Screeding Over Underfloor Heating: Complete Guide
Underfloor heating (UFH) has become standard in modern construction. Whether you're installing water-based (wet) or electric systems, the screed you choose and how it's applied will fundamentally determine thermal performance, energy efficiency, and long-term reliability. This guide covers everything you need to know about screeding over UFH systems.
Why Screed Choice Matters for UFH
The screed serves as the thermal mass that stores and distributes heat from the UFH pipes or cables. Its thermal conductivity directly affects how efficiently heat transfers from the system to the room above. A screed with poor conductivity acts as an insulator — the opposite of what you want.
Thermal conductivity is measured in watts per metre-kelvin (W/mK). The higher the value, the better the screed conducts heat:
- Traditional sand & cement: 0.4-0.8 W/mK
- Cementitious flowing screeds: 0.8-1.2 W/mK
- Anhydrite (calcium sulphate): 1.2-1.5 W/mK
Anhydrite screeds conduct heat approximately twice as efficiently as traditional cement, meaning faster warm-up times, lower energy consumption, and more even heat distribution across the floor surface.
Water-Based vs Electric UFH
Water-Based (Wet) Systems
Pipes carrying heated water are laid across the floor in loops, typically at 150-200mm centres. The screed must fully encapsulate the pipes with no air voids — any gap between pipe and screed creates a thermal barrier that dramatically reduces heat transfer.
Minimum screed cover over the top of the pipes is typically 25-30mm, with total thickness commonly 65-75mm depending on insulation and pipe diameter. Flowing screeds are strongly preferred for wet UFH because they naturally fill around and under pipes, eliminating void risk.
Electric Systems
Heating cables or mats are laid on the substrate. These systems are thinner than wet UFH, often requiring only 15-25mm of screed cover. They heat up faster but have higher running costs than water-based systems.
The Critical 28-Day Curing Rule
Before commissioning any UFH system, the screed must be fully cured. This is absolutely non-negotiable. Heating uncured screed causes rapid moisture loss, leading to severe shrinkage cracking, curling, and permanent damage.
For cement-based screeds, the standard curing period is 28 days minimum. For anhydrite screeds, this may be reduced to 14-21 days depending on the product, but always follow manufacturer guidance.
Commissioning Procedure
After curing, the UFH system must be commissioned gradually:
Day 1: Set water temperature to 25°C
Days 2-3: Increase by 5°C per day
Days 4-7: Continue increasing until operating temperature is reached (typically 45-55°C)
Days 7-14: Maintain operating temperature to drive out residual moisture
This gradual process prevents thermal shock and allows remaining moisture to escape evenly.
Avoiding Air Voids
Air voids around UFH pipes are the single biggest cause of poor heating performance. Traditional screeds, which rely on manual compaction, are prone to voids — particularly in tight spaces between pipe loops and around manifold connections.
Flowing screeds eliminate this risk entirely. Their liquid consistency means they naturally encapsulate pipes completely, filling every gap and achieving full contact between pipe and screed. This is why virtually all UFH specialists now recommend flowing screeds.
Product Recommendations for UFH
Our Tekcem anhydrite range is specifically formulated for UFH applications, delivering outstanding thermal conductivity (1.2-1.5 W/mK) with reliable C25-C35 strength. For projects where anhydrite isn't suitable (for example, areas with moisture exposure risk), our Ardex cementitious flowing screeds offer excellent UFH performance with C25-C30 strength and superior moisture tolerance.
Getting Expert Advice
Related Reading
UFH screeding requires careful specification. Call us on 0118 370 2060 to discuss your system type, insulation requirements, and product selection. We offer free delivery on orders over £600 ex-VAT.
