Underfloor Heating is fast becoming a popular home heating method. Its many advantages include an even distribution of heat around the room, it is easy to install, leaves no restriction on furniture placement and allows individual room temperature control.

There are two types of underfloor heating, warm water heating (wet) and electric (dry). Warm water heating is the only system that works with wood floors, as the alternative method, electric system, is too fast for wood floors (this is due to the fact that wood expands, and if heated too quickly, as with the electric underfloor heating system, the wood may split.)

All hardwood flooring is to be installed according to industry standards and as per manufacturers instructions. Shrinkage can and will occur in most hardwood regardless of the heating system chosen. However, due to the presence of a low temperature heated mass in direct contact with the hardwood, the normal shrinkage which may take place over a 6 to 12 month period can be greatly accelerated with a radiant floor heating system if the proper installation techniques have not been followed.

There are many types of suitable hardwood flooring for underfloor heating available including solid planks as well as semi-Solids. In most regions of North America, semi-solid flooring is gaining acceptance as more types/brands and finishes become available. Semi-Solid flooring has several advantages over other types of hardwood flooring especially when used in conjunction with a radiant floor heating system. Due to the layers being manufactured at right angles to each other (i.e. Similar process as plywood sheathing) shrinkage is nearly eliminated.

If solid hardwood stripping is desired, and an acceptable semi-solid flooring cannot be used, it is imperative that the hardwood has been "acclimatized" to the region where it is being installed. Sometimes a flooring supplier will receive a shipment of hardwood from a manufacturer and then send it out to a job site within several days or weeks. This may not be sufficient time for the hardwood to acclimatize to the particular region, especially in a "dry" climate area. Hardwood must not exceed 6% to 8% moisture content at the time of installation.

Control selection is especially critical for installations with hardwood to ensure that proper modulation of the supply water temperature is possible. Modulating controls will provide the lowest possible supply water temperature for the given outside temperature. As well, maximum floor surface temperature should not exceed 85-90F (30-32C). With the variance in humidity levels through the various seasons in a given year, supplemental humidification may have to be provided to ensure relative humidity can be maintained at 40% to 45%.

 

Additional Information

To minimize the effect that rapid changes in temperature will have on the floor, it is recommended that an outside thermostat be installed. If one is not present, suggest to your customer that this should be considered. Unlike conventional heating systems whereby when it becomes cold, the heat is switched on, the radiant systems work most effectively and with less trauma to the wood floor if the heating process is gradual, based on small increment increases in relation to the outside temperature.

The subfloor should have a proper moisture test. A 6-8 mil polyethylene vapor barrier should be installed over slab radiant heat systems. Tape all seams to be sure the barrier is tight. When the slab has cured, turn the heat on, regardless of season, and leave it on for at least 5-6 days before installation of the wood flooring.

Installation and Underfloor Heating

1. Prevention of Movement and Shrinkage: Lay out wooden floors over the floor heating (usually for about 6 weeks) to dry out or 'acclimatise' the timber, before final fixing. This will help to stop gaps appearing in between the boards. Moisture content will get down to around 7% when right. Floor heating is a gentle heat, like sunshine in a room, and will not adversely effect wood once it is correctly dried. (Typical floor heating temperatures are only in the region of 25-28 degrees)

2. Moisture in New Concrete Floors: Be careful not to fix timber over a new concrete floor construction without first running the system to dry out the moisture still trapped in the structure.

3. Wooden Floor Thickness: The floorboard thickness should not exceed 22mm. I.e. do not cover a floor with plywood and then expect to add another layer for your boards or your heat will have a difficult job to get through.

4. Heating suspended timber floors: Ensure that the insulation boards are set at the correct height on supporting battens (top of board is usually 25mm from the top of the joist) and properly sealed from the cold air below. The insulation type and heights will vary depending on whether the floor is at ground level (50mm) or an intermediate floor (25mm) and also whether an in-fill (8:1 sand & cement) material is to be used for increased output, soundproofing, etc. Foil faced polyurethane insulation is recommended as it reflects heat if an in-fill is not used.
NOTE: A cavity must not be left between the concrete floor and the timber flooring.

5. Semi-Solid flooring: Because they are so stable and dry by nature, they generally do not need to be acclimatised. The benefit of using this type of floor is that it can be simply glued on the tongue and groove and laid out as a "floating Floor" over the concrete on a thin foam mat. This eliminates the need for battens or special fixings.

Useful Tips

Once the underfloor heating is installed you must wait until the concrete slab is completely dry before the flooring is brought into the house.
Once the underfloor heating is installed and ready to be used, turn it on to a typical temperature.
Lay out wooden floors in the unopened packaging over the floor heating, turning the boxes every couple of days, and leave for 6 weeks.
After 6 weeks lay the floor as per correct fitting instructions.
In week one, adjust the temperature to 12 degrees.
In week two, adjust the temperature to 13 degrees.
Repeat this procedure, by turning the heating up 1 degree a week for 6 weeks bringing the temperature up to 18 degrees.
Finally, set the heating system at the desired temperature.
Cautionary Note
In any nail-down installation, wood flooring installers must take care that nails don't puncture the radiant-heat tubing. It may be a question of accuracy: The location of the tubing must be identified and avoided, or nails should be driven into the sleepers / joists. Or it may be a question of nail length: They should be short enough not to penetrate the subfloor above the tubing. If your nailing only into joists or sleepers, be aware that joists are normally placed 450mm on-centre and sleepers are often placed 350mm on-centre.

Radiant heat beneath wood flooring usually involves hot water running through tubes embedded in concrete or stapled to plywood subfloors, but there are virtually endless variations.

Radiant heating systems are now designed to run cooler than they did years ago, with current water temperatures generally ranging from 100 degrees to 120 degrees. Hotter water temperatures in the past often traumatised wood fibres, damaging the floor. With glue down floors, repeated heating and cooling also broke down the adhesive that bonded the wood to the slab.

Radiant-heat systems can be installed in either frame or slab construction involves stapling tubes to the underside of the wood subfloor, between the floor joists.

The wood finish floor is then nailed, as usual, to the wood subfloor, with shorter nails, if necessary, to avoid puncturing the tubing.

In a sandwich system, the pipes are laid between sleepers over an existing wood subfloor. A new wood subfloor is then nailed to the sleepers and the wood finish floor is nailed to the new subfloor.

A variation on the sandwich system is to pour concrete between the sleepers. In effect creating a thin radiant slab that encases the tubing.

In all of the above cases, a layer of insulation is normally placed below the tubes or the existing subfloor to ensure that most of the heat from the pipes is directed upward.

Radiant heat in slab construction takes many forms. In new construction the radiant heat tubing is often referred to as a "cast-in-slab" construction.

In retrofit installations, sleepers may be placed over existing slab with the tubing run between the sleepers and embedded in new concrete.

In either case the wood flooring can then be installed above the slab by a variety of methods