Air Tightness and Mechanical Ventilation for London Home Renovation: A Practical Guide

TRADITIONAL LONDON VICTORIAN TERRACES ARE TYPICALLY VERY LEAKY — AIR PERMEABILITY OF 10-25 M³/H/M² AT 50 PA IS COMMON (COMPARE WITH THE PART L 2021 MAXIMUM FOR NEW BUILD: 8 M³/H/M² AT 50 PA; AND PASSIVHAUS: 0.6 ACH AT 50 PA). THIS LEAKINESS PROVIDES NATURAL VENTILATION BY ACCIDENT — THROUGH GAPS IN FLOORBOARDS, AROUND WINDOW FRAMES, VIA CHIMNEY STACKS, AND THROUGH PARTY WALLS.

WHEN RENOVATION SEALS THE BUILDING: A RENOVATION THAT SEALS GAPS (NEW WINDOWS, INSULATION, MODERN JOINERY) DRAMATICALLY REDUCES AIR PERMEABILITY — OFTEN TO 3-5 M³/H/M² AT 50 PA. THIS IS GOOD FOR ENERGY — BUT IF VENTILATION IS NOT PROVIDED, THE RESULT IS: - ELEVATED CO2 (POOR AIR QUALITY, SLEEPINESS, REDUCED CONCENTRATION) - HIGH RELATIVE HUMIDITY (CONDENSATION ON COLD SURFACES — PARTICULARLY IN BATHROOMS, BEDROOMS, AND KITCHENS) - MOULD GROWTH (CLADOSPORIUM, ASPERGILLUS, STACHYBOTRYS IN EXTREME CASES) - OFF-GASSING OF VOLATILE ORGANIC COMPOUNDS (VOCS FROM NEW MATERIALS RETAINED IN AIR)

AIR TIGHTNESS TESTING (BLOWER DOOR TEST): THE BLOWER DOOR TEST (PRESSURISATION METHOD, AS PER BS EN ISO 9972:2015) MEASURES THE BUILDING\'S AIR PERMEABILITY AT 50 PA PRESSURE DIFFERENTIAL. REQUIRED FOR NEW DWELLINGS AND EXTENSIONS CLAIMING BETTER THAN THE DEFAULT ASSUMPTION IN SAP. FOR SIGNIFICANT RENOVATIONS IN LONDON, AIR TIGHTNESS TESTING IS INCREASINGLY RECOMMENDED (NOT MANDATORY FOR EXISTING BUILDING RENOVATIONS BUT OFTEN REQUIRED BY ARCHITECTS AND ENERGY ASSESSORS ON HIGH-SPEC PROJECTS).

AIR TIGHTNESS TARGETS: - NEW BUILD PART L 2021: MAXIMUM 8 M³/H/M² AT 50 PA (5 M³/H/M² IF USING THE TARGET EMISSION RATE CALCULATION APPROACH) - RETROFIT GOOD PRACTICE: AIM FOR 3-5 M³/H/M² AT 50 PA - PASSIVHAUS: 0.6 ACH AT 50 PA (APPROXIMATELY 0.5-1.0 M³/H/M²) — REQUIRES SPECIALIST DETAILING

FOUR MAIN VENTILATION STRATEGIES APPLY IN UK RESIDENTIAL RENOVATION:

1. MVHR — MECHANICAL VENTILATION WITH HEAT RECOVERY: THE PREMIUM WHOLE-HOUSE VENTILATION SYSTEM. A CENTRAL HEAT EXCHANGER EXTRACTS STALE AIR FROM WET ROOMS (KITCHEN, BATHROOMS, UTILITY, WC) AND SUPPLIES FRESH FILTERED AIR TO HABITABLE ROOMS (BEDROOMS, LIVING ROOMS) — SIMULTANEOUSLY RECOVERING 85-93% OF THE HEAT FROM THE EXTRACTED AIR TO PRE-HEAT THE INCOMING SUPPLY.

PRODUCT EXAMPLES: - ZEHNDER COMFOAIR Q350 (93% THERMAL EFFICIENCY, MODBUS COMMS, EC MOTOR): THE PREMIUM RESIDENTIAL CHOICE. SUITABLE FOR 3-5 BEDROOM HOMES. ZEHNDER COMFOPOST DUCT SYSTEM (CIRCULAR PLASTIC MANIFOLD SYSTEM). - VENT-AXIA SENTINEL KINETIC PLUS (89%): UK MANUFACTURER, STRONG SERVICE NETWORK, POPULAR ON LONDON RENOVATION PROJECTS. - TITON HRV1.5 Q (85%, LOWER COST): SUITABLE FOR 2-3 BEDROOM HOMES OR BUDGET PROJECTS. - PAUL NOVUS 300 (94% PHI-CERTIFIED): PASSIVHAUS-CERTIFIED; FOR ULTRA-LOW ENERGY PROJECTS.

WHEN MVHR IS MOST APPROPRIATE: NEW EXTENSIONS + WHOLE-HOUSE RENOVATION (AIR TIGHTNESS LIKELY TO IMPROVE SIGNIFICANTLY); LOFT CONVERSIONS WITH FULL HOUSE REFURBISHMENT; BASEMENTS; PASSIVHAUS-STANDARD RENOVATIONS. MVHR REQUIRES DUCTWORK THROUGHOUT THE HOUSE — BEST INSTALLED DURING RENOVATION WHEN CEILINGS ARE OPEN. COST: £4,000-9,000 SUPPLY AND INSTALL (DUCT AND UNIT).

2. dMEV — DECENTRALISED MECHANICAL EXTRACT VENTILATION: INDIVIDUAL CONTINUOUSLY-RUNNING LOW-FLOW FANS IN EACH WET ROOM — NO DUCTWORK EXCEPT WITHIN THE ROOM. EACH FAN HAS A BOOST MODE (TRIGGERED BY HUMIDITY SENSOR OR PULL SWITCH) FOR HIGHER FLOW WHEN STEAM IS PRODUCED. PRODUCTS: VENT-AXIA LO-CARBON TEMPRA (HUMIDITY-SENSING, LED INDICATOR); NUAIRE CYFAN (HUMIDITY-CONTROLLED, QUIET, 16 DBA); TITON TRIMVENT XCITE 4000 (CONTINUOUS TRICKLE + BOOST). WHEN dMEV IS APPROPRIATE: PART F 2021 COMPLIANT SOLUTION FOR EXISTING HOUSES WITH MODERATE IMPROVEMENTS TO AIR TIGHTNESS; COST-EFFECTIVE AND SIMPLE TO INSTALL WITHOUT MAJOR DUCTWORK RUNS; SUITABLE WHERE MVHR IS NOT PRACTICAL. COST: £100-300 PER ROOM UNIT, SUPPLY AND INSTALL.

3. PIV — POSITIVE INPUT VENTILATION: A SINGLE UNIT IN THE LOFT SPACE THAT GENTLY PUSHES FILTERED FRESH AIR INTO THE HOUSE THROUGH THE CEILING — DILUTING MOISTURE AND POLLUTANTS BY SLIGHTLY PRESSURISING THE HOUSE (AIR LEAKS OUT THROUGH GAPS RATHER THAN DAMP OUTSIDE AIR LEAKING IN). PRODUCTS: NUAIRE DRIMASTER ECO (POPULAR, HUMIDITY-SENSING); VENT-AXIA POSITIV (EQUIVALENT). WHEN PIV IS APPROPRIATE: RETROFIT SOLUTION FOR HOUSES WITH PERSISTENT CONDENSATION AND MOULD THAT HAVE A LOFT SPACE; CHEAP TO INSTALL (NO MAJOR DUCTWORK); PART F 2021 MAY NOT ACCEPT PIV AS THE SOLE VENTILATION STRATEGY FOR SIGNIFICANTLY MODIFIED BUILDINGS. COST: £500-900 SUPPLY AND INSTALL.

4. NATURAL VENTILATION (BACKGROUND TRICKLE VENTILATORS + RAPID VENTILATION): TRICKLE VENTILATORS BUILT INTO WINDOW FRAMES (RAVEN AIRMASTER 4000, TITON TRIMVENT XCITE 4000) PROVIDE BACKGROUND VENTILATION. OPENABLE WINDOWS PROVIDE RAPID/PURGE VENTILATION. ONLY APPROPRIATE FOR BUILDINGS WHERE AIR TIGHTNESS HAS NOT IMPROVED SIGNIFICANTLY BEYOND THE PRE-EXISTING LEVEL. PART F 2021 SPECIFIES MINIMUM FREE AREA OF TRICKLE VENTILATORS BASED ON ROOM SIZE.

PART F (2021) OF THE BUILDING REGULATIONS COVERS VENTILATION OF BUILDINGS. THE KEY CHANGES FROM THE 2010 EDITION THAT AFFECT LONDON RENOVATION:

EXISTING BUILDINGS — WHEN PART F APPLIES: PART F APPLIES TO AN EXISTING BUILDING UNDERGOING RENOVATION IF THE RENOVATION AFFECTS VENTILATION PROVISION (E.G. NEW WINDOWS REMOVE TRICKLE VENTILATORS; OR NEW KITCHEN/BATHROOM EXTRACTION IS INSTALLED). WHERE WORKS TRIGGER BUILDING REGULATIONS PART F, THE BUILDING MUST BE UPGRADED TO PART F STANDARDS IN THE AFFECTED AREA.

WET ROOM EXTRACTION RATES (MINIMUM): - KITCHEN: 60 L/S (ADJACENT TO HOB) OR 30 L/S (ELSEWHERE IN KITCHEN) - BATHROOM: 15 L/S CONTINUOUS OR 30 L/S BOOST (FOR INTERMITTENT FANS) - WC (NO BATH OR SHOWER): 6 L/S - UTILITY ROOM: 30 L/S

MINIMUM WHOLE-DWELLING VENTILATION RATES (CONTINUOUS): BASED ON FLOOR AREA AND NUMBER OF BEDROOMS: - 2-BED FLAT (75M²): 25 L/S WHOLE-DWELLING CONTINUOUS - 3-BED TERRACE (90M²): 29 L/S WHOLE-DWELLING CONTINUOUS - 4-BED HOUSE (130M²): 38 L/S WHOLE-DWELLING CONTINUOUS THESE RATES INFORM THE SIZING OF MVHR UNITS AND dMEV SYSTEMS.

NOISE REQUIREMENTS (NEW IN PART F 2021): FANS IN ROOMS (BATHROOM, WC, KITCHEN) MUST NOT EXCEED 30 DB(A) AT 1M FROM THE FAN IN CONTINUOUS OPERATION MODE. THIS REQUIREMENT EFFECTIVELY RULES OUT MANY OLDER FAN MODELS AND REQUIRES EC MOTOR FANS (TITON, VENT-AXIA LO-CARBON, NUAIRE CYFAN).

COMMISSIONING AND TESTING: MVHR AND CENTRALISED MEV SYSTEMS MUST BE COMMISSIONED (AIRFLOW BALANCED AND MEASURED AT EACH TERMINAL) AND A COMMISSIONING RECORD PROVIDED TO THE OCCUPIER.

ACHIEVING A SIGNIFICANT IMPROVEMENT IN AIR TIGHTNESS REQUIRES IDENTIFYING AND SEALING THE MAIN AIR LEAKAGE PATHS. IN A TYPICAL LONDON VICTORIAN TERRACE, THESE ARE:

1. SUSPENDED TIMBER GROUND FLOOR: THE GAP BETWEEN THE SUSPENDED TIMBER FLOOR AND THE VOID BELOW (FRONT TO BACK THROUGH THE UNDERFLOOR VOID TO THE OUTSIDE AIR) IS OFTEN THE LARGEST SINGLE AIR LEAKAGE PATH. SOLUTIONS: SEAL AROUND PIPES AND SERVICES PENETRATING THE FLOOR WITH FIRE-RATED ACOUSTIC SEALANT (HILTI CP606 FIRESTOP SEALANT); INSULATE THE FLOOR VOID AND INSTALL A VCL (VAPOUR CONTROL LAYER) AT JOIST LEVEL (ISOVER VARIO KMTM DUPLEX ADAPTIVE MEMBRANE).

2. CHIMNEY BREAST AND FLUES: UNUSED CHIMNEYS ARE SIGNIFICANT AIR LEAKAGE AND HEAT LOSS PATHS. OPTIONS: CHIMNEY BALLOON (THERMACAP, WOOLLY MAMMOTH CHIMNEY PILLOW) — TEMPORARY SOLUTION; CHIMNEY CAP (EXTERNAL LOUVRED CAP TO PREVENT RAINWATER ENTRY — RETAIN TRICKLE VENTILATION OF FLUE); PROFESSIONALLY SEALED FLUE FOR DISUSED CHIMNEY BREASTS.

3. WINDOW AND DOOR JUNCTIONS: GAPS BETWEEN WINDOW/DOOR FRAME AND REVEALS ARE TYPICALLY FILLED WITH EXPANDING FOAM (SOUDAL PU FOAM) BUT FOAM ALONE DOES NOT PROVIDE A CONTINUOUS AIR BARRIER UNLESS TAPED. USE EPDM AIRTIGHTNESS MEMBRANE TAPE (PRO CLIMA TESCON VANA, ISOVER VARIO TAPE) TO SEAL THE FRAME PERIMETER TO THE WALL MEMBRANE/VCL.

4. LOFT HATCH: A POORLY SEALED LOFT HATCH IS A MAJOR CONVECTIVE HEAT LOSS AND AIR LEAKAGE POINT. SPECIFICATION: PREMIER LOFT LADDERS PREMIUM INSULATED HATCH (40MM INSULATED PANEL, RUBBER GASKET SEAL) OR FAKRO LTK THERMO HATCH (U-VALUE 0.49 W/M²K, TRIPLE-SEAL GASKET).

5. PARTITION-TO-EXTERNAL-WALL JUNCTIONS: WHERE INTERNAL PARTITIONS MEET EXTERNAL WALLS OR ROOFS, SEAL THE JUNCTION WITH AIRTIGHTNESS TAPE AND/OR INTUMESCENT ACOUSTIC SEALANT (HILTI CP617 OR TREMCO SPECTRUM).