Trickle Vents and Room-Side Condensation: What To Tell Customers

A classic call, two weeks after new windows go in: “the old ones never steamed up.” True — because the old ones leaked. Understanding this saves awkward conversations.

Why better windows can mean more mist

Room-side condensation needs two ingredients: humid air and a cold surface. Draughty frames ventilated the house accidentally, carrying moisture away. Airtight new glazing keeps that moisture indoors, so it finds the coldest surface available — usually the glass edge on a winter morning.

The role of trickle vents

Trickle vents restore controlled background ventilation without the draughts. Since the 2022 Part F changes, replacement windows in most habitable rooms need equivalent or better ventilation than what they replace — in practice, that usually means fitting them. Advise customers to leave them open in winter; closing them to “keep heat in” traps the moisture that then fogs the glass.

Glass choices that help

• Warm edge spacers lift edge-of-glass temperature — the first place mist forms
• Low-E coatings keep the inner pane warmer across its whole face
• Argon fill adds a further margin on cold nights

The habits that finish the job

Lids on pans, bathroom extractor run-on, dry washing outdoors or in a vented space, furniture pulled a little off cold external walls. Glazing sets the surface temperature; the household sets the humidity. Both sides of the equation matter — and a unit specced with the three upgrades above (compare them in the U-value calculator) gives the household the most forgiving starting point.