Underfloor Heating and Floor Finishes: Systems, Screeds and Levelling

Underfloor heating adds comfort and value to residential and commercial spaces, but it also introduces extra movement and stress into the floor build-up. Getting the interaction between heating systems, screeds, levelling compounds and floor finishes right is essential for a long-lasting result.

In this article, ARDEX Technical Services Advisor Chris Brady outlines the two main types of underfloor heating, how they are typically installed, and the key considerations for tilers and flooring contractors.

Two main types of underfloor heating

While there are many proprietary systems and brands, most underfloor heating falls into two broad categories:

– Electric underfloor heating – usually strip or cable systems powered by electricity.
– Hydronic underfloor heating – systems of water pipes that circulate heated water through the floor.

Both can work well with tiles and other finishes when the correct screeds, adhesives and levelling compounds are used.

Electric underfloor heating systems

Electric systems typically use:

– Loose heating cables laid out and taped to the substrate, or
– Pre-formed mats where cables are already woven into a mesh framework.

These systems can be embedded in:

– Tile adhesives – useful where falls are already formed and you don’t want a self-leveller to run away and flatten them.
– Levelling compounds – often the preferred option where you’re working to a level plane and want a smooth, protected surface over the heating wires.
– Screeds – in some cases, cables may be encased within screeds as part of the floor build-up.

When embedding in levelling compounds, it’s important to ensure there is sufficient cover – at least a few millimetres of leveller above the highest point of the cable. Simply touching the top of the wire is not enough; thin, brittle sections can crack if there isn’t enough material above the cable.

For frameworks or mat systems with small perforations, fluid levelling compounds can flow through the holes and lock the mat in place. Tile adhesives are usually too thick to achieve this, which limits their use over these types of mats.

Hydronic underfloor heating systems

Hydronic systems use larger-diameter water pipes embedded within:

– Concrete slabs,
– Engineered screeds, or
– Thicker levelling compound build-ups (with substantial cover).

Because of the larger pipe size and the overall mass of these systems, they are typically treated as a heavier-duty construction. Adequate headspace above the pipes is critical to:

– Distribute loads evenly.
– Protect the pipes from damage.
– Provide a uniform surface for tiles or other finishes.

Manufacturers of hydronic systems often provide minimum cover requirements; ARDEX product literature and technical bulletins also give guidance on compatible screeds and levelling compounds.

Key considerations for both systems

Regardless of whether a system is electric or hydronic, installers should keep the following in mind:

– Sufficient cover over heating elements – To prevent cracking of thin sections and to protect cables or pipes from damage.
– Compatibility of products – Screeds, levellers, adhesives and membranes must all be rated for use with underfloor heating and thermal cycling.
– Movement and expansion – Heating and cooling cycles cause expansion and contraction.

Movement joints and flexible adhesives/grouts, where appropriate, help manage those stresses.
Where acoustic underlays or sound reduction mats are used, any heating wires installed above them may need to be embedded in tile adhesive rather than leveller, due to the way levellers flow.

Information and responsibilities

Underfloor heating systems must be correctly designed, wired and commissioned – responsibilities that typically sit with the electrician or hydronic specialist. However, tilers and flooring installers are responsible for:

– Ensuring compatible materials are used over the heating system.
– Following manufacturers’ instructions for minimum curing times before the heating is switched on.
– Protecting cables and pipes during installation of screeds, levellers and tiles.

ARDEX Technical Bulletins – such as TB017 and TB176 – provide more detailed guidance on underfloor heating systems, including recommended build-ups and product combinations.

Key takeaways for underfloor heating projects

To avoid problems and deliver comfortable, durable heated floors:

– Identify whether the project uses electric or hydronic heating.
– Choose screeds, levelling compounds and adhesives that are approved for heated floors.
– Ensure adequate cover over wires or pipes and avoid thin, fragile sections.
– Respect curing times before bringing systems up to temperature.
– Consult ARDEX technical literature and Technical Services for project-specific advice.

With careful planning and the right products, underfloor heating can be integrated smoothly into high-performance tiling and flooring systems.

ARDEX Technical Hotline: 1800 224 070

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