Airtightness and Heat Pumps — Why It Matters

Why air leakage undermines a heat pump

Uncontrolled air leakage can rival the heat lost through the walls of an older home. Warm air rising in a heated house creates a 'stack effect' that draws cold air in low down and pushes warm air out high up — through floor perimeters, loft hatches, chimneys, service penetrations and worn window seals. Every cubic metre of warm air lost must be replaced and reheated.

A boiler can mask this by blasting heat in at 70°C. A heat pump cannot — it relies on a steady, low-temperature output, so a constant cold draught is the difference between a room that holds temperature and one that never quite warms up. Reducing leakage directly lowers the design heat loss the heat pump has to meet, which means a smaller unit and a lower flow temperature.

What good airtightness does for a heat pump

Lowers the design heat loss, allowing a smaller, cheaper heat pump
Enables a lower flow temperature and therefore a higher SCOP
Improves comfort immediately by removing cold draughts at floor level
Reduces running cost regardless of the heating system
Makes room temperatures more stable and easier for the heat pump to maintain

Airtightness is not the same as 'no fresh air'

A common worry is that sealing a home makes it stuffy or damp. The opposite is true when it is done properly. Accidental leakage is uncontrolled — it under-ventilates in still weather and over-ventilates (wasting heat) in wind. Replacing it with designed, controlled ventilation — continuous extract or MVHR — gives reliable fresh air and humidity control while stopping the heat loss. Tightening without ventilating is the mistake; tightening with the right ventilation is the goal.

How airtight does a retrofit need to be?

There is no fixed retrofit target the way there is for Passive House (0.6 ACH₅₀) or new build. The point is direction of travel: the lower the leakage, the lower the heat demand and the better the heat pump performs. Many older homes start at 10–15 ACH₅₀ or worse; bringing that down meaningfully through targeted sealing is one of the best-value steps towards readiness.

How RetrofitIQ measures airtightness for readiness

A blower door test to measure total air leakage (ACH₅₀) and the air-change rate that feeds into the heat-loss calculation
Smoke tracing and thermal imaging under depressurisation to locate every leak path with photographs
A prioritised air-sealing plan, ranked by impact on the heat demand
A controlled-ventilation strategy matched to the tightened fabric
A re-test to verify the reduced leakage before the heat pump is sized

Common questions from clients.

Can a leaky house still have a heat pump?

Yes, but it will need to be larger, run hotter and cost more to run. Air-sealing first is one of the cheapest ways to improve readiness and efficiency, and it pays back through lower bills whatever heating you have.

Will sealing the house cause condensation?

Only if you seal without ventilating. We always pair air-sealing with a controlled ventilation strategy — continuous extract or MVHR — so you avoid trapping moisture and actually improve air quality.

How airtight does it need to be for a heat pump?

There is no single retrofit target. Lower leakage means lower heat demand and a more efficient heat pump, so we measure your starting point and reduce it as cost-effectively as possible rather than chasing an arbitrary number.

Is a blower door test worth it before a heat pump?

Very much so. It quantifies a major part of the heat demand that is otherwise guessed, locates the draughts so sealing is targeted, and lets us verify the improvement — all of which improve the accuracy of the heat-pump sizing.

What are the biggest leakage paths in a typical UK home?

Suspended floor perimeters and gaps between boards, loft hatches and ceiling penetrations, open or disused chimney flues, and the gaps around window and door frames behind the trim. A smoke-traced blower door test finds them all.

Airtightness and heat pumps — the overlooked half of readiness.