Screed Over Concrete (Most Common)
- Concrete slab
- DPM (1200 gauge)
- Insulation (EPS 50–100mm / PIR 50–75mm)
- Pipe clipped to insulation / grip rail
- Sand/cement screed 65–75mm
- Floor finish
U-value target: 0.15–0.18 W/m²K (Part L)
Underfloor Heating Design Guide
Technical reference for UFH system design. Principles, floor structures, finishes, pipe spacing, manifold placement. For installers, designers, and self-builders.
Guide Sections
System Principles
Heat transfer, flow/return temps, ΔT, mean water temperature, output calculations.
Floor Structures
Screed, timber suspended, floating, overlay systems. Build-ups & U-values.
Floor Finishes
Tile, wood, carpet, vinyl, polished concrete. R-value impact on output.
Pipe Layout Patterns
Snake, spiral, double-spiral, zone patterns. Spacing: 100–300mm.
Manifold Placement
Centralised vs distributed, access requirements, cover boxes.
Controls Design
Zoning strategy, thermostat placement, wiring routes, actuator selection.
System Principles
Heat Output Formula
Q = k × A × (MWT − Tr)
- Q = heat output (W)
- k = floor construction factor (W/m²K)
- A = heated area (m²)
- MWT = mean water temperature = (Flow + Return) / 2
- Tr = room set temperature (°C)
Typical Design Conditions (UK)
| Heat Source | Flow °C | Return °C | MWT °C | ΔT |
|---|---|---|---|---|
| Gas/Oil Boiler | 55–65 | 45–55 | 50–60 | 10–15 |
| Air Source Heat Pump | 35–45 | 30–40 | 32.5–42.5 | 5–10 |
| Ground Source Heat Pump | 35–45 | 30–40 | 32.5–42.5 | 5–10 |
Pipe Spacing vs Output (16mm pipe, 45°C MWT, 21°C room)
| Spacing | Output (W/m²) | Typical Use |
|---|---|---|
| 100mm | ~140 | High heat loss / conservatories |
| 150mm | ~110 | Standard residential |
| 200mm | ~85 | Well-insulated / heat pump |
| 250mm | ~70 | Low heat loss / large areas |
| 300mm | ~55 | Industrial / agricultural |
Floor Structures & Build-Ups
Timber Suspended Floor
- Joists (200mm centres typical)
- PIR insulation between joists (50–75mm)
- Alu-foil faced for reflectivity
- Alu-PEX or PEX in aluminium spreader plates
- Chipboard / plywood deck
- Floor finish
Output: ~60–80 W/m² at 45°C MWT
Floating Floor (Overlay)
- Existing solid floor
- Thin insulation (10–20mm EPS/PIR)
- Routed insulation boards for pipe
- 12–16mm pipe in grooves
- Floating chipboard / Fermacell
- Floor finish
Build height: 30–45mm total. Ideal for retrofit.
Joisted with Battens (Retrofit)
- Existing floorboards
- Battens (50mm) at 400mm centres
- Insulation between battens
- Pipe on insulation / spreader plates
- Floorboards / engineered wood
Floor Finishes — R-Value Impact
| Finish | Typical Thickness | R-Value (m²K/W) | Output Factor* | Max Surface Temp |
|---|---|---|---|---|
| Ceramic/Porcelain Tile | 8–12mm | 0.01–0.02 | 1.00 (baseline) | 29°C |
| Natural Stone | 10–20mm | 0.02–0.05 | 0.95–0.98 | 29°C |
| Polished Concrete | 10–15mm | 0.01–0.02 | 1.00 | 29°C |
| Engineered Wood | 14–20mm | 0.08–0.12 | 0.80–0.90 | 27°C |
| Solid Wood (quarter-sawn) | 18–22mm | 0.12–0.15 | 0.75–0.85 | 27°C |
| Laminate | 8–12mm | 0.05–0.08 | 0.85–0.92 | 27°C |
| Luxury Vinyl (LVT) | 4–6mm | 0.03–0.05 | 0.92–0.95 | 27°C |
| Carpet + Underlay (TOG ≤1.5) | 8–12mm | 0.15–0.25 | 0.60–0.75 | 27°C |
*Output factor relative to tile at same MWT. Multiply design output by factor.