Is Natural Ventilation Possible? – Can I Just Ask? – Episode 6

Natural vs Mechanical Ventilation: What Really Works?

The idea of ventilating your home naturally – simply opening windows and letting fresh air in – sounds appealing. It’s low-tech, intuitive, and has that wholesome, back-to-basics feel. But in well-sealed, high-performance buildings, is it enough?

For many, especially those who care about indoor air quality, the answer is increasingly no. Natural ventilation might work in theory, but in practice it often turns out to be patchy, seasonal, and labour-intensive. Mechanical systems, especially MVHR (Mechanical Ventilation with Heat Recovery), provide consistent, measurable performance that natural methods rarely match.

If you’d rather hear Andy and Chris talk through the ideas in this article, you can watch Episode 6 of Can I Just Ask? below. 👇

The Reality of Manual Ventilation

Living in a home without mechanical ventilation puts the responsibility squarely on the occupant to keep air quality in check. That means remembering to open windows at the right times, monitoring humidity and CO2 levels, and responding to changing weather conditions. It’s not impossible, but it’s a burden.

One of the team at Back to Earth lives in such a home and takes air quality seriously, with CO2 monitors and humidity sensors dotted throughout. But even with vigilance, the challenges are clear. At night, for instance, CO2 levels in the bedroom can rise above 1400 ppm – well into the range that can cause drowsiness and reduced cognitive performance. To keep levels down would require getting up several times a night to open windows. That’s neither practical nor comfortable.

And it’s not just about CO2. Drying clothes indoors, cooking, or even just breathing adds moisture and pollutants to the air. Without constant, effective extraction, these build up over time. Unless it’s windy and dry outside—and you remember to cross-ventilate—natural ventilation often comes up short.

The Quiet Reliability of MVHR

Compare this with a home using MVHR, and the difference is stark. With an MVHR system running continuously, you get a steady supply of filtered, pre-warmed air, and extraction from moisture-heavy rooms like kitchens and bathrooms. That means no build-up of cooking smells, no damp from drying laundry, and no high overnight CO2 levels.

There’s a misconception that mechanical ventilation must mean noise or forcefulness. In reality, a well-designed MVHR system is nearly silent in operation. It runs in the background, balancing humidity, filtering out mould spores and pollen, and ensuring that even if the occupants do nothing, the building still breathes.

Even in places like Devon, where outdoor air quality is relatively good, MVHR filters show surprising levels of trapped particles—especially mould spores. It’s a reminder that ‘fresh air’ isn’t always as clean as it seems.

Why ‘Natural’ Isn’t Always Better

There’s a psychological pull to the word “natural”, especially in sustainable building. But when it comes to ventilation, natural often just means uncontrolled. It means relying on trickle vents and hoping for wind. It might work in an old, leaky house, but in a modern, airtight home, it’s rarely enough.

Passive ventilation systems do exist—using stack effect and pressure differences to drive airflow without fans—but they depend heavily on building geometry and external conditions. In most cases, they’re too inconsistent to rely on as a primary strategy.

Conclusion: Let the Building Do the Work

At its best, mechanical ventilation with heat recovery takes the burden off the occupant and ensures a baseline of indoor air quality that supports health and comfort. Natural ventilation has its place, but as a primary strategy in airtight, energy-efficient homes, it too often falls short.

If the goal is healthy, low-energy living, then MVHR isn’t just a good option—it’s increasingly the essential one.