Draught-proofing mistakes that can leave you colder, damper — or no better off

Draught-proofing is the poster child of cheap energy fixes — brush strips, compression seals and a tube of sealant against heat that the Energy Saving Trust's published estimates put at meaningful money off a typical bill (figures vary by property and year — roughly in the £50-£125 annual band for a thorough job on windows and doors). Done well, it's the best hours-to-savings ratio in the house. Done badly, it either does nothing, or worse, seals in moisture and starves heaters of air. Here is where it goes wrong.

Mistake one: sealing the ventilation that's there on purpose

Some gaps are load-bearing for the house's health. Air bricks and underfloor vents keep the timber floor structure dry — blocking them trades a draught for the damp diagnosis guide. Trickle vents in modern window frames are the ventilation system of a sealed house — the condensation guide explains what happens when homes stop exchanging air. And the non-negotiable: vents serving fuel-burning appliances — boilers, gas fires, stoves — exist because combustion needs air and exhaust needs somewhere to go; blocking one is how you manufacture a CO problem. Gas Safe engineers flag sealed appliance vents on every visit, and they're right to.

Mistake two: the chimney

An open chimney can pull heat out of a room like an open window — and capping or blocking it without thought creates both of mistake one's problems: no airflow for the structure, and if anything is ever burned or the boiler's flue shares the stack, a safety failure. The workable middle: a chimney balloon or vented cap for disused flues, professionally fitted caps where any doubt exists, and a ventilation route deliberately kept.

Mistake three: sealing the wrong gaps, leaving the real ones

The obvious offenders — letterbox, keyhole, loft hatch (the hatch is routinely the biggest single leak in the ceiling), door bottoms — are cheap and worth it. The bigger losses hide in the boring places: gaps around pipe penetrations under sinks, the skirting-to-floor junction in older homes, and — the classic — drafty sash windows treated with foam strips that fall out in a season instead of proper brush seals. Draught-proofing old sashes correctly is a known, cheap win (EST has long put sash draught-proofing among the best value retrofit moves); doing it with the wrong kit twice is more expensive than doing it right once. And if a room feels cold despite the work, check the heating itself — some heaters work against the room, and an unserviced boiler works against the bill.

Mistake four: over-sealing a home that needs to breathe

Homes — older UK ones especially — were built to leak deliberately; moisture from living leaves the way draughts enter. Seal every gap without providing ventilation and the moisture stays: condensation on windows, mould behind furniture, the whole ventilation-guide problem. The rule of thumb professionals use: draught-proof the planned openings — doors, windows, hatches, letterbox — and never the designed ventilation — vents, air bricks, trickle vents, flues. If whole-house sealing is the ambition (insulation, membranes, mechanical ventilation), that's a proper retrofit project with professional design, not a tube of sealant.

Sources: Energy Saving Trust draught-proofing guidance (savings estimates vary by property and publication year); Gas Safe ventilation requirements for fuel-burning appliances. Reviewed September 2026 · general information, not professional advice.