Condensation, Trickle Vents and Part F: A Householder's Guide

New windows make homes dramatically more airtight — that is much of their point. But the moisture a household produces (cooking, showers, breathing: litres per day) no longer leaks out through rattly frames, so without a ventilation plan it condenses on the coldest surfaces left. This is why Building Regulations Part F ties window replacement to ventilation, and why trickle vents went from optional to normal.

What Part F requires when you replace windows

Since the 2022 changes in England, replacement windows must generally provide ventilation that is no worse than what was there before — in practice: if the old windows had trickle vents, the new ones need them at least as large; if the old windows had none, vents of a prescribed minimum size are usually still required unless the home has another compliant ventilation system (such as continuous mechanical extract or MVHR). Background ventilators are sized in equivalent area (mm²), typically 8,000mm² per habitable room and 4,000mm² for kitchens and bathrooms alongside extract fans.

Trickle vents, honestly

Nobody buys slim aluminium frames dreaming of a plastic vent — so specify them properly. Modern through-frame vents are slim, colour-matched to the profile, closable, and available in acoustic versions for noisy roads. On our window systems they mount discreetly at the head; on doors and sliders, glazed-in or frame-head options keep faces clean. A closed vent still leaks its rated background air path — that is by design; ventilation is not a tap to shut permanently.

Reading condensation like a surveyor

• Inside face of glass: humidity too high or surface too cold. New thermally broken glazing with warm-edge spacers usually ends it; persistent cases point to lifestyle moisture or inadequate extract.
• Between panes: the sealed unit has failed; the glass needs replacing, not the household's habits.
• Outside face at dawn: counter-intuitively a good sign — the outer pane is cold because so little heat escapes. It clears as the day warms.
• Walls and cold corners after new windows: the moisture moved to the next-coldest surface; the answer is ventilation and heating balance, not blaming the windows.

Getting the spec right first time

Tell us room-by-room use when you enquire — kitchens, bathrooms, bedroom occupancy — and whether extract fans or MVHR exist. The quotation then pairs vent sizes and locations with the frames, keeps sightlines tidy, and gives building control what it needs. It is a five-minute conversation through the quote form that prevents the classic January phone call.