Ground Floor Heating for House Extensions: UFH vs Radiators vs Electric

**What wet UFH is**:

Wet underfloor heating (also called hydronic UFH) circulates warm water through a network of polyethylene pipes embedded in or laid on top of the floor construction. The pipes connect to a manifold (distribution point), which connects to the boiler or heat pump. Water temperature in a UFH system is typically 35–45°C — lower than the 65–80°C of a conventional radiator circuit, which makes it significantly more efficient when paired with a modern condensing boiler and essential when paired with a heat pump (which cannot efficiently achieve high flow temperatures).

**How wet UFH is installed in a ground floor extension**:

For a new extension on a concrete slab, the standard wet UFH installation sequence is:

1. *Concrete slab (oversite)*: Cast to the designed level, incorporating any drainage penetrations 2. *DPC (damp-proof course/membrane)*: 1200-gauge polyethylene DPM laid on the concrete slab 3. *Rigid insulation below slab or above slab*: - Part L (Approved Document L1A/B, 2021 edition) requires the total thermal resistance of the floor construction, including the insulation, to achieve a U-value of 0.13 W/m²K for ground floors in a new extension - To achieve 0.13 W/m²K with a standard 100mm concrete slab and screed, approximately 100–150mm of PIR (polyisocyanurate) rigid insulation is required below the screed - In practice: 100mm PIR under slab (laid before the slab is poured, often impractical for retrofit) OR 75–100mm PIR above the slab (laid after the slab, before the screed) 4. *UFH pipe installation*: Pipes fixed to the insulation at 150–200mm centres using clip rails or staple systems; connected to the manifold at the wall 5. *Screed overlay*: Poured over the insulation and pipes to encase the pipes and provide the finished floor substrate

**Screed types for wet UFH**:

*Sand and cement screed (traditional)*: - Thickness: minimum 65mm above the top of the pipe (to ensure the pipe is fully encased and the screed has sufficient structural integrity) - Total screed depth (pipe + cover): typically 75–85mm - Drying time: approximately 1mm per day — so 65mm of screed takes 65 days to dry to a level suitable for most floor finishes (less for tiles) - Advantage: well-understood, low material cost - Disadvantage: slow to dry, which is a significant programme constraint; poor thermal conductivity (0.6 W/mK) means the screed needs to be at a higher temperature to transfer heat

*Anhydrite (calcium sulphate/gypsum) screed*: - Thickness: minimum 30mm above the top of the pipe (the better bond and self-levelling properties of anhydrite allow a thinner coverage) - Total screed depth: typically 40–50mm - Drying time: approximately 1mm per day initially, but anhydrite screeds can often be force-dried and primed within 7–14 days using accelerated drying protocols - Advantage: faster programme, better thermal conductivity (0.9–1.0 W/mK vs 0.6 for sand/cement), self-levelling (no tolerance issues), thinner overall build-up - Disadvantage: more expensive material; must be protected from moisture during installation (not suitable for rooms with standing water during construction); requires specialist laitance removal before tiling - Programme saving: 6–8 weeks shorter than traditional screed for a typical extension — significant on projects with tight programmes

**Floor finish compatibility**:

| Floor finish | Sand/cement screed | Anhydrite screed | |---|---| | Porcelain/ceramic tiles | ✓ (dry fully, prime) | ✓ (remove laitance, prime) | | Natural stone | ✓ | ✓ | | Engineered wood | ✓ (manufacturer's minimum temp) | ✓ | | Solid wood | ✗ (not recommended — movement) | ✗ | | LVT/vinyl | ✓ | ✓ (with primer) | | Carpet | ✓ (client-supplied per RCB terms) | ✓ |

**Wet UFH costs for a typical single-storey rear extension (30m²)**:

| Item | Cost range | |---|---| | UFH pipe supply and installation (manifold, pipe, clips) | £1,200–£2,200 | | Sand/cement screed (65mm, 30m²) | £900–£1,500 | | Anhydrite screed (40mm, 30m²) | £1,200–£2,000 | | PIR insulation 100mm (30m²) | £400–£700 | | Commissioning and pressure test | £150–£300 | | **Total (wet UFH, sand/cement screed)** | **£2,650–£4,700** | | **Total (wet UFH, anhydrite screed)** | **£2,950–£5,200** |

**Boiler and heat pump compatibility**:

- *Existing gas boiler*: Most modern condensing gas boilers can supply a UFH system; a mixing valve (to limit flow temperature to 45°C max) is required if the boiler is also supplying upstairs radiators at higher temperatures - *ASHP (air source heat pump)*: Heat pumps are specifically suited to UFH — they operate most efficiently at low flow temperatures (35–45°C), which matches UFH requirements exactly; the Boiler Upgrade Scheme provides £7,500 toward ASHP installation (2025 figure) - *Combi boiler*: Compatible with UFH, but a combi boiler heating a UFH system in a large extension may need its capacity checked — some combis are not sized for large UFH areas combined with domestic hot water demand

**What electric UFH is**:

Electric underfloor heating uses a resistance heating element (either a mat (thin woven cable on a mesh) or a loose cable) installed in the floor build-up, connected to the mains electricity supply and a thermostat. Unlike wet UFH, there is no water, no boiler connection, no manifold, and no screed — the electric element is typically installed directly in the tile adhesive bed or within a thin self-levelling compound.

**When electric UFH is appropriate for an extension**:

• Electric UFH is best suited to:
• •*Small floor areas* (under 10–15m²) — single bathroom additions, a utility room within an extension, a small ground floor WC
• •*Renovation retrofits* where the existing floor level cannot accommodate the build-up of screed and insulation
• •*Supplementary heating* in a single room rather than a whole-zone heating requirement
• •*Conservation Areas or listed buildings* where a wet UFH system would require structural alteration not permitted

For a standard 25–40m² rear extension, wet UFH is almost always the better choice — the running cost of electric UFH (all-electric resistance heat) versus gas or heat-pump-heated wet UFH is significantly higher over time.

**Running cost comparison (2025 energy prices)**:

For a 30m² extension heated to 21°C in London (approximately 1,500 heating hours per year, UK climate):

| System | Approximate running cost per year | |---|---| | Electric UFH (100W/m² at 30m²) | £600–£900 | | Gas boiler wet UFH (condensing combi) | £180–£300 | | ASHP wet UFH (COP 2.8) | £280–£450 |

Electric UFH costs 2–3× more to run than a gas-heated wet system and 1.5–2× more than a heat pump system at current energy prices. For a permanent living space, this difference accumulates significantly over the life of the system.

**Electric UFH installation**:

Typical specification for a tiled floor: - 150W/m² heating mat laid in the adhesive bed below porcelain or ceramic tiles - Thermostat and programmer wired to fused spur (Part P notifiable electrical work — must be carried out by a registered electrician or notified to Building Control) - No additional floor build-up required beyond the tile adhesive - Cost for a 15m² bathroom: £400–£800 supply and install including thermostat

**When radiators are appropriate in an extension**:

Despite the attractiveness of UFH for a new extension, radiators remain appropriate when:

• •*The floor build-up cannot accommodate screed*: If the extension floor level is constrained by existing thresholds, external levels, or existing drainage, the 100–150mm build-up of insulation plus screed may not be feasible without raising the floor level significantly. In this case, electric UFH or radiators are more viable.
• •*The extension is connected to an existing system without a wet UFH zone*: If the extension uses the existing first-fix pipework connected to the existing radiator circuit, adding a mixed (UFH + radiator) system requires a mixing valve — the added complexity may not be justified for a small space.
• •*A fast-heating response is required*: UFH is a slow-response system — it can take 1–3 hours to bring the floor to temperature from cold. Radiators heat a space faster. If the extension is used intermittently (rather than continuously heated), radiators may be more comfortable in practice.
• •*Cost constraint*: Radiators are cheaper to install than wet UFH — a designer panel radiator costs £200–£600 per unit plus plumbing, compared with £2,500–£5,000+ for a wet UFH system including screed.

**Radiator sizing for an extension**:

A correctly sized radiator for a well-insulated single-storey rear extension (2.4m ceiling, triple-glazed, well-insulated roof and walls, Part L 2021 compliant) is significantly smaller than for an older, poorly insulated room. The heat load calculation (Q = U-value × area × ΔT for each element) should be carried out for the specific extension rather than using rules of thumb — an over-sized radiator is ugly and wastes space; an under-sized one leaves the room cold.

**Designer radiators and exposed pipework in extension design**:

In an open-plan kitchen-dining-living extension, radiators are often unwanted — they conflict with the desire for uninterrupted wall-to-wall glazing, kitchen unit runs, and open-plan aesthetics. Where radiators are specified in an open-plan extension, consider: vertical radiators (narrower, take less horizontal wall space); kickspace heaters under kitchen units; or concealed convectors under windows or beneath glazing.