Common air-leakage pathways in UK homes — where buildings actually leak.

Under a blower door test in depressurisation, you can literally feel cold air streaming in at the leaks. After locating thousands of them, clear patterns emerge: leakage concentrates at junctions, penetrations and changes of material — anywhere the air barrier is interrupted or was never continuous in the first place. Here is where to look, element by element.

Service penetrations — the worst offenders

Every pipe, cable, duct and waste that passes through the envelope is a potential leak, and they're often the largest single contributors:

• Soil and waste pipes passing through floors and external walls.
• Incoming services — water, gas, electricity, broadband — at the point of entry.
• Extract ducts, flue penetrations and boiler/heat-pump pipework.
• Kitchen and bathroom plumbing penetrating floor decks into joist voids.
• Recessed downlights puncturing the ceiling air barrier into a cold loft — dozens of small holes that add up dramatically.

The loft and ceiling plane

• The loft hatch — frequently un-gasketed and a major warm-air escape route via the stack effect.
• Downlights and ceiling-mounted fittings penetrating into a cold loft.
• The wall head / ceiling-to-wall junction, where the ceiling air barrier meets the wall.
• Service risers, soil-vent pipes and extract ducts passing up through the ceiling into the roof void.

Floor-to-wall junctions and suspended timber floors

Ground floors are a classic leakage zone, especially older suspended timber floors:

• Suspended timber ground floors with an underfloor void open to outside air via airbricks — air leaks up between floorboards and around the perimeter.
• The skirting-to-floor and skirting-to-wall junction, where shrinkage gaps run the length of every wall.
• The wall-to-floor junction at the base of external walls, where the wall air barrier meets the floor.
• Gaps around radiator pipes, where they drop through the floor.

Windows, doors and openings

• The gap between the window/door frame and the structural opening — often only foam-filled, not sealed airtight.
• Worn or missing weatherseals on opening lights and doors.
• Letterboxes, cat flaps, keyholes and trickle vents (the latter are intentional, but should be closable).
• Bay windows, with their complex junctions and multiple frame-to-structure interfaces.

Electrical and wall penetrations

• Socket and switch boxes set into external walls — air leaks through and around the back box, especially on dot-and-dab plasterboard.
• Dot-and-dab (drylined) plasterboard itself, where air can circulate in the void behind the board and escape at any unsealed edge — the so-called 'perimeter sealing' problem.
• Consumer units, meter boxes and external lighting fittings.

Chimneys, flues and the stack effect

Open or disused chimneys are direct holes to the sky and a powerful driver of the stack effect — warm air rises and escapes up the flue, drawing cold air in elsewhere. Capping (with controlled ventilation to keep the flue dry) is often one of the highest-impact single measures in an old house.

Hidden bypasses — intermediate floors and party walls

The trickiest leaks are the ones you can't see, where air moves through concealed voids:

• Intermediate (first) floor voids open at the perimeter into the external wall cavity or build-up — air enters the floor zone and escapes through the external wall.
• Party-wall bypass — the cavity within a party wall can act as a chimney, with air entering at the bottom and leaking out at the eaves (a significant, often-ignored heat-loss path in terraces and semis).
• Cavity walls open at the top into the roof void.
• Behind-the-bath and behind-kitchen-unit panels, where boxed-in areas hide unsealed penetrations.