Pillar Guide · Ventilation & IAQ

Ventilation & Indoor Air Quality

Once you make a building more airtight, ventilation stops being optional. This guide covers every strategy — and the numbers that tell you whether your indoor air is actually healthy.

11 in-depth articles·By George Sora · Certified Passive House Designer

Ventilation is the deliberate, controlled supply of fresh air and extraction of stale, moist and polluted air. As homes get tighter — through retrofit, new windows or insulation — the accidental ventilation we used to rely on disappears, and a designed strategy becomes essential to control humidity, CO₂ and pollutants. This pillar compares the options and explains how to measure the result.

Good ventilation does three jobs at once: it controls humidity (preventing condensation and mould), it removes pollutants (CO₂, particulates, VOCs), and — done well with heat recovery — it does both without throwing away your heating. The right strategy depends entirely on how airtight the building is. The articles below work through every option, the air-quality numbers that matter, and how to match ventilation to a retrofit.

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In this guide

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MVHR explained — mechanical ventilation with heat recovery.

How the heat exchanger recovers 75–95% of ventilation heat, why airtightness is its prerequisite, and what separates a silent, healthy system from a noisy, ineffective one.

6 Jun 202612 min

MVHR vs PIV — which ventilation strategy is right for your home?

MVHR, PIV, dMEV and intermittent extract — four ventilation strategies with very different running costs, performance and suitability.

20 Mar 202610 min

PIV explained — positive input ventilation, and where it fits.

How PIV works, what it's good at (and not), how it compares to MVHR and extract, its lack of heat recovery, and when it's the right ventilation strategy.

7 Jul 20269 min

Extract ventilation explained — intermittent, dMEV and MEV.

Intermittent extractor fans, continuous decentralised extract (dMEV), centralised MEV, humidity-sensing controls, Part F, and where each fits in a moisture-control strategy.

8 Jul 202610 min

Trickle vents and background ventilation — what they do, and don't.

What background ventilators are for, how they fit Part F, why people block them and shouldn't, and how background ventilation interacts with airtightness and mechanical systems.

9 Jul 20269 min

CO₂ monitoring — the simplest measure of whether you're ventilating enough.

Why indoor CO₂ is the best single proxy for ventilation adequacy, what the ppm thresholds mean, the effect on sleep and cognition, and how to use a CO₂ monitor.

10 Jul 20269 min

PM2.5 and particulates indoors — sources, health and filtration.

What PM2.5 and PM10 are, where indoor particulates come from (cooking, wood burning, candles, outdoor air), their health effects, WHO guideline levels, and how MVHR filtration and source control reduce them.

11 Jul 202610 min

VOCs and indoor pollutants — formaldehyde, off-gassing and source control.

What VOCs are, common indoor sources (paints, furnishings, MDF/formaldehyde), how they affect health, what TVOC readings mean, and why source control plus ventilation is the answer.

12 Jul 202610 min

Indoor air quality — the four numbers that actually matter.

CO₂, humidity, PM2.5 and VOCs. Get these four right and almost every other 'air-quality' worry resolves itself.

26 Mar 20268 min

Ventilation and mould prevention — the humidity connection.

How ventilation controls indoor humidity, the moisture households generate, the link between ventilation rate and surface RH, and why ventilation plus warm surfaces is the durable mould fix.

13 Jul 202610 min

Ventilation strategy for retrofit — matching ventilation to airtightness.

How to choose the right ventilation as airtightness improves: when extract, PIV or MVHR is appropriate, 'build tight ventilate right', commissioning, Part F, and avoiding the post-retrofit mould trap.

14 Jul 202611 min

Also being written

Expanding this guide

We publish carefully — these topics are in production rather than thin placeholder pages.

Single-room heat recovery (dMVHR) for constrained retrofits
Radon and ground-gas ventilation
Ventilation for allergies and asthma
Summer bypass and overheating control
Ventilation investigations — IAQ before and after