The heating is on, yet the chair beside the window still feels chilly. Before buying a roll of foam, find out where the air is coming from. A leaking opening joint may need attention; a ventilator has a job to do. The first useful decision is knowing which opening to leave alone.

On a breezy day, close the external door or window normally and move your hand around its edges. Note where you feel moving air. Daylight through an unintended gap, a rattling sash or a whistle can help locate the problem. Check from a safe position indoors; this job does not require leaning out or climbing.

Record the opening, the affected edge, whether it closes easily and any split, loose or missing seal. Photograph the joint and the old seal so a supplier can help identify a suitable replacement. If the frame is damaged or the opening already jams, put repair ahead of adding material.

Energy Saving Trust distinguishes uncontrolled leaks from ventilation that exchanges indoor air. Leave airbricks, wall vents and extractor outlets unobstructed. Trickle vents, usually near the top of window frames, should generally stay open, including in winter. Where a mechanical system supplies and extracts air continuously, follow its instructions or ask its installer before changing the ventilation arrangements.

The gap beneath an ordinary internal door also lets air move between rooms. Do not add a draught excluder there as part of this job. If the home has no identifiable ventilation, persistent condensation or mould, get its ventilation assessed before making it more airtight.

Fuel-burning appliances need particular care. Never obstruct their ventilation or flues. Ask a suitably qualified appliance professional before sealing around a room whose combustion-air provision you do not understand. For gas appliance work, use a Gas Safe registered engineer qualified for that work. Chimneys and flues are outside this DIY exercise.

Fire doors are a separate stop point, including a flat entrance door opening onto a shared corridor. Their seals form part of a fire-protection assembly. London Fire Brigade advises residents against carrying out alterations or repairs themselves. Ask the building manager or a qualified fire-door specialist about leaks or closing problems; do not add ordinary weatherstrip, remove seals or adjust the closer. If you are unsure whether a door is a fire door, establish that first.

Measure the gap to be sealed, following the product maker’s measuring instructions, and check it at several points. A frame may not give you the same clearance all the way round. Take the measurements, the photographs and the opening type to the supplier if the right profile is unclear. Do not choose by the strip’s width alone.

Stormguard’s Sureseal 3 provides a useful example of the distinction. Its drawing shows a strip 6 mm wide and 3 mm high; the product is specified for gaps of 1–3 mm. Those are different measurements doing different jobs. This is an illustration of how to read a specification, not a recommendation for every window, and we have not tested the product.

Read the working gap as well as the dimensions. Stormguard’s Sureseal 3 drawing shows the strip’s cross-section; its stated sealing range is 1–3 mm. Diagram: Stormguard.

For a compatible self-adhesive seal, follow the specific pack instructions. The surface needs to be clean and dry, with dust, grease and loose material dealt with appropriately for its finish. Use a cleaning method the maker permits; a solvent that suits one frame may damage another. Do not disturb suspect hazardous coatings or turn a small sealing job into unplanned joinery repair.

The seal has to sit where the closing action compresses it. Stormguard’s instructions for this example specify working down the vertical frame, smoothing the strip into place and cutting it to length. Other profiles and systems may fit differently. Keep the maker’s diagram beside you, and leave drainage openings, vents and operating hardware clear. Check one completed opening before buying enough material for the whole house.

Open and close the door or window gently. It should still move freely and latch normally; you should not have to slam it or force the handle. If the new strip fouls the movement, stop and correct the fit using the product instructions or ask the supplier for help. Do not solve a tight fit by altering the lock or trimming the door.

Return to the recorded leak on another breezy occasion. Has moving air at that joint reduced? This is a simple household check, not a measurement of airtightness or a prediction of bill savings. Keep the note so that a remaining problem can be described precisely.

Over the following days, pay attention to recurring window condensation, persistent smells or mould. These are reasons to investigate moisture and ventilation, rather than carry on sealing more openings. The absence of visible condensation does not prove that air quality is good. Continue using the home’s ventilation as intended, including extraction when cooking and bathing.

Persistent condensation is a reason to check moisture and ventilation. This illustrative window photograph does not diagnose a particular home. Source: Energy Saving Trust; individual photographer unconfirmed.

In an older house, original windows deserve an assessment of their condition before being written off. Historic England identifies draught-proofing and well-designed secondary glazing as ways of improving comfort while retaining existing windows. A joiner experienced in traditional windows can distinguish a fitting problem from decay or movement. Secondary glazing needs its own design and ventilation assessment; it is not the automatic next item in the basket.

In England, Historic England says simple self-adhesive draught strips do not need listed building consent, while invasive methods such as grooving may. Check proposals affecting historic fabric with the relevant local authority; heritage controls differ across the UK. Leave structural cracks, uncertain service penetrations and appliance-related gaps for appropriate professional assessment.

The window handle should be as easy to turn as it was before you began. If warmth requires a struggle to shut the window, the job needs another look.