Ventilation is one of the most commonly underspecified elements of a renovation or new build. In an older, draughty house, natural air leakage provides enough fresh air through gaps in the fabric — but improving insulation and airtightness without addressing ventilation creates a serious risk of condensation, mould, and poor indoor air quality. The right ventilation strategy depends on how airtight your building is and how much you are willing to spend.
MVHR (mechanical ventilation with heat recovery) is the gold standard for airtight buildings: it supplies fresh air to habitable rooms and extracts stale, humid air from wet rooms, recovering up to 95% of the heat in the extracted air and transferring it to the incoming supply. For a three-bedroom house, an MVHR system costs £2,500–£6,000 supply and fit. For retrofits with less stringent airtightness, simpler ventilation strategies may be more appropriate.
Ventilation Strategies Compared
Natural ventilation
Relies on background ventilators (trickle vents in windows, wall vents) and intermittent extract fans in kitchens and bathrooms. No mechanical supply; relies on wind and stack effect. Suitable only for buildings with significant air permeability. Does not meet Part F requirements as a standalone strategy in new builds.
dMEV — decentralised mechanical extract ventilation
Individual continuously-running, low-speed extract fans in each wet room. No heat recovery. Cheaper to install than MVHR but recovers no heat. Typical installation cost: £600–£1,500 for a full house. Running costs: £30–£60/year. Suitable for moderately airtight properties where MVHR ducting is impractical.
cMEV — centralised mechanical extract ventilation
A single central fan unit serving all wet rooms via a duct network. No heat recovery. Simpler than MVHR; slightly more efficient than dMEV. Cost: £1,000–£2,500 installed. Suitable for moderately airtight refurbishments.
MVHR — mechanical ventilation with heat recovery
A balanced ventilation system: the unit simultaneously supplies filtered fresh air to living rooms and bedrooms and extracts stale air from kitchens and bathrooms. A heat exchanger transfers warmth from the exhaust air to the incoming supply air. A high-quality unit achieves 85–95% heat recovery efficiency (Passivhaus Institute certified units must exceed 75%).
MVHR is appropriate when:
- Air permeability is below approximately 5 m³/(h·m²) at 50 Pa (tested by a blower door test)
- The building is a new build or deep retrofit aiming for high energy performance
- The homeowner wants controlled, filtered ventilation regardless of outdoor conditions (pollen filtration, noise reduction)
Comparison table
| Strategy | Heat recovery | Best suited to | Installed cost (3-bed) | Running cost/year |
|---|---|---|---|---|
| Natural ventilation | None | Older leaky properties | £200–£600 (trickle vents) | Negligible |
| dMEV (fans per room) | None | Moderately airtight refurbs | £600–£1,500 | £30–£60 |
| cMEV (central fan) | None | Moderately airtight; simple layout | £1,000–£2,500 | £40–£80 |
| MVHR | 70–95% | Airtight new builds; deep retrofits | £2,500–£6,000 | £30–£80 |
| MVHR + summer bypass | 70–95% + free cooling | Airtight, any climate | £3,000–£7,000 | £40–£100 |
How MVHR Works
The MVHR unit is typically installed in a loft, utility room, or plant room. Two duct networks run throughout the house:
- Supply ducts from the unit to living rooms, bedrooms, and hallways — delivering fresh, filtered, pre-warmed air
- Extract ducts from kitchens, bathrooms, toilets, and utility rooms back to the unit — removing moisture, odours, and CO₂
Inside the unit, extract air and supply air pass through a heat exchanger (counter-flow, cross-flow, or rotary type) without mixing. Heat from the warm extract air warms the incoming cold outdoor air. In summer, a bypass mode allows cool outdoor air to enter without passing through the heat exchanger, providing passive cooling.
Key unit components:
- Heat exchanger — counterflow plate type achieves the highest efficiency (85–95%); rotary units transfer some moisture as well as heat (enthalpy recovery)
- Supply and extract fans — dual fans for balanced pressure; EC (electronically commutated) motors are most efficient
- Filters — G4 filters on supply and extract are standard; F7 (ePM2.5) on the supply side filter pollen and fine particles for allergy sufferers
- Condensate drain — moisture condensing from extracted air must drain; must be connected to a drain or soak-away
Building Regulations: Part F
Ventilation is governed by Approved Document F (ADF) of the Building Regulations for England and Wales. The 2022 revision of ADF introduced stricter requirements and brought domestic ventilation rates in line with industry expectations:
- New dwellings must have a whole-building ventilation strategy designed to provide adequate air quality. MVHR, cMEV, dMEV, or natural ventilation with background ventilators are all acceptable routes depending on the calculated air permeability
- For an MVHR system, minimum whole-building ventilation rates are set by the number of bedrooms and floor area (minimum 0.3 litres/second/m² of habitable floor area as a baseline)
- Commissioning and testing is required: air flow rates must be measured at each terminal and recorded on a commissioning sheet
- Renovation trigger: if an extension or renovation changes the thermal envelope significantly, a ventilation strategy review is required
For properties aiming for Passivhaus certification, the Passivhaus Institute requires an MVHR unit with a measured heat recovery efficiency of at least 75% and a specific fan power (SFP) of no more than 0.45 Wh/m³.
Installation: What to Expect
MVHR installation in a new build during construction is straightforward — rigid circular duct (typically 125 mm or 160 mm) is run within the structure before plasterboard. Retrofit MVHR in an existing occupied house is significantly more complex and expensive:
- Rigid ducts may need to be surface-mounted in service voids, ceiling slots, or concealed in joinery
- Alternatively, semi-rigid and flexible duct is used for retrofits, with reduced efficiency and higher noise risk
- Expect 3–5 days of installation work for a competent team in a typical three-bedroom house retrofit
- Airtightness testing (blower door test, £300–£600) should be carried out before and after to verify performance
Design is critical. Poor duct layout, inadequate duct sizing, or improper balancing causes noise, draughts, or poor air quality. An MVHR system should be designed by someone familiar with ADF and CIBSE guidance — not just adapted from a generic template.
Cost Breakdown
| Item | Typical cost |
|---|---|
| MVHR unit (mid-range, 250–550 m³/h capacity) | £800–£2,000 |
| Ductwork materials (new build) | £400–£800 |
| Ductwork materials (retrofit) | £600–£1,500 |
| Installation labour (new build) | £800–£1,500 |
| Installation labour (retrofit) | £1,500–£3,500 |
| Commissioning and air flow balancing | £200–£400 |
| Total (new build, 3-bed) | £2,500–£4,500 |
| Total (retrofit, 3-bed) | £3,500–£7,000 |
Maintenance
MVHR is low-maintenance but not no-maintenance:
- Filter replacement every 3–12 months depending on outdoor air quality and system usage. G4 filters: £10–£25 per set; F7 filters: £25–£50. Neglecting filters sharply reduces efficiency and air quality
- Heat exchanger cleaning: check annually; clean every 2–3 years. Most units allow the exchanger to be removed and rinsed
- Duct inspection: inspect every 5–10 years for dust accumulation in supply ducts (extract ducts in kitchens are more prone to grease build-up)
- Annual check: listen for increased fan noise (bearing wear) and verify flow rates at terminals are still balanced
A properly maintained MVHR system has a design life of 15–25 years. Units from established manufacturers carry 2–5 year warranties extendable to 10 years in some cases.
Is MVHR Worth It?
For an airtight new build or deep retrofit, MVHR is essential — without it, the building will suffer condensation and poor air quality regardless of how good the insulation is. The question of “worth it” in those cases is not really optional.
For a standard cavity-wall semi with only moderate insulation improvements, dMEV or cMEV at a fraction of the cost may be more appropriate. The rule of thumb: if your blower door test result is above 5 m³/(h·m²) at 50 Pa, passive and semi-mechanical solutions are viable; below that, MVHR becomes the preferred solution. Below 1 m³/(h·m²) (Passivhaus territory), MVHR is effectively mandatory.
