Anhydrite (calcium sulfate) screed

Why is anhydrite screed the dominant choice for underfloor heating in Slovakia?

Anhydrite, or calcium sulfate, screed has become the standard binder for radiant floor heating installations across Slovakia because it solves a critical problem with traditional cement screeds: shrinkage. When a large floor area is heated, a conventional cement-based screed shrinks as it cures, creating hairline cracks and requiring movement joints every 4-5 metres. Anhydrite shrinks very little in comparison, allowing a 100+ square-metre floor to be poured as a single pour without internal joints. For underfloor heating, this is not merely aesthetic; a floor without joints is better for the heating circuit itself, which would otherwise have to route around them, creating cold spots and inefficient control zones.

Beyond shrinkage, anhydrite offers two additional advantages specifically tailored to heat pump heating. It is self-levelling and pumped, which means it flows around embedded heating pipes and fills the voids between them. An air pocket trapped under a heating pipe is a dead zone that conducts no heat, wasting energy and creating cold areas in the room. Self-levelling anhydrite minimizes this risk. Equally important, anhydrite has superior thermal conductivity compared with typical cement screed, in the range of 1.6 to 2.0 watts per metre-kelvin. This allows the screed to be laid thinner over the pipes, which matters greatly for a low-temperature heat pump system: the floor responds faster to a control signal, and the system reaches its target temperature quicker with less overshoot.

This combination of qualities explains why anhydrite dominates underfloor heating in new residential construction. The case is not about a single property but a chain of reasoning: low shrinkage means fewer joints, thinner application means faster response, and the pumped, self-levelling nature means consistent density. For a heat pump delivering warmth at 35-40 °C rather than the 55-60 °C of a traditional boiler, every fraction of a degree of efficiency matters.

What are the disadvantages that make anhydrite unsuitable for many locations?

The decisive disadvantage is moisture sensitivity. Calcium sulfate absorbs water and, if exposed to persistent dampness, softens and loses its strength. It is entirely unsuitable for bathrooms, kitchens, laundry rooms, or any location where water pooling is even a possibility. This constraint applies to the underfloor heating layer itself; the damp-proof layer below must be absolutely continuous. Any breach in this membrane allows groundwater or construction moisture to rise into the screed, and the entire floor fails structurally. In renovation projects, when an old, failing damp-proof course is not replaced, anhydrite screed will inevitably fail within a few years.

A second disadvantage is equally common but more easily remedied: laitance. As anhydrite cures, a weak, friable surface film forms, composed of cement minerals and fine dust. This laitance must be mechanically removed by sanding before any adhesive-bonded finish is applied. If this step is skipped, a tiled or stone floor will debond within months or years, because the adhesive bonds to the laitance, not to the solid screed beneath. The laitance is not strong enough to hold the bond, and the finish simply lifts away. This is one of the most commonly encountered failures in tiled anhydrite floors in Slovakia, almost always because the contractor or installer did not sand the surface.

The third disadvantage is chemical and specific. Calcium sulfate and Portland cement, in the presence of moisture, undergo a reaction that forms expansive ettringite crystals. These crystals expand inside the screed and the tile adhesive, creating internal stresses that crack the floor from below. A water droplet on a ceramic tile that sits for days can trigger this reaction if a standard Portland cement adhesive is applied directly to an unsanded anhydrite screed. The solution is to use a sulfate-compatible adhesive or to apply an appropriate primer before laying a standard adhesive. Many installers in Slovakia are unaware of this requirement, and when a tiled anhydrite floor fails in a cluster pattern, this is often the cause.

How does anhydrite screed compare with cement screed?

Both bind aggregates and level a floor, but they differ in the properties that matter for underfloor heating and long-term durability.

The critical trade-off is visible: anhydrite excels in performance for underfloor heating but demands stricter conditions regarding moisture and surface preparation. A cement screed is more forgiving in damp locations and requires no laitance sanding, but it shrinks more and conducts heat less efficiently. The choice of screed significantly affects which finish flooring materials can be used above it.

What is the proper drying and commissioning sequence?

Residual moisture in the screed must be verified, not assumed. A calcium carbide meter or capacitive moisture meter should be used to check the moisture content before any finish is applied. The target is typically below 1.5 percent by mass for anhydrite, though the finishing material's tolerance varies. Once the screed is dry and the laitance has been sanded, the underfloor heating system is commissioned by gradually raising the water temperature over several days. This controlled heat-up cycle drives off any remaining surface moisture and prevents rapid thermal stress cracking. Only after this process is complete should the finish flooring (tile, vinyl, wood) be installed.

The commissioning phase is frequently omitted or rushed, particularly in fast-track projects. Skipping it leaves residual moisture trapped under the finish, which then causes debonding and chemical degradation of the adhesive.

What does pumped, self-levelling installation entail?

Unlike traditional screeds that are laid and manually troweled, anhydrite is typically delivered as a slurry by truck pump and poured across the floor. The self-levelling property means it flows to a consistent level without troweling, and it automatically fills voids around the heating pipes. This method is faster than manual laying, reduces labour cost, and is far more likely to produce a void-free mass. The trade-off is that pumping equipment is required, making small projects less economical. In Slovakia, most new residential floors with underfloor heating are now installed this way.

When should cement screed be chosen over anhydrite?

Cement screed remains appropriate for locations where moisture risk is moderate to high, where laitance sanding is not feasible, or where the budget precludes the cost of specialist pumping equipment. Basements, ground floors in flood-risk zones, and commercial kitchens are better served by a cement-based product. For any floor that combines underfloor heating with genuine dampness risk, neither anhydrite nor cement screed alone is sufficient; the solution is to upgrade the damp-proof layer below, or to reconsider the placement of the heating system altogether. Anhydrite is optimal for underfloor heating in dry, heated residential spaces; everywhere else, the conditions of use must drive the choice.