Ventilation in London Renovations: Natural Ventilation, MEV, and MVHR

Ventilation is among the least glamorous decisions in a London renovation and among the most consequential. A well-insulated, well-draught-proofed home that lacks adequate ventilation will accumulate moisture and CO₂. Condensation forms on cold surfaces, mould follows, and indoor air quality deteriorates. Building Regulations Part F requires all new dwellings and significantly altered existing dwellings to meet ventilation standards — the question is which system meets those standards for a given project.

This guide explains the three principal ventilation strategies used in London residential renovations and when each is appropriate.

Why ventilation is more important in modern renovations

Older London housing stock — Victorian terraces, Edwardian flats — was leaky by design and by age. Air infiltrated through gaps in floors, windows, and walls, providing background ventilation that prevented moisture accumulation. A renovation that replaces windows with double or triple-glazed units, insulates walls, and draughtproofs the building envelope removes that background ventilation. Without a replacement strategy, the result is a tight building with insufficient air exchange.

The irony is that the improvements that make a renovation more energy-efficient and comfortable also create the conditions for moisture problems if ventilation is not addressed alongside them.

Strategy 1: Natural ventilation with intermittent extract

The simplest and cheapest approach. Trickle vents in windows provide background fresh air; extract fans in wet rooms (bathrooms, kitchens, utility rooms) remove moist air when triggered.

Appropriate for: older housing stock with moderate air permeability (i.e., not fully airtight), or projects where only wet rooms are being renovated.

Requirements under Part F: trickle vents must be provided in habitable rooms (living rooms, bedrooms, kitchens) at a minimum equivalent area. Extract fans must meet minimum flow rates: - Bathroom (intermittent): 15 l/s (54 m³/h) - Kitchen (adjacent to hob): 30 l/s (108 m³/h) intermittent, or 13 l/s continuous - Utility room: 15 l/s

Limitations: trickle vents are draughty in cold weather if they are properly sized. Occupants tend to close them, which defeats the system. Intermittent extract fans address moisture at source but do nothing for background CO₂ levels in living areas. This strategy is a compliance minimum, not a quality ventilation outcome.

Strategy 2: Mechanical extract ventilation (MEV)

A centralised extract fan (typically located in the loft or a cupboard) draws air continuously from wet rooms through ductwork, at low background rates with a boost function triggered by humidity sensors or manual switches. Fresh air enters through trickle vents or purpose-provided openings in habitable rooms.

Appropriate for: whole-house renovations where the building envelope is being improved, particularly where ductwork can be concealed in a new ceiling void or service corridor.

Advantages over intermittent extract: - Continuous low-rate extraction prevents background moisture accumulation - Single fan unit rather than multiple extract fans in wet rooms - Quieter in operation than individual fans

Limitations: MEV extracts stale air but does not recover heat from it. In a well-insulated building, the air extracted by MEV carries heat out of the building — this is an energy penalty. MEV does not provide filtered fresh air to habitable rooms; incoming air still passes through trickle vents (which are unfiltered and untempered).

MEV is appropriate where budget does not extend to MVHR, or where ductwork routing makes MVHR impractical.

Strategy 3: Mechanical ventilation with heat recovery (MVHR)

MVHR is a whole-house system that extracts stale air from wet rooms and living areas while simultaneously supplying filtered fresh air to bedrooms and living rooms. A heat exchanger in the central unit recovers 70–95% of the heat from the extract air and transfers it to the incoming supply air. The result is continuous whole-house ventilation with minimal heat loss.

Appropriate for: new-build equivalents, deep retrofits targeting Passivhaus or low-energy standards, or any project where the building is being made substantially airtight and the client wants both good indoor air quality and energy efficiency.

Why MVHR is increasingly common in quality London renovations: London's housing stock, when thoroughly renovated, can achieve air permeability figures that make MVHR not just beneficial but necessary. An airtight building without adequate mechanical ventilation is a damp building. MVHR addresses both sides of this equation — continuous ventilation and heat recovery.

Requirements for effective MVHR:

1. 1.Airtight building envelope: MVHR only recovers heat efficiently if the building is reasonably airtight. In a leaky building, uncontrolled infiltration bypasses the system. A target of ≤5 m³/h/m² at 50Pa is the minimum for MVHR to be worthwhile; ≤3 m³/h/m² is better.

1. 2.Ductwork design: MVHR requires a duct network connecting the central unit to each wet room (extract) and each habitable room (supply). In London's terraced houses, routing this ductwork is the primary challenge. A whole-house MVHR retrofit typically requires a new ceiling void (20–30mm) for duct runs, or service bulkheads in corridors. This must be coordinated with the architect at design stage — not retrofitted during construction.

1. 3.Commissioning: the system must be balanced (supply and extract flows measured and adjusted) after installation. An unbalanced system performs poorly and is a common failure mode in residential installations.

Limitations: higher installation cost (£6,000–£14,000 installed for a 3–4 bed London terrace), requires filter maintenance every 6–12 months, and ductwork routing is a design constraint that must be resolved early.

Humidity-controlled vs constant flow

Both MEV and MVHR can operate at constant low flow (background ventilation) with a boost mode triggered by humidity sensors (in bathrooms and kitchens) or CO₂ sensors (in habitable rooms). Humidity-controlled systems adapt to actual conditions rather than running at maximum flow continuously, which reduces energy use and noise.

For a quality renovation, specify humidity-controlled boost in wet rooms and CO₂-controlled boost in bedrooms as a minimum.

Acoustic specification

Ventilation systems that are heard are turned off. Background noise from a poorly specified fan or unlagged ductwork in a bedroom is one of the most persistent complaints in new renovations. Key acoustic considerations:

• —Fan unit location: the MVHR or MEV unit should be located away from sleeping areas (loft space or plant cupboard on a different floor)
• —Duct velocity: target ≤3 m/s in main duct runs and ≤2 m/s at terminal grilles to limit flow noise
• —Acoustic attenuators: specify attenuator sections in ductwork runs serving bedrooms where the fan unit is close
• —Anti-vibration mounts: the central unit should be mounted on anti-vibration brackets, not fixed directly to the structure

For a well-specified MVHR installation, the system should be inaudible in any habitable room. If it is audible at background flow rates, either the system is oversized, the ductwork is undersized, or the unit is poorly located.

Coordination with other trades

Ventilation ductwork competes for the same ceiling and wall void space as electrical conduit, plumbing pipework, and structural elements. In a London terrace with limited floor-to-floor height, gaining even 50mm for ductwork may require careful coordination with the structural engineer (to avoid notching joists excessively) and the architect (to maintain acceptable ceiling heights).

Resolve ductwork routing before first fix commences. A ventilation design drawing showing duct routes, section sizes, and unit location should be produced and reviewed against structural and architectural drawings before the project breaks ground.