If My House Is Airtight, Do I Need MVHR? Airtightness and Ventilation Explained
It's the question every airtightness installer hears eventually: "If you seal the building up this tightly, where does the air come from?" It's a fair challenge, and the honest answer is that airtightness and ventilation aren't opposing goals; they're a pair. Get one without the other, and you have a problem. Get both right, and you have a building that's both efficient and healthy to live in.
Why "airtight" doesn't mean "sealed shut"
An airtight building stops uncontrolled air leakage: draughts through gaps, cracks, and poorly detailed junctions that waste heat and let moisture-laden air migrate into places it shouldn't. It says nothing about how fresh air gets in deliberately. Those are two separate systems: the air barrier controls where air can move, and the ventilation strategy controls how much air moves, on purpose, to clear moisture and pollutants.
Leaky older buildings get away without much thought on ventilation because draughts do the job badly but constantly. Tighten the envelope and that accidental ventilation disappears, which is exactly the point, since it was wasting heat, but it means ventilation now has to be designed in rather than left to chance.
What Approved Document F actually says
Part F doesn't mandate a specific ventilation system purely on the basis of how airtight a building is. It gives designers four options (background ventilators with intermittent extract fans, passive stack ventilation, continuous mechanical extract ventilation, or MVHR) provided the chosen system hits the performance rates in its Appendix A.
In practice, though, airtightness level makes some of those options unworkable. Guidance generally points to whole dwelling MVHR becoming the practical default once a building is designed to achieve an air permeability below roughly 5 m³/(h·m²) at 50 Pa, because background ventilators and intermittent extract fans struggle to pull enough air through a very tight envelope without causing noticeable draughts or simply failing to meet the required rates. Below about 3 m³/(h·m²), the range most Passive Purple and Airtight White projects are targeting, natural and extract only ventilation strategies genuinely don't cope well, and Part F's own guidance flags that buildings tested tighter than this may need extra scrutiny to confirm ventilation is adequate.
So what's the actual pairing?
For anything at or near Passivhaus level airtightness (0.6 ACH or tighter), MVHR is the standard answer, not an optional upgrade. It supplies filtered fresh air continuously, extracts stale, moist air from kitchens and bathrooms, and recovers the heat from the outgoing air to warm the incoming supply, so you're not trading your airtightness energy savings away through an open trickle vent. For less extreme retrofits that still land above roughly 5 m³/(h·m²), a well-specified continuous mechanical extract system or background ventilators can still comply, but it's worth checking the numbers rather than assuming.
The detail that actually matters on site
MVHR only performs as designed if the envelope it's ventilating is as airtight as it was specified to be. An MVHR system fighting unplanned leaks is working against itself, pulling in unfiltered, untempered air through gaps instead of through its own ducting. That's the real argument for getting the air barrier right first: a continuous, tested layer of Passive Purple or Airtight White gives the MVHR system a sealed envelope to actually work within, rather than asking it to compensate for a leaky one.
Bringing Airtightness and Ventilation Together
Airtightness and ventilation aren't a trade-off. A tight building needs a deliberate ventilation strategy, and MVHR is usually that strategy once you're building to anything like Passivhaus standards. Specify both together, and get the airtightness detailing right first, so the MVHR system can do the job it was designed for.