Sealing Draughts: The Cheapest Energy Work in Any House

Insulation slows conduction through a surface. Air sealing stops warm air leaving through gaps. In an older house the second is frequently the bigger loss, and it costs a fraction as much to fix.

Find the leaks without a blower door

Pick a cold, windy day. Close all windows and external doors, turn on extractor fans to depressurise the house slightly, and walk the perimeter with a lit incense stick. Where the smoke pulls sideways, air is moving.

Where the leaks actually are

Do not seal everything

Houses need ventilation. Keep trickle vents, extractor fans, and airbricks serving suspended floors or combustion appliances clear. The goal is to eliminate uncontrolled leakage while keeping controlled ventilation intact — sealing without ventilating causes condensation and mould.

Worked example: sealing a typical three-bedroom house

A loft hatch with no draught strip can leak air roughly equivalent to leaving a small window cracked open all winter. Sealing it, adding brush seals to two exterior doors, fitting a chimney balloon in an unused fireplace, and sealing skirting gaps in three rooms typically costs less than a single radiator replacement and is usually a half-day job for one person with basic tools.

A sensible order to work through the house

1. Start in the loft: seal the hatch and any pipe or cable penetrations through the ceiling.
2. Move to unused chimneys and flues and fit a removable balloon or cap, noting it clearly so it is removed before lighting a fire.
3. Fit brush or compression seals to external doors, checking the fit with a piece of paper — if it pulls free easily, the seal is too loose.
4. Seal skirting and floorboard gaps in ground-floor rooms with flexible filler.
5. Check letterboxes, keyholes, and cat flaps for brush seals.
6. Finish with window perimeters, using compression seals rather than adhesive foam strip, which perishes within a season or two.

• Common mistake: sealing an extractor fan or trickle vent by accident, which then causes condensation and mould in the room it served.
• Common mistake: using expanding foam around a chimney or flue, which is a fire risk near heat sources.
• Common mistake: assuming new double glazing has solved air leakage when the actual gaps are around the frame, not through the glass.
• Common mistake: sealing only the rooms that feel cold and skipping the loft, which is usually the largest single source.

Materials worth having on hand

How to tell the job actually worked

The honest test is comfort near the treated areas, not a single dramatic number on a bill. A hallway that used to have a noticeable cold stripe under the front door, or a loft hatch that used to whistle in wind, should simply stop doing that. Heating bills fall too, but they are affected by weather and occupancy from one winter to the next, so treat comfort at the specific gaps as the more reliable signal that the work held.

Is it worth sealing an old sash window rather than replacing it?

Often yes. Brush and compression seals fitted into the sash channels recover a large share of the draught-proofing benefit of full replacement — enough that many conservation officers require this route rather than allowing uPVC replacement in listed or period properties. A professional draught-proofing job on a set of sash windows typically costs a small fraction of full double-glazed replacement and can be done without disturbing the original glass or joinery.

What professional air-sealing adds that DIY cannot

A blower door test depressurises the house with a calibrated fan and measures actual air changes per hour, then a smoke pencil finds leaks with far more precision than an incense stick on a windy day. This service costs more than the materials for a DIY draught-proofing weekend, but it is worth booking on an older, larger, or unusually draughty house where the incense-stick method keeps finding leaks faster than they can be sealed, or before a major retrofit where the airtightness target needs verifying rather than guessed at.