How Underfloor Heating Works
Underfloor heating (UFH) warms a room by heating the floor surface, which then radiates warmth upward. Because the heat comes from a large surface (the whole floor) at a relatively low temperature (25–35°C for wet systems), UFH provides even, comfortable warmth without the hot spots and cold spots associated with radiators.
Wet (Hydronic) Underfloor Heating
Wet UFH circulates warm water through flexible plastic pipes (typically polyethylene) embedded in or fixed below the floor construction. The pipes connect to a manifold, which connects to the boiler or heat pump.
Installation Methods
- Screed system: Pipes are clipped to insulation and covered with a levelling screed (typically 65–75mm). The screed takes 28+ days to cure before floor finishes can be installed. Standard in new extensions with concrete slab floors.
- Low-profile overlay system: Pipes are laid in a profiled board (typically 15–20mm thick), which sits on the existing floor. Suitable for retrofit in rooms where raising the floor is acceptable.
- Suspended timber floor system: Pipes are clipped between floor joists with aluminium diffuser plates, heating from below. Lower efficiency than screed systems but possible without major floor alterations.
Advantages of Wet UFH
- Lower running cost than electric UFH (especially when powered by a heat pump)
- Ideal for large open-plan areas
- Works well with all floor finishes (tile, stone, wood, vinyl, carpet — though check the floor's thermal resistance rating)
- Compatible with smart heating controls and zoning
Disadvantages of Wet UFH
- Higher installation cost (pipework, manifold, connection to boiler or heat pump)
- Screed systems require a significant floor build-up (adds to structural loading and may affect floor-to-ceiling height)
- Slower response time than electric UFH — not ideal for rooms used intermittently
Typical Costs (London, Extension)
| System | Approximate Cost (30 sqm) |
|---|---|
| Wet screed system (supply and install) | £2,500–£5,000 |
| Connection to boiler/manifold | £500–£1,200 |
| Low-profile overlay retrofit | £1,500–£3,500 |
Electric Underfloor Heating
Electric UFH uses a thin heating mat or loose resistance wire installed under the floor finish. It is powered directly from the electrical supply, converting electricity to heat.
Installation Methods
- Heating mat: A pre-spaced resistance wire on a mesh backing, rolled out and tiled over (or other floor finish). Very thin (3–4mm) — no significant change in floor level.
- Loose wire: More flexible — can be configured for any room shape. Embedded in a flexible tile adhesive or levelling compound.
- Foil mats: Ultra-thin foil systems suitable for installation under engineered wood or LVT.
Advantages of Electric UFH
- Very low installation cost
- No pipework, manifold, or boiler connection
- Fast response — heats up in 20–30 minutes
- Ideal for bathrooms, en-suites, and individual rooms
- Very thin — minimal effect on floor level
Disadvantages of Electric UFH
- Higher running cost than wet UFH (especially vs. heat pump)
- Not economical as the primary heating for a large open-plan space
- Requires sufficient electrical circuit capacity
Typical Costs
| System | Approximate Cost (10 sqm bathroom) |
|---|---|
| Electric mat (supply and install) | £400–£800 |
| Thermostat and programming | £100–£250 |
Which System for a London Extension?
| Scenario | Recommended System |
|---|---|
| New extension with concrete slab floor | Wet screed UFH |
| Large open-plan area (20+ sqm) | Wet UFH (lower running cost) |
| Bathroom or en-suite addition | Electric mat |
| Retrofit into existing room (timber floor) | Electric foil or wet overlay system |
| Heat pump primary heating system | Wet UFH (ideal match) |
| Gas boiler heating system | Either — wet preferred for large areas |
Floor Finishes and Thermal Resistance
UFH works best under floor finishes with low thermal resistance:
- Tile and stone: Excellent — low resistance, fast heat transfer
- Engineered timber: Good — check manufacturer's approved UFH products
- Vinyl/LVT: Good — check thermal resistance rating
- Solid timber or thick carpet: Poor — high resistance, significantly reduces UFH effectiveness
RCB Design & Build
RCB specifies and installs wet UFH systems as part of extension projects, coordinating the floor build-up, screed specification, manifold position, and boiler or heat pump connection within the main contract. We advise on electric mat systems for bathrooms and smaller areas within refurbishment projects.