Successful installation over floor heating systems using an uncoupling membrane

By: Joseph P. Joy

Heated floors, whether for space heating or for a more comfortable floor temperature, are not a new idea. Ceramic tile and natural stone is an ideal conductive floor covering and is therefor particularly suitable for radiant heat systems. Ceramic tile and natural stone allows easy transfer of heat from the heating system.

Floor heating systems typically fall into two categories; heated water circulated beneath the floor in tubes or conductive coils. The tube system can be installed in the concrete slab or installed over an existing sub-floor and covered using a gypsum material. The floor covering is then applied. For the heated water system, to achieve a room temperature of 72 ° F (22 ° C), the water temperature must be 140 ° F (60 ° C) and will result in a surface temperature of 85 ° F (29 ° C).

Precaution should be taken when installing ceramic tile and natural stone over heated substrates. The effect of temperature fluctuations during the heating and cooling processes induces thermal movement that without proper compensations will lead to failure in the surface covering. Applying an uncoupling membrane has proven to be a successful technique for a variety of floor heating systems.

An uncoupling membrane is typically a corrugated, polyethylene sheet with an anchoring webbing (scrim) laminated to its underside. The corrugated ribs are cut back in a dove-tail configuration. The uncoupling membrane was designed for the installation of ceramic tiles and natural stone over problematic substrates on walls and floors. All substrates that tend to change shape, are sensitive to moisture, or display cracks must be regarded as problematic.

The scrim laminated to the underside of the sheet establishes a mechanical bond to the adhesive applied to the substrate. Using the thin-set method, the tiles are then laid and solidly embedded on top of the membrane. The air channels provided by the corrugated ribs allow any residual moisture in the substrate to evaporate, thus neutralizing vapor pressure.

Multi-purpose Function

An uncoupling membrane serves several functions in successful tile installations and is particularly suitable for floor heating systems.

The membrane uncouples the tile covering from the substrate and prevents the transfer of stresses to the tile flooring. As a result, stresses caused by deformation of the substrate through thermal movement, shrinkage of the curing screed or concrete, deflection, etc. are neutralized.

The appropriate application of the membrane, in conjunction with the stone covering, produces an alternative to conventional waterproofing. It creates a waterproofing layer over a gypsum based screed and protects it from moisture related damage, as well as other harmful substances such as acids, oils and solvents.

The membrane bridges stress cracks and, because it uncouples the finished surface from the substrate, prevents the transmission of cracks to the surface covering.

The positive effects of an uncoupling membrane over floor heating systems

Mortar screeds must cure a minimum of 28 days prior to the installation of tile so that any changes in the shape of the screed (due to shrinkage as excess water is given off) are concluded. Floating screeds and heated screeds, in particular, tend to change shape or develop cracks even later, e.g. through load stresses or the influence of temperature. An uncoupling membrane allows the tile to be installed on fresh screed when it is hardened to the point that it can be walked upon. Even with heated floors, tiles can be installed over the membrane when the screed can be walked on.

As a general rule, when installing tiles over gypsum screeds, the residual moisture should not exceed 0.5% (percentage by volume). In addition, this substrate requires specific pre-treatment, as well as a special tile adhesive. With the application of an uncoupling membrane, tiles can be installed without specific pre-treatment, even if the residual moisture is approximately 2.0% (percentage by volume). The sheet will allow residual moisture from the substrate to escape through the rib cavities and prevent moisture penetration from the surface. The sheet is applied using a thin-set mortar.

Tile application:

Applying an uncoupling membrane is straightforward. The substrate must be clean, even, and load-bearing. The adhesive used to apply the membrane is determined by the substrate. The adhesive must bond to the substrate and anchor the webbing. Depending upon the surface of the substrate, the adhesive is applied with a 1/4" x 1/4" (10 cm x 10 cm)-notch trowel. The direction in which the mortar is combed does not affect the adhesion and function of the membrane.

Cut the membrane to size. Apply the tile adhesive to the substrate using a 1/4" x 1/4"(10 cm x 10 cm)-notched trowel. Press the sheet into the adhesive with the support webbing facing down so that its entire surface is adhered. Work the sheet into the adhesive in one direction using a float or screed trowel. The individual strips are overlapped along the flat edges while the ends of two courses are cut straight and abutted.

To install tile on the membrane, match the trowel size and mortar to the type and format of the tile. Trowel the tile adhesive over the top surface, filling the open ribs completely. Lay the tiles directly onto the tile adhesive and firmly embed them in the mortar. Complete coverage is important. The tile format should always be greater than 2" x 2" (5 cm x 5 cm). Wait 48 hours after installation of the tiles before turning on the heating system.

When using an uncoupling membrane, movement joints, as well as flexible edge, corner, and junction joints, should be installed at the usual intervals in the tile covering, in accordance with technical standards. Perimeter joints are particularly important since the sub-floor is normally insulated at the edges. Movement joints in the tile covering must be positioned directly over movement joints in the substrate. Generally, slight deviations (max. +/- 2/5" or 1 cm ) can be bridged by an uncoupling membrane. If there are no movement joints in the substrate, it is sufficient to place movement joints over the membrane in the tile covering only. These joints should be placed in accordance with technical standards. Regarding structural expansion joints, the entire structure, including the membrane and the tile covering, must be separated.

Ceramic tile and natural stone is an ideal floor covering for radiant heat systems. An uncoupling membrane is a simple addition that will insure a successful tile installation with any floor heating system.

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