Ventilation first: why retrofit starts with airflow

Illustrative photo.

Written by Miles Sharples, Managing Director

Retrofit programmes delivered at scale across the UK for councils, housing associations and energy suppliers. About Miles · LinkedIn

Insulation and airtightness make a home warmer, and without enough ventilation they can also make it wetter. This guide sets out what PAS 2035 and Approved Document F expect before fabric work starts, where Awaab’s Law fits, and a ventilation check to run on every home.

Insulation changes how a home breathes

Most older homes were never ventilated by design. PAS 2035:2023 notes that infiltration and air leakage provide approximately three quarters of the ventilation in UK dwellings, and that without adequate ventilation, adding insulation or airtightness anywhere in the building, or blocking an existing ventilator, cuts that leakage and raises the risk of poor indoor air quality, condensation and mould. It adds that a ventilation system being present is not proof that a home is adequately ventilated.

The government's damp and mould guidance for landlords makes the same point: some energy efficiency work reduces overall ventilation, and if ventilation is not addressed the risk of damp and mould, poor indoor air quality and overheating goes up. The cost of getting it wrong is on record. The National Audit Office reported in October 2025 that an estimated 22,000 to 23,000 homes with external wall insulation installed under the government's Energy Company Obligation (98% of the total) and 9,000 to 13,000 with internal wall insulation (29%) had major issues that will cause problems such as damp and mould.

The standards already put ventilation first

Approved Document F, Volume 1 says that when energy efficiency work is done on an existing home, its ventilation should either meet the standards or be no less satisfactory than before (paragraph 3.2), and that an assessment should decide what extra ventilation is needed (paragraph 3.7). Its simplified method counts the measures fitted since the home was built plus those planned, classes each as minor or major, and places the work in Category A, B or C. Category A is likely to need nothing more; B and C need added ventilation.

Approved Document F accepts following PAS 2035 as an adequate way to show the work has not left the home under-ventilated (paragraph 3.6), and PAS 2035 builds on the same categories. The retrofit assessment must inspect the existing ventilation against a checklist, recording the position, type, make and model of each fan where possible, and the size and location of every background ventilator. If the package falls into Category B or C, or there is evidence of condensation or mould, the retrofit designer must confirm whether the existing ventilation is adequate and, if not, specify an upgrade.

Wherever ventilation is upgraded, every habitable room needs purge ventilation, air must be able to move between rooms, continuous systems must be controlled automatically rather than only from a light switch, and the system must be inspected, with new mechanical systems tested and commissioned.

Awaab's Law makes the cause part of the fix

Awaab's Law has applied to social landlords in England since 27 October 2025, with fixed timescales for damp and mould and for emergency hazards. It sets deadlines for hazards rather than an energy standard, but the government guidance for social landlords is explicit about causes: where a damp and mould hazard results from the absence of ventilation or poor insulation, the landlord must take appropriate remedial action, such as installing mechanical ventilation or effective insulation. The same guidance warns that ventilation work must not damage fire compartmentation.

Phase 2, which adds excess cold and six other hazard groups, is due to come into force on 30 November 2026, subject to Parliamentary approval. The Phase 2 guidance says Awaab's Law is not intended to drive larger scale fabric improvements. Those sit with the new Decent Homes Standard: social rented homes at EPC C on one of three metrics (fabric performance, smart readiness or heating system) by 1 April 2030, a new Criterion E requiring homes to be free of damp and mould, and mechanical ventilation, where present, added to the components landlords must keep in repair, with the full standard applying from 2035. Government expects that meeting it should mean fewer cases reach Awaab's Law. Our Awaab's Law guide sets out the deadlines in full.

A ventilation check to run before any fabric work

This adapts the checklist in Appendix D of Approved Document F, with points from the government damp and mould guidance. A no on any line means the ventilation design needs attention before the insulation goes in.

• A working extract fan in every wet room, rated for at least 30 l/s in a kitchen with a cooker hood extracting outside (60 l/s without one), 30 l/s in a utility room, 15 l/s in a bathroom and 6 l/s in a WC.
• Background ventilators in each habitable room, sized to Approved Document F and left open.
• Fans and background ventilators in the same room at least 0.5 m apart.
• Internal doors undercut to 10 mm above the floor finish, or 20 mm above the floor surface where no finish is fitted.
• Purge ventilation, usually an opening window, in every habitable room.
• A manual override on any automatic fan controls.
• A visual check for condensation and mould, recorded with photographs.
• A count of the measures already fitted and those planned, to place the home in Category A, B or C.
• Residents shown how the system works, and asked whether running costs stop them using it.

How we sequence a programme

On whole house programmes for housing providers and councils our order does not change: assess and design every home under PAS 2035 with a ventilation strategy, put the ventilation right, then the fabric, then windows, doors and roofs, then heating and solar PV sized for the home the fabric has made. Ventilation upgrades ran through the programmes our team has delivered for local authorities and housing associations. The Durham County Council LAD2 programme included ventilation so the warmer homes stayed dry, and the North Star Housing programme in Darlington and the City of York Council HUG2 programme both included full ventilation upgrades. Each was led by Miles Sharples as project lead, with a skilled, accredited workforce.

What to do next

Take the homes with the most damp and mould reports, or the most measures planned, and run the ventilation check on a sample of each archetype. Record what is there, what works and what is missing before the fabric package is fixed. If you want a second pair of eyes, send us the addresses, what residents are reporting and what has been installed before, especially cavity fill, through the programme enquiry form.

About the author

Miles Sharples, Managing Director, Energy Giants

Miles has led retrofit programmes at scale across the UK, with long-standing relationships with local authorities including Kirklees Council, Durham County Council and Ribble Valley Borough Council. He has project-directed programmes for housing associations and delivery partners including Aspire Housing, Sureserve, LivGreen and BroadOak, and for energy suppliers including E.ON Next, Utilita, British Gas and Octopus, and for tier one contractors including UKEM.

Worked with:Kirklees CouncilDurham County CouncilRibble Valley Borough CouncilAspire HousingSureserveLivGreenBroadOakE.ON NextUtilitaBritish GasOctopusUKEM

More about Miles and the teamMiles on LinkedIn