Old building in Dortmund

Underfloor heating in renovation projects

In new residential or office buildings, hot water underfloor heating systems are the state of the art when it comes to heat distribution. The large heat-emitting surface guarantees comfort and low heating medium temperatures. It creates the basis for efficient heat generation. The cost-effectiveness of a heat pump is significantly better in combination with underfloor heating. However, the conditions for planning, setting up, installing and operating underfloor heating in old buildings are different and sometimes more complex than in new buildings.

Photo © Philip Kistner

The usual and comparatively inexpensive construction of underfloor heating in new buildings consists of thermal and impact sound insulation, to which the heating pipes are attached and then covered with a wet screed. The heating pipes are embedded in a wet cement or anhydrite screed. This type of construction poses problems for many renovation projects. On the one hand, the installation height of such a “wet” underfloor heating system is approx. 120 to 150 mm. This construction height may require further structural measures.

Lintels and stairs have to be adapted. The most serious problem, however, lies in the statics. A heating screed of 60 mm, as required by the standard, weighs > 120 kg/sqm. The proven solution for implementing underfloor heating in old buildings is therefore dry construction systems.

Prefabricated screeds, often also called dry screeds, are load distribution panels that are joined together from dry and prefabricated elements to form a screed surface and can be covered immediately after installation without any waiting time. The advantages of prefabricated screeds are their low weight of just 20 to 50 kg/sqm, depending on the material, their rapid readiness for covering immediately after installation and their low construction height of 20 to 30 mm. Due to the homogeneous panel consistency, there are generally no shrinkage cracks in the screed panel.

Precast screeds always have a constant thickness. It is therefore not possible to level out unevenness in the unfinished floor to a certain extent, as with a wet screed. For this reason, the condition of the unfinished floor must be meticulously checked before work begins. An important criterion for It is also important to include the desired surface covering in the planning. In the case of ceramic coverings, the safe tile size must be taken into account. At the very least, any necessary additional measures must be agreed.

Resilient floor coverings should only be applied to fully leveled prefabricated screeds. Otherwise, there is a risk of the tile joints marking out on the top layer.

The complete heating system should be planned in line with measures to optimize the building’s insulation, and the maximum heating load and therefore the maximum heating output of 100 W/sqmapplies for the comfortable operation of underfloor heating. Replacing the windows or taking measures to insulate the building is advisable at the latest when this value is obviously exceeded in the building. In this report, we document our contribution to the refurbishment of an old building from the 1970s in Dortmund.

Photo © Philip Kistner

Photo © Philip Kistner

Photo © Philip Kistner

We carried out all the steps involved in installing a modern underfloor heating system, from planning the underfloor heating to measuring the height and installing the complete floor with leveling and dry screed.

The unfinished floor
The type and condition of the unfinished floor must be meticulously recorded. Wooden beam ceilings are one of the most common substructures in renovation projects. They must be checked in particular for their structural load-bearing capacity. In the course of further work, the system elements of the underfloor heating and the prefabricated screed must be in full contact. For low levelling heights, Knauf levelling or filling compounds can be used after suitable priming. To create a level and easy to work on substrate, the first step in this project was to level the surface by 20 to 50 mm using a leveling mortar with EPS aggregate. Proven systems are EPO-Leicht from Knauf or the eco 400 Speed system from Thermozell used here. The leveling is ready for further work after approx. 24 hours.

Underfloor heating
Dry building materials must also remain dry! For this reason, installation of the underfloor heating and screed should only begin when there is no longer any excess moisture in the building. Wall and ceiling plaster should have reached their equilibrium moisture content. The ClimaLevel drywall system is based on the ClimaTE25 drywall panel. This is a system panel made of EPS polystyrene rigid foam with a nominal thickness of just 25 mm. There is a significant difference compared to the system panels for underfloor heating systems for wet screeds. These panels have no impact sound insulation effect. This is not structurally possible in combination with dry screeds. Basically, impact sound insulation boards made of EPS DES are not suitable for dry systems. Impact sound insulation made of mineral wool is possible, but must be agreed with the manufacturer of the screed in each individual case.

The installation of the underfloor heating begins with the attachment of the edge insulation strip. A continuous edge insulation strip must be attached to all vertical components such as walls, pillars or door frames. The height must extend at least to the upper edge of the planned floor covering. The edge insulation strip ensures acoustic decoupling of the floor from the walls and prevents cracking. The ClimaTE25 system panels are then laid. The panels are laid using a stepped hook seam and joined together. As the heating pipes in this type of underfloor heating are located inside the insulation, heat conducting plates must be installed first. These improve the heat transfer from the heating pipe to the screed on top. With the dry construction system, we usually use an installation distance of 125 mm for the heating pipes. This must be harmonized with the system hydraulics and the heating medium temperatures when planning the heating surface. To ensure that this works as planned in the end, the heating water quantities are set precisely at the manifold.

The screed
To decouple the prefabricated screed from the underfloor heating, we initially use a PE sliding foil. The dry screed we prefer is the Brio prefabricated screed from Knauf, which we also used in Dortmund. Brio consists of monolithic stepped gypsum fiber elements with a high bulk density. Variants with a thickness of 18 or 23 mm are available. The weight of each is less than 30 kg/sqm. Brio can also be used in bathrooms, kitchens or rooms with similar moisture exposure. In this case, a full-surface seal must be provided in accordance with the Knauf installation guidelines.