood flooring over radiant heat.

How It Works
For many builders, the reluctance to install hardwood floors over radiant heat stemmed from problems associated with the original technology introduced more than 40 years ago. Back then, floors were heated excessively to compensate for poor building insulation. Those high temperatures exaggerated expansion and contraction in hardwood flooring, causing irreparable damage to both the floors and builders' reputations.

Today, modern insulation and building techniques allow a radiant floor to stay cooler than the floor of the average sunroom. Radiant heat systems use a three-stage process to convey heated water to its destination. (See diagram).

-- A water heating system, comprising either a standard boiler, water heater, a geothermal heat pump or even solar panels, warms the water.

-- A series of controls then pumps the heated water through a tubing network that is installed in the subfloor.

-- As the warm water moves through the tubing network, it releases its energy and returns to the boiler system to be reheated.

Installing a Radiant Heat Floor

Pay Attention to Details
Installing hardwoods over radiant heat isn't much different from laying a typical hardwood floor.

Most important is good communication with the radiant heat system designer. It's critical that everyone is notified of any work pertaining to the installation, especially if specifications are changed.

To ensure a superior end product, pay attention to the following factors before, during and after installation:

-- Floor Temperature
-- Subfloor
-- Tube Installation
-- Climate Controls
-- Moisture Content

Floor Temperature
Provide the radiant heat system designer with the hardwood flooring dimensions and species, subfloor style and the desired temperature of each room. This will give him/her the information needed to calculate the necessary system supply water temperature.

Work with the system designer to choose the subfloor option (see illustrations) that best meets both of your needs. The heat system designer is responsible for the subfloor installation, but you will want to be familiar with some of the styles. Direct contact of the tubing with the flooring is not recommended. The subfloors shown here are recommended for hardwood floor installations.

Subfloor Options

Installers use this system quite often. The tubes are stapled onto the subfloor which continuously releases heat to the hardwood flooring.

An approach used when you don't have access under the existing floor or when the underside of the floor can't be used, such as a second story over beamed ceilings.

This is a method often used when installing hardwood flooring, yet which also provides the fire resistance, sound dampening and thermal mass of a thin slab.

This is another technique used for hardwood flooring when access to the underfloor is impossible. It also offers fire resistance, sound dampening and thermal mass.

Tube Installation
Consult with the system designer to determine the tube network layout, so you'll know where the tubes are before you nail down the floor. It is best to have the tubing spaced evenly down the joist cavity (between the sleepers). Then you can nail down the finished flooring onto the sleepers on eight-inch centers. When the tubing circuits are crossed over the center of the joist cavity, have the system designer use nail plates to protect the radiant circuits from being punctured.

Climate Controls
The following climate controls should be operational to minimize expansion and contraction during and after installation of the floor.

Mechanical Humidity Control: The heating/ventilation/air conditioning (HVAC) system should have mechanical humidity control. This will monitor the room and keep the relative humidity at an even level, which will keep the equilibrium moisture content of the floor stable.

Heat Transfer Point Control: The system designer also should install a set point control that will monitor the wood floor temperature. The set point control should either reduce the system water temperature or temporarily cycle the system off to prevent overheating the flooring in case of an equipment malfunction.

Exterior Thermostat: An exterior thermostat is recommended to protect the perimeter of the system from condensation absorption during the spring and fall when rapid temperature changes may occur.

Moisture Content

Once the subfloor, tubing and climate controls have been installed, the heating system is fully operational and should be run for at least 72 hours to balance the house's moisture content.

Now follow the customary procedures for installing any hardwood floor. Be sure to monitor the moisture content of both the subfloor and flooring strips, because this will have a profound effect on the end result of your installation. Remember, do not install the flooring over green concrete or wet plywood. Even if the wood is dry, it will pick up moisture from the wet subfloor.

If you're using a concrete slab -- The slab should be well-aged before installing a hardwood floor. Never install wood flooring over concrete or gypsum cement until you've turned on the floor heating system to remove any residual moisture from the slab.

Here is a simple procedure to check for the presence of excessive moisture in the slab: Tape a 4' X 4' section of polyethylene plastic sheeting to the slab and turn on the heat. If moisture appears under the plastic, heat the slab for another day or so. Repeat this test until no moisture is visible.

Install an 8 mil polyethylene vapor barrier on top of all slabs in contact with the ground or over fill. Make sure the vapor barrier isn't damaged before the finished floor is put in.

Plywood subfloor -- Turn on the permanent heat source if the plywood is wet. Don't deliver the flooring until the room has reached the proper relative humidity.

Plywood (5/8") or oriented strand board (3/4") make good candidates for subfloor materials in radiant installations. Particle board subfloors are not recommended by radiant heat companies.

Moisture Content in the Hardwood Flooring Strips -- Monitor the moisture content of your flooring before installation. Most wood is shipped and stored at an equilibrium moisture content between 6 an 9 percent, which matches average levels in North America. But you are well-advised to either borrow or buy a wood moisture meter and check samples to determine the flooring's moisture content at the job site.

Consult with your local hardwood flooring supplier about the average conditions in your area and match the flooring's moisture level to that. If the wood is too wet, turn on the floor heat or a secondary heat source, spread the flooring out, and let it dry while maintaining adequate ventilation in the room. Keep a close eye on the moisture level and keep written records daily.

Make sure you do not dry the wood too much or it will cause installation problems. If the wood is too dry when you remove it from its packaging, return it. Once the wood reaches the desired moisture content, you are ready to install the flooring.


Avoid Wide Plank Flooring
Solid North American hardwood flooring is available in parquet patterns or pieces that are either less than 3" (strip flooring), or more than 3" wide (plank flooring). All of these types of solid flooring will expand and contract with changes in moisture content. Parquet floors are readily used in radiant heat applications. With flooring strips, the wider the board, the greater the potential for gaps between the boards when they experience these seasonal changes.

It isn't recommended to use radiant floor heating under plank flooring wider than 3". Despite all your precautions, there is a high probability the user will not be satisfied.

Follow these additional tips to prevent shrinkage cracks:

- Tongue-and-groove strips are recommended.
- Beveled edge strips show fewer seasonal cracks.

Pre-Installation Heating
Once the heating system is installed, turn it on and let it run for at least 72 hours to bring the house to the desired relative humidity. Temporary, unvented sources of heat such as propane-fired "salamanders" are not appropriate, because they can put excessive amounts of water vapor into the building. If they must be used, windows must be left open so excessive humidity can be vented.

Technical assistance and/or photos provided by: HeatLink USA Inc., Grand Rapids, Mich.; Heatway, Springfield, Mo.; Wirsbo, Apple Valley, Minn.

1999 The Hardwood Council

Reprinted with permission from The Hardwood Council website.