How much does a blower door test cost, and is it worth it?

What this usually means

There are two broad reasons to have a blower door test, and they price differently. The first is compliance — a single air-permeability figure required for Building Regulations on a new build or extension, which is a relatively quick, standardised test producing a number. The second is diagnostic — finding out where a home leaks so you can fix it — which involves depressurising the house and then walking it with smoke pencils, an anemometer and often thermal imaging to trace each leakage path. The diagnostic test takes longer and costs more, but it is the one that actually saves a homeowner money, because the number alone tells you little about what to do.

What drives the cost is mainly the size and complexity of the property, the type of test, travel, and whether leak-location and a written report are included. A small flat is quicker than a large, rambling house; a bare compliance test is cheaper than a full diagnostic survey; and adding thermal imaging or a detailed report adds value and cost. It is worth asking exactly what is included — some quotes are for the number only, others for a full leak-finding survey with a prioritised list of fixes — because comparing a bare test with a full diagnostic on price alone is misleading.

Whether it is worth it comes down to what the leakage is costing. In an older or draughty home, infiltration — air leaking through gaps around floors, skirtings, services, loft hatches, windows and doors — can be one of the single largest heat losses, larger than people expect, and it is the cause of those draughts you can feel but cannot find. A diagnostic test reveals exactly where that air is moving, so sealing is targeted at the leaks that matter rather than sprayed around at random. Because effective, prioritised sealing is one of the cheapest ways to cut heat loss and improve comfort, the test usually pays back quickly — and it stops you wasting money sealing the wrong things or over-sealing a home that needs the ventilation.

Common causes

Signs and symptoms

A blower door fits a calibrated fan into an external doorway and either pressurises or depressurises the house to a reference pressure, typically 50 pascals. The fan's airflow needed to hold that pressure is a direct measure of the total leakage, normally expressed as air changes per hour at 50 Pa or as air permeability in cubic metres per hour per square metre of envelope. That figure quantifies the problem; but the money-saving part is what happens while the house is depressurised — air is drawn in through every gap, so smoke pencils, a hand held at suspect junctions and a thermal camera (which sees the cold streaks where outside air enters) reveal precisely where the leakage is and how significant each path is.

This matters because infiltration heat loss scales with the leakage rate and the temperature difference, and in a draughty home it can rival or exceed the fabric loss through the walls. Crucially, leakage is concentrated — a relatively small number of paths often account for most of it — so locating and sealing those few worst offenders captures most of the benefit cheaply, whereas random sealing wastes effort on minor gaps and may miss the big ones. The test also guards against over-sealing: a home still needs controlled ventilation, so the diagnostic distinguishes the uncontrolled leakage worth sealing from the purpose-provided ventilation that must remain. That targeting — fixing the leaks that matter while protecting the ventilation — is why the test repays its cost.

How to get value from a blower door test

Decide whether you need a compliance number or a diagnostic survey, get the leak-finding included, then seal the prioritised leaks while keeping the ventilation the home needs.

1. 01

   Define what you need

   Choose a compliance test for regs, or a diagnostic survey to find and fix leaks.

2. 02

   Compare like with like

   Check whether quotes include smoke, thermal imaging and a written report.

3. 03

   Get the leaks located

   Have the worst leakage paths identified and ranked by impact.

4. 04

   Seal by priority

   Fix the largest leaks first for the greatest gain per pound.

5. 05

   Protect ventilation

   Keep purpose-provided ventilation while sealing uncontrolled leakage.

6. 06

   Re-test to verify

   Confirm the sealing worked with a follow-up measurement where worthwhile.

How to prevent it coming back

• Test before sealing so the budget targets the real leaks.
• Don't pay for a number when you need a leak-finding survey.
• Avoid over-sealing — keep the ventilation the home needs.
• Prioritise the few worst leaks rather than chasing every minor gap.