Why is my house so draughty?

What this usually means

When a house feels draughty, air is moving through it without being controlled. Warm indoor air leaks out through gaps high in the building, and as it leaves, cold outdoor air is pulled in through gaps lower down to replace it. You feel the incoming cold air as a draught, but the underlying problem is the whole circulation: every litre of warm air that escapes is replaced by a litre of cold air that has to be heated from scratch. That is why a leaky home is both uncomfortable and expensive to run.

Crucially, the draughts you can feel are only a fraction of the story. The obvious gaps around old windows and doors are real, but the largest leakage paths are usually hidden — the junction where the floor meets the external wall, gaps behind skirting boards, an uninsulated and unsealed loft hatch, holes where pipes and cables pass through the structure, the backs of kitchen and bathroom units, recessed downlights, and the route up through a suspended timber floor. Air finds every one of these, and together they dwarf the window gaps.

Air leakage is also dynamic, which is why a house feels draughtier on some days than others. The movement is driven by pressure differences across the building: warm air is buoyant and rises, creating the 'stack effect' that pushes air out at the top and draws it in at the bottom, while wind adds pressure on the windward side and suction on the leeward side. Both effects are stronger in cold, windy winter weather — exactly when you most notice the draughts and most want to keep the heat in.

Common causes

Signs and symptoms

Air moves through a building because of pressure differences across its envelope, and there are two main drivers. The first is the stack effect: warm indoor air is less dense than cold outdoor air, so it rises and exerts pressure at the top of the building, escaping through high-level gaps. As it leaves, it lowers the pressure lower down, drawing cold air in through low-level gaps. The taller the warm column and the bigger the temperature difference, the stronger the effect — which is why draughts are worst in cold weather and on stairs and upper floors.

The second driver is wind. As wind hits a building it creates positive pressure on the windward face and negative pressure (suction) on the leeward and side faces. Air is pushed in on one side and pulled out on the other, and the effect rises sharply with wind speed. Stack and wind effects combine and vary constantly, which is exactly why a draughty house feels different from day to day and why chasing individual draughts by feel is so unreliable — the pattern changes with the weather.

Because leakage depends on both the size of the gaps and the pressure across them, two things follow. First, many small, hidden gaps distributed around the envelope can add up to far more leakage than a few obvious ones at the windows. Second, you cannot reliably judge the total or its distribution by hand on a normal day, because the pressures are low and shifting. This is the problem the blower door test solves: it puts the whole house under a controlled, steady pressure so that every leakage path reveals itself at once, and the total can be measured.

A blower door fan pressurises or depressurises the house to a standard 50 pascals and measures the airflow needed to hold that pressure — giving the air permeability and air change rate (such as ach50). Under depressurisation, cold air is drawn in through every leak simultaneously, so smoke pens and thermal imaging can map each one precisely. This converts a vague, weather-dependent feeling of 'draughtiness' into a measured number and a located list of leaks, which is what makes targeted, effective sealing possible.

How to fix a draughty house properly

The aim is to stop uncontrolled leakage while keeping the controlled ventilation a home needs. That means finding the real leaks, sealing them in the right order, and not simply blocking up the windows.

1. 01

   Measure and locate the leakage first

   A blower door test quantifies the total air leakage and, with smoke and thermal imaging, pinpoints every significant path — so effort goes where the air actually moves.

2. 02

   Seal the big, hidden paths first

   Address floor-to-wall junctions, service penetrations, the loft hatch, suspended floor gaps and the backs of fitted units before fine-tuning windows and doors. These usually deliver the biggest gains.

3. 03

   Draughtproof windows, doors and letterplates

   Renew worn seals and fit brush or compression seals to windows, doors and letterboxes — worthwhile, but as part of the whole picture rather than the whole job.

4. 04

   Deal with chimneys and unused flues

   Cap or fit a draught excluder to unused flues to stop the permanent stack-effect draught they create, while keeping any required background ventilation.

5. 05

   Protect and provide controlled ventilation

   As the home becomes more airtight, ensure deliberate ventilation — trickle vents, extract or MVHR — so moisture and stale air are removed without relying on leaks.

6. 06

   Re-test to verify

   A follow-up blower door test confirms how much the leakage has fallen and whether any paths remain, proving the work succeeded.

How to prevent it coming back

• Seal new penetrations as soon as pipes, cables or extractors are installed, rather than leaving open holes.
• Maintain window and door seals and replace them before they perish.
• Keep the loft hatch insulated and sealed, and cap unused flues.
• Plan controlled ventilation alongside any airtightness work so the home stays healthy.
• Re-test airtightness after major works to catch new leakage paths early.