Underfloor Heating and Screed for London Home Extensions: A Complete Guide

UNDERFLOOR HEATING SYSTEMS DIVIDE INTO TWO FUNDAMENTAL TYPES — WET (HYDRONIC) AND ELECTRIC (DRY). THE CHOICE DETERMINES THE ENTIRE FLOOR BUILD-UP AND RUNNING COST PROFILE:

WET (HYDRONIC) UFH SYSTEMS: WATER IS HEATED BY THE EXISTING BOILER OR HEAT PUMP AND CIRCULATED THROUGH PLASTIC PIPE LOOPS EMBEDDED IN OR LAID ABOVE THE FLOOR SCREED. THE HEATED WATER TEMPERATURE IS TYPICALLY 30-45°C (MUCH LOWER THAN RADIATOR SYSTEMS AT 55-75°C) — MAKING WET UFH IDEAL FOR CONDENSING BOILERS (BETTER CONDENSING EFFICIENCY AT LOWER RETURN TEMPERATURES) AND ESSENTIAL FOR HEAT PUMPS (WHICH OPERATE OPTIMALLY AT LOW FLOW TEMPERATURES).

KEY WET UFH PRODUCTS FOR LONDON EXTENSIONS: - NU-HEAT LOOPSTRATE: FACTORY-ASSEMBLED LOW-PROFILE PANEL SYSTEM WITH PRE-GROUTED PIPES AT 150MM CENTRES — SCREED DEPTH REDUCED TO 65-75MM TOTAL. WIDELY USED IN LONDON EXTENSION PROJECTS. - UPONOR MLCP PIPE 16MM BARRIER PIPE (UPONOR COMBI PIPE): ALUMINIUM-BARRIER MULTILAYER COMPOSITE PIPE — OXYGEN DIFFUSION BARRIER PREVENTS CORROSION OF STEEL HEATING COMPONENTS. LAID AT 150-200MM CENTRES IN CONVENTIONAL SCREED. - HEP2O FLEXE BARRIER PIPE: POLYETHYLENE BARRIER PIPE ALTERNATIVE. - POLYPIPE UFH PANEL: CHANNELLED EPS PANELS THAT HOLD PIPE AT CORRECT SPACING WITHOUT SCREED STAPLING — SUITABLE WHERE FASTER INSTALLATION IS NEEDED. - MANIFOLD: GIACOMINI R557 OR POLYPIPE PUMA — THERMOSTATIC FLOW VALVES FOR EACH LOOP (ZONE); GIACOMINI R462 OR POLYPIPE ALPHA ACTUATORS CONNECTED TO ROOM THERMOSTATS.

ELECTRIC UFH SYSTEMS: ELECTRIC HEATING ELEMENTS (CABLES OR MATS) EMBEDDED IN THE FLOOR FINISH ADHESIVE OR SCREED. NO PIPEWORK TO THE BOILER. INSTALLATION IS FASTER AND CHEAPER. RUNNING COSTS ARE SIGNIFICANTLY HIGHER THAN WET SYSTEMS (ELECTRICITY VS GAS) — SUITABLE FOR BATHROOMS AND SMALL AREAS AS A COMFORT TOP-UP, NOT AS A PRIMARY HEATING SOURCE FOR A WHOLE EXTENSION.

KEY ELECTRIC UFH PRODUCTS: - WARMUP 4IE ELECTRIC UFH CABLE: 150W/M² HEATING DENSITY, LAID IN TILE ADHESIVE UNDER PORCELAIN OR CERAMIC TILES - NUHEAT SIGNATURE ELECTRIC MAT: PRE-SPACED ELECTRIC CABLE MAT, 150W/M², ROLLED OUT AND TILED OVER - WARMUP DCM-PRO: DECOUPLING CRACK MEMBRANE WITH INTEGRATED ELECTRIC HEATING — SUITABLE FOR TILES OVER PROBLEMATIC SUBSTRATES

FOR LONDON EXTENSIONS WHERE WET SYSTEM PIPEWORK IS ALREADY BEING INSTALLED, ELECTRIC UFH IN BATHROOMS IS OFTEN ADDED AS A SIMPLE COMFORT UPGRADE WITH VERY LOW ADDITIONAL COST (£150-400 FOR A BATHROOM ELECTRIC MAT).

THE SCREED THAT ENCAPSULATES OR COVERS THE UFH PIPES IS AS IMPORTANT AS THE PIPE SYSTEM ITSELF. SCREED SELECTION AFFECTS: FLOOR BUILD-UP HEIGHT (WHICH AFFECTS DOOR HEIGHTS AND THRESHOLDS); THERMAL MASS (WHICH AFFECTS HOW QUICKLY THE FLOOR HEATS AND COOLS); STRENGTH; AND DRYING TIME (WHICH AFFECTS THE PROJECT PROGRAMME).

TRADITIONAL SAND AND CEMENT SCREED: - MIX: BS EN 13813 — TYPICALLY CT-C25-F5 (COMPRESSIVE STRENGTH 25 N/MM², FLEXURAL STRENGTH 5 N/MM²) FOR DOMESTIC FLOORS - AGGREGATE SIZE: SHARP WASHED SAND, MAXIMUM AGGREGATE SIZE 4MM FOR SCREEDS 25-40MM THICK; 6MM FOR 40-75MM - WATER:CEMENT RATIO: 0.45-0.55 — LOWER RATIO GIVES HIGHER STRENGTH - MINIMUM DEPTH OVER UFH PIPES: 50MM IN TRADITIONAL SCREED (ALLOWING FOR 16MM PIPE DIAMETER — MINIMUM 35MM COVER) - TOTAL SCREED DEPTH WITH 16MM UFH PIPE: 65-75MM MINIMUM - DRYING TIME: APPROXIMATELY 1MM PER DAY UP TO 40MM THICKNESS, THEN SLOWER — A 75MM SCREED REQUIRES APPROXIMATELY 75+ DAYS BEFORE FLOOR FINISHES CAN BE APPLIED AND UFH COMMISSIONED - CRACKING RISK: HIGH FOR FLOATING SCREED IF JOINTS ARE OMITTED OR MOVEMENTS EXCEED ALLOWABLE LIMITS

LIQUID CALCIUM SULPHATE (ANHYDRITE) SCREED — MOST POPULAR FOR UFH: - PRODUCTS: HANSON CEMFLOOR STANDARD AND LIGHTWEIGHT; LAFARGE AGILIA SCREED A; LEVEL IT BY ARDEX - FLOW-APPLIED — SELF-LEVELLING — NO COMPACTION REQUIRED — MUCH FASTER TO LAY - MINIMUM DEPTH: 30MM OVER PIPES (35-40MM TOTAL INCLUDING 16MM PIPE) VS 50MM FOR SAND:CEMENT — SIGNIFICANT HEIGHT SAVING IN EXISTING BUILDINGS WHERE FLOOR-TO-CEILING HEIGHT IS ALREADY RESTRICTED - THERMAL CONDUCTIVITY: APPROXIMATELY 0.9-1.5 W/MK (SIMILAR TO SAND:CEMENT ~1.0-1.5 W/MK) — COMPARABLE THERMAL PERFORMANCE - DRYING: SIMILAR TO SAND:CEMENT (1MM/DAY) BUT MORE CONSISTENT — ENHANCED DRYING AVAILABLE WITH DEHUMIDIFIERS AND FORCE-DRYING PROTOCOL - CRITICAL: CALCIUM SULPHATE SCREED MUST BE SEALED BEFORE TILING (WITH DILUTED PVA COAT OR ARDEX P51 PRIMER) AS IT IS WATER-SOLUBLE IF REPEATEDLY WETTED — NOT SUITABLE WHERE PROLONGED WETTING RISK EXISTS - NOT COMPATIBLE WITH CEMENT-BASED TILE ADHESIVES WITHOUT PRIMER — USE MAPEI MAPELASTIC, BAL ADHESIVE OVER PRIMER, OR ARDEX PRODUCTS SPECIFICALLY RATED FOR CALCIUM SULPHATE SUBSTRATE

SCREED CRACK PREVENTION: - PERIMETER INSULATION STRIPS (COMPRIBAND / ACOUSTIFOAM PERIMETER STRIP) AT ALL WALLS AND COLUMN BASES — ALLOWS THE SCREED TO EXPAND WITHOUT CONSTRAINT - SCREED CONTROL JOINTS AT DOOR OPENINGS AND WHERE SCREED AREA EXCEEDS 40M² OR EITHER DIMENSION EXCEEDS 8M - CORRECT WATER:CEMENT RATIO — OVERWET SCREED SHRINKS MORE ON DRYING - NOT STARTING UFH COMMISSIONING TOO EARLY — MINIMUM 21-28 DAYS CURING BEFORE BEGINNING THE GRADUAL COMMISSIONING PROCESS

A CORRECTLY DESIGNED WET UFH SYSTEM REQUIRES HEAT LOSS CALCULATIONS FOR EACH ZONE TO ENSURE THE SYSTEM CAN MEET THE DESIGN HEAT DEMAND.

HEAT LOSS CALCULATION: FOR A LONDON EXTENSION WITH PART L 2021 SPECIFICATION (WALL U-VALUE ~0.18-0.25 W/M²K, ROOF ~0.15-0.18 W/M²K, GLAZING ~1.0-1.6 W/M²K), TYPICAL HEAT LOSSES FOR A KITCHEN-DINING EXTENSION ARE 40-80 W/M² OF FLOOR AREA. THIS IS SIGNIFICANT BECAUSE UFH HAS A MAXIMUM SURFACE TEMPERATURE OF 29°C (INHABITED AREAS, PART L — BASED ON EN ISO 11855-2) AND A MAXIMUM HEAT OUTPUT OF APPROXIMATELY 50-80 W/M² DEPENDING ON FLOW TEMPERATURE AND FLOOR FINISH.

FLOOR FINISH THERMAL RESISTANCE: THE FLOOR FINISH ON TOP OF THE SCREED ADDS THERMAL RESISTANCE THAT REDUCES UFH OUTPUT: - PORCELAIN TILE (10MM): R = 0.01 M²K/W — NEGLIGIBLE, UFH WORKS WELL - NATURAL STONE (15MM GRANITE/MARBLE): R = 0.01-0.02 M²K/W — EXCELLENT - ENGINEERED TIMBER (18MM ENGINEERED OAK): R = 0.05-0.10 M²K/W — ACCEPTABLE IF UFH-RATED PRODUCT USED (MAX MOISTURE CONTENT VARIATION) - SOLID TIMBER FLOOR: NOT RECOMMENDED OVER WET UFH — MOISTURE MOVEMENT - CARPET + UNDERLAY: R = 0.10-0.20 M²K/W — SIGNIFICANTLY REDUCES OUTPUT — ONLY THIN LOW-TOG CARPET SUITABLE (MAXIMUM 1.5 TOG)

PIPE SPACING AND LOOP LENGTH: - STANDARD SPACING: 150MM CENTRES (MID-RANGE HEAT OUTPUT) - EDGE ZONE SPACING: 100MM CENTRES AT PERIMETERS AND UNDER LARGE WINDOWS WHERE HEAT LOSS IS HIGHER — APPROXIMATELY 1.0-1.5M WIDE EDGE ZONE - MAXIMUM LOOP LENGTH: 80-100M FOR 16MM PIPE AT 0.5 BAR PRESSURE DROP — LONGER LOOPS REQUIRE HIGHER PUMP PRESSURE OR LARGER PIPE DIAMETER - BALANCING: EACH LOOP ON THE MANIFOLD IS INDIVIDUALLY BALANCED — FLOW METERS ON GIACOMINI R557 MANIFOLD OR EQUIVALENT ALLOW VISUAL BALANCING

CONTROLS: - ROOM THERMOSTAT PER ZONE: HEATMISER NEOSTAT (WIRED) OR HEATMISER NEOHUB (WIFI CONTROLLED); TADO SMART THERMOSTAT; HONEYWELL EVOHOME (MULTI-ZONE WIRELESS SYSTEM) - ACTUATORS: ONE THERMAL ACTUATOR PER MANIFOLD LOOP CONNECTED TO THE ZONE THERMOSTAT — OPENS WHEN HEAT REQUIRED, CLOSES WHEN SATISFIED - PUMP OVERRUN: BOILER OR HEAT PUMP MUST HAVE ADEQUATE PUMP-OVERRUN PERIOD AFTER FINAL ZONE CLOSES — PREVENTS HEAT BUILD-UP IN CLOSED SYSTEM

UFH IN AN EXTENSION MUST COMPLY WITH BUILDING REGULATIONS PART L (CONSERVATION OF FUEL AND POWER) AND PART J (COMBUSTION APPLIANCES IF A NEW BOILER IS INSTALLED).

PART L 2021 COMPLIANCE FOR UFH IN NEW EXTENSIONS: - U-VALUE OF FLOOR: INSULATION UNDER UFH MUST ACHIEVE 0.15 W/M²K OR BETTER FOR NEW EXTENSION GROUND FLOORS - FOR A CONCRETE RAFT OR BEAM AND BLOCK FLOOR WITH UFH: MINIMUM 100MM PIR INSULATION (CELOTEX GA4000, XTRATHERM THIN-R, IKO ENERTHERM) BELOW THE SCREED - THERMAL BRIDGING AT PERIMETER: PERIMETER INSULATION STRIP MUST EXTEND THE FULL DEPTH OF THE SCREED (TYPICAL 50-75MM) AND IDEALLY CONTINUE DOWN THE EXTERNAL WALL INNER FACE TO THE FOUNDATION — THIS BREAKS THE THERMAL BRIDGE WHERE FLOOR SLAB MEETS EXTERNAL WALL

TYPICAL FLOOR BUILD-UP FOR A LONDON EXTENSION WITH WET UFH: FROM BOTTOM TO TOP: 1. GROUND (LONDON CLAY OR HARDCORE) — COMPACTED TYPE 1 GRANULAR FILL 150MM MINIMUM 2. BLINDING LAYER — 50MM SHARP SAND OR LEAN-MIX CONCRETE 3. DPM — VISQUEEN SUPER FLEXIBLE PLUS 1200 GAUGE (300 MICRON) LAPPED AND TAPED 4. PIR INSULATION — CELOTEX GA4000 OR KINGSPAN TF70 100MM (TWO LAYERS OF 50MM STAGGERED JOINTS) 5. UFH PANELS / PIPE FIXING SYSTEM — NU-HEAT LOOPSTRATE OR CHANNELLED PANELS 6. UFH PIPE — UPONOR 16MM MLCP AT 150MM CENTRES 7. SCREED — HANSON CEMFLOOR 65-75MM OR SAND:CEMENT 75MM 8. PRIMING COAT (CALCIUM SULPHATE ONLY) 9. TILE ADHESIVE AND FINISH TILE

TOTAL CONSTRUCTION DEPTH FROM TOP OF INSULATION TO TOP OF TILE: APPROXIMATELY 90-110MM. FROM EXISTING GROUND LEVEL TO TOP OF FINISHED FLOOR: DEPENDS ON EXCAVATION DEPTH AND SLAB LEVEL.

COMMISSIONING PROTOCOL (PART OF BUILDING CONTROL HANDOVER): UFH MUST BE COMMISSIONED IN ACCORDANCE WITH BSEN 1264-4: GRADUAL TEMPERATURE INCREASE — START AT 25°C FLOW, INCREASE 5°C PER DAY TO DESIGN FLOW TEMPERATURE, HOLD FOR 2-3 DAYS, THEN COOL GRADUALLY. MINIMUM 21 DAYS AFTER SCREED LAYING BEFORE COMMISSIONING. CONTRACTOR MUST PROVIDE WRITTEN COMMISSIONING RECORD TO BUILDING CONTROL.

TYPICAL UFH AND SCREED COSTS FOR LONDON EXTENSIONS (2025-2026):

UFH SUPPLY ONLY (PIPE, PANELS, MANIFOLD — EXCLUDED CONTROLS): - 20M² KITCHEN EXTENSION: £600-1,200 - 30M² KITCHEN-DINING EXTENSION: £900-1,800 - 50M² LARGE OPEN-PLAN EXTENSION: £1,400-2,800

UFH INSTALLATION (LABOUR, PIPE LAYING, CONNECTIONS TO MANIFOLD AND CONTROLS): - 20M² EXTENSION: £400-800 - 30M² EXTENSION: £600-1,200 - 50M² EXTENSION: £1,000-2,000

SCREED (SUPPLY AND INSTALLATION): - HANSON CEMFLOOR LIQUID SCREED, 70MM DEPTH: £25-40/M² INSTALLED (INCLUDES PUMP DELIVERY AND APPLICATION) - TRADITIONAL SAND:CEMENT SCREED, 75MM: £20-35/M² INSTALLED - FOR A 30M² EXTENSION: £750-1,200 SCREED SUPPLY AND INSTALLATION

TOTAL UFH + SCREED PACKAGE FOR 30M² EXTENSION (SUPPLY + INSTALL): £2,200-5,000

HEAT PUMP OR BOILER CONTROLS AND MANIFOLD UPGRADE (IF NOT ALREADY UFH-COMPATIBLE): £300-800

THINGS THAT ADD COST: - MULTIPLE ZONES (MORE MANIFOLD BRANCHES AND ACTUATORS) - THICK FLOOR FINISH (REDUCING OUTPUT AND REQUIRING CLOSER PIPE SPACING) - EXISTING BOILER NOT COMPATIBLE WITH UFH FLOW TEMPERATURES (MAY NEED BLENDING VALVE — £200-400) - HEAT PUMP INSTALLATION AS PART OF UPGRADE (ADDS £5,000-12,000)