Retrofit Ventilation Strategy: Matching It to Airtightness

Ventilation strategy for retrofit — matching ventilation to airtightness.

The single most damaging retrofit mistake is improving a home's airtightness and insulation without upgrading its ventilation. As a building gets tighter, the accidental ventilation it relied on disappears — and unless a designed strategy replaces it, condensation and mould follow. This article explains how to match ventilation to the airtightness you're creating.

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:

Ventilation strategy vs airtightness

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

Common questions from clients.

I'm insulating and draught-proofing my home — do I need to change the ventilation?

Almost certainly yes. Insulation and draught-proofing reduce the accidental leakage that was ventilating your home, so unless you add a designed ventilation strategy (humidity-sensing extract, PIV, or MVHR depending on how tight the home becomes), humidity will build up and you risk condensation and mould you didn't have before. Designing the ventilation alongside the fabric works is essential, not optional.

Do I have to install MVHR when I retrofit?

Not always — the right strategy depends on the airtightness you achieve. A light retrofit that leaves the home relatively leaky may be fine with continuous humidity-sensing extract and open trickle vents; a deep retrofit that makes the home airtight should have MVHR to ventilate it and recover the heat. Where whole-house ducting is impractical, single-room heat-recovery units can bridge the gap. We match the system to the building.

Why do so many retrofits end up with mould?

Because the fabric was upgraded (windows, insulation, draught-proofing) without upgrading the ventilation, so the leakage that used to ventilate the home was removed with nothing put in its place — humidity builds up and condenses on cold surfaces. It's entirely avoidable by designing the ventilation alongside the airtightness ('build tight, ventilate right') and commissioning it properly. That's standard on every retrofit we design.