It seems backwards. A brand new house, built to current insulation standards, still gets streaming windows every morning. Here's why, and what actually fixes it.
It's one of the more counterintuitive calls we get. Someone's moved into a new-build on one of the newer Ashford estates, expecting a warm, dry, energy-efficient home, and instead they've got condensation running down the bedroom window every morning and a musty smell developing in the wardrobe. Surely a new house shouldn't have this problem at all?
The answer is that modern building regulations solved one problem and quietly created another. New homes are built to be extremely airtight, that's what gets them their energy performance certificate rating. Good insulation, sealed joints, high-performance double or triple glazing. All of that keeps heat in. It also keeps moisture in.
A typical family of four produces somewhere between 10 and 15 litres of water vapour a day just from ordinary living. Showering, cooking, boiling the kettle, drying washing on radiators, even breathing overnight in a closed bedroom. In an older, draughtier house, a lot of that moisture used to leak out through gaps around windows, floorboards and chimneys without anyone noticing. In a modern airtight house, it has nowhere to go unless it's actively removed.
Most new-build windows come with trickle vents built into the frame, a small slot designed to let a bit of background air exchange happen. In theory that's the ventilation strategy. In practice, a lot of homeowners close them because they let in cold air or noise, and even fully open, they were never sized to cope with a full day of cooking, showering and a house full of drying laundry. Building control sign-off checks that the vents exist and meet Part F, it doesn't check that anyone will actually leave them open or that they're enough for how a family really lives.
Bedrooms are usually the coldest room overnight, with the door shut and two people breathing out warm, moist air for eight hours. That warm air hits the coldest surface in the room, almost always the window glass or the reveal around the frame, and the moisture drops out as condensation. By morning there can be a genuine puddle on the sill. Do that every night for a few months and you get black mould starting in the corners and around the frame, exactly where the airflow is worst.
The fix isn't more trickle vents or leaving a window cracked open all winter, undoing the insulation you paid for. It's mechanical ventilation that removes the right amount of moist air continuously and, ideally, recovers the heat from it before it leaves. That's exactly what a whole-house system does: a central unit runs quietly in the background, pulling humid air out and either replacing it with filtered fresh air or actively dehumidifying it, without you needing to leave a window open or remember to switch anything on.
New-build condensation isn't a fault with the house, it's a mismatch between how airtight modern homes are and how much moisture ordinary daily life produces. Trickle vents alone rarely close that gap. A whole-house dehumidification or ventilation system does the job the vents were supposed to do, properly and automatically.