Insulation for Electric Underfloor Heating Systems

Cross section of floor heating cable embedded in thinset

Use of insulation in underfloor heating systems

Proper insulation is a critical element in any underfloor heating system and electric radiant floor heating systems are not an exception. A common misbelieve is that the heat emitted by the cable can only rise up and therefore there’s no need for insulation. This could not be further from the truth. In fact, as seen on the image (cross-section of the floor with heating cable embedded in thinset), heat emitted by the cable radiates in all directions, including the portion of the floor underneath the floor heating cable. Installing insulation underneath the cable effectively separates the flooring into heated and unheated section therefore reducing thermal mass (read more about thermal mass below).


Thermal mass of a radiant heated floor

Thermal mass of heated floors with and without insulation In context of radiant floor heating systems, thermal mass is the volume (size) of the flooring heated by the electric floor heating cable (or mats). When cable is installed over a concrete slab without insulation, the slab consequentially becomes a part of the overall thermal mass and will intake the heat produced by the cable.
With insulation in place, however, only the finished floor (such as tile) becomes thermal mass, as thermal barrier between a concrete slab and the heating system (including the finished floor) is created.
See image for a general representation and comparison.


Impact of insulation on heating costs and system efficiency

1. Size of the thermal mass affects how long it will take the system to heat the floors. The larger the thermal mass is, the longer it will take to heat it to a desired temperature. By reducing thermal mass to only a portion of the flooring which requires heat, the heat-up time can be effectively reduced.

2. Larger thermal mass would require more energy to reach a pre-set temperature and therefore will result in higher energy costs. With proper insulation installed, on the other hand, the system can operate at very efficiency, transferring nearly all the heat to the surface of the flooring.

Types of insulation compatible with electric floor heating systems

1/4 (6mm) cork in a roll

Cork

Cork is among the most popular choices of insulation for underfloor heating for many reasons:
It is a natural product and does not emit harmful vapors when heated.
It is approved by the TCA (Tile Council of America).
It has low moisture absorption and expansion ratios.
Relatively low cost at about $1.70 per Sq. Ft for 1/4” (6mm) thick cork (data from various internet retailers as of Dec. 2012)
A 1/4" thick cork has about the same R value as 3/4" plywood
Common cork thickness used for electric floor heating insulation ranges from 1/8” to 3/8” with 1/4" (6mm) being among the most popular sizes. Cork flooring insulation usually comes in rolls, but is also available in tiles.
Surface of a plywood sheet

Plywood

Plywood itself is not a very effective thermal insulator and has a relatively low R value of about 1.0 for a 3/4" thick sheet. However, in many residential settings, installing an underfloor electric system over plywood is completely acceptable given that the insulation beneath the plywood subfloor is adequate (such as fiberglass - typical for residential homes).
1 square foot of 3/4" thick plywood costs about $1.00 (data from local home improvement stores) with prices varying depending on wood grade, finish, etc.
EPS (extruded polystyrene) boards/panels

Polystyrene

Polystyrene panels are among the best type of insulation for electric heated floors, with an impressive R value around 5.0 per inch of thickness (compare to 3.0/inch for cork). Polystyrene is also an effective waterproofing material and prevents the growth of mold and mildew. This type of insulation offers flat finished surface (when using boards) which is especially helpful for installation of tile or stone floors. The downside of this material, however, is that it tends to loose its’ thermal resistance properties over time.

After the insulation is in place, heating cable or mats can be installed. HeatTech underfloor heating mats are well suited for installation over plywood, cork and polystyrene since they come with an adhesive backing and therefore don’t have to be glued or taped.

Today’s market offers a wide selection of other flooring insulation types which are compatible with electric underfloor heating systems. Consult with your insulation of choice manufacturer whether it’s compatible with HeatTech floor heating products.