Airtightness and ventilation: build tight, ventilate right

What does build tight, ventilate right mean?

Build tight, ventilate right is the idea that a home should be made airtight to stop uncontrolled heat loss, and then given controlled ventilation to supply fresh air on purpose. The two go together. Sealing a home without adding ventilation traps moisture and pollutants; ventilating a leaky home wastes heat. Modern building regulations require both.

The phrase comes from retrofit and low-energy building practice and is now central to how UK homes are designed. It is the reason Part L, which governs energy and airtightness, and Part F, which governs ventilation, have to be read together. Improving one without the other creates a problem: a warm home that is damp and stuffy, or a well-ventilated home that is expensive to heat.

What airtightness must a new home reach?

Under Part L of the Building Regulations, the 2021 edition, a new dwelling in England must reach an air permeability no worse than 8 cubic metres per hour per square metre at 50 pascals, tightened from the previous limit of 10. The target used in the underlying energy calculation is tighter still, at 5, so many homes aim well below the limit.

Air permeability is measured with a blower door test that pressurises the home to 50 pascals and measures the leakage through the envelope. A lower number means a tighter home. Because the notional building in the energy calculation assumes 5, designing only to the 8 limit often makes the energy targets harder to hit, so builders tend to aim lower.

Sources: GOV.UK

Why can a very airtight home not rely on natural ventilation?

Natural ventilation depends on gaps, trickle vents and wind to move air. As a home is sealed to modern airtightness levels, those gaps shrink, so natural airflow becomes unreliable and often insufficient. Below a certain airtightness, a mechanical system such as continuous extract or MVHR is needed to guarantee the fresh air the home requires.

The tighter the home, the less it can depend on chance leakage to bring in fresh air and carry away moisture. That is why airtight new builds almost always use a mechanical system, and why sealing up an old home during a retrofit without adding ventilation is a common cause of new damp and condensation problems.

Is airtightness testing required for new homes?

Yes. Under the 2021 Part L, air permeability testing is mandatory for new dwellings, and averaging the result across a sample of homes is no longer allowed. Each home is tested individually. The result feeds into the energy calculation and confirms the home meets the airtightness limit.

Requiring every home to be tested closed a loophole where a developer could test a few sample plots and apply the result across a site. Now the measured figure for each home is real, which also matters for ventilation: a home that turns out tighter than expected may need its ventilation strategy checked to make sure it still delivers enough fresh air.

Sources: GOV.UK

How does airtightness decide which ventilation system I need?

The more airtight the home, the more it needs mechanical ventilation. Very airtight homes, such as new builds and deep retrofits, typically use MVHR so they can ventilate without losing heat. Moderately airtight homes may use continuous extract. Leakier older homes have more natural airflow, which is why positive input ventilation can suit them.

This is the practical link between the two regulations. Your air permeability figure is one of the main inputs into choosing a system. It is not the only factor, since cost, disruption and whether you are building or improving all matter, but it is usually the starting point. See our guide to the four system types for how they compare.