Every retrofit changes a home's ventilation whether you plan it or not. New windows, draught-proofing, insulation and air-sealing all reduce the uncontrolled leakage that used to ventilate the house by accident. That's good for energy — but it means the deliberate ventilation strategy has to step up to replace what's been removed. 'Build tight, ventilate right' isn't a slogan; it's the rule that prevents retrofits from causing the very damp they were meant to cure.
Match the strategy to the airtightness
There's no one-size-fits-all answer — the right ventilation depends on how airtight the home is, or will become after the works:
| Airtightness after retrofit | Appropriate strategy |
|---|---|
| Still leaky (light retrofit) | Trickle vents + humidity-sensing extract (dMEV); PIV for condensation |
| Moderately tight (deeper retrofit) | Continuous extract (dMEV/MEV); consider MVHR |
| Airtight (deep retrofit / EnerPHit) | MVHR — heat-recovered, filtered, balanced supply + extract |
Decentralised vs whole-house
Where ducting a whole-house system is impractical (occupied homes, awkward layouts, single rooms), decentralised options can bridge the gap: single-room heat-recovery units (dMVHR) that fit through an external wall and recover heat for one room, or humidity-sensing dMEV in each wet room. They're not as efficient or as balanced as a ducted whole-house MVHR, but they're far better than leaving a tightened home under-ventilated, and they suit phased or constrained retrofits.
Don't forget commissioning
A ventilation system only works if it actually delivers its design flow rates — and an alarming proportion of installed systems never do, because they're not commissioned. Every system (especially MVHR) must have its flow rates measured and balanced after installation, with a commissioning record. An un-commissioned MVHR can be noisy, under-ventilate bedrooms, or run unbalanced and undermine its own heat recovery. Commissioning is not optional; it's what turns installed equipment into a working system. (See the MVHR article.)
Part F and getting it signed off
Building Regulations Part F governs ventilation, including for some retrofit works, and the 2021 edition strengthened the requirement to maintain adequate ventilation when energy-efficiency measures are installed — precisely to stop the post-retrofit mould trap. A compliant retrofit demonstrates that the ventilation provision matches the (now tighter) building. Even where Part F isn't strictly triggered, designing to its principles is the responsible approach.
The joined-up approach
Sequencing ventilation in a phased retrofit
Most homes are retrofitted in stages as budget allows, and ventilation has to keep pace with airtightness at every step rather than being left to a final phase. The principle is that the ventilation provision should always match the airtightness the home has reached so far — so a light first phase (loft insulation, draught-proofing) may only need open trickle vents and good humidity-sensing extract, while a later phase that adds wall insulation and new windows tips the balance toward continuous mechanical extract or MVHR. The danger of a phased approach is the gap: a homeowner draught-proofs and replaces windows one year, intending to add ventilation 'later', and spends an intervening winter with a tightened envelope and no upgraded ventilation — exactly the post-retrofit mould trap. The discipline is to treat ventilation as part of each phase that increases airtightness, never as a deferred extra. Where MVHR is the eventual goal, planning the duct routes early (even if the unit is fitted later) avoids tearing finished work apart to retrofit ducting.
PAS 2035 and the ventilation assessment
The UK retrofit standard, PAS 2035, makes this joined-up thinking a formal requirement rather than good intention. It mandates a whole-dwelling assessment, a ventilation assessment and moisture-risk management whenever energy-efficiency measures are installed, precisely because uncoordinated retrofit causing condensation and mould had become a recognised problem. In practice this means a PAS 2035 retrofit must check the existing ventilation, assess how the planned measures will change the home's airtightness and moisture behaviour, and specify ventilation that keeps the dwelling adequately ventilated afterwards — with the work overseen by a retrofit coordinator. For a homeowner, the value of this framework is that it forces the ventilation question to be answered before the fabric work is done, not discovered after the mould appears. Even on smaller, non-funded projects that fall outside the standard's strict scope, designing to its principles — assess, coordinate, ventilate, verify — is the responsible way to retrofit.
