Your Home Is Sealed Tight — So Where Does the Fresh Air Actually Come From?

There's a strange irony in modern British housing. We've spent thirty years making homes harder for air to escape, then wondered why so many of them feel stuffy.

It isn't a fault in the building so much as a gap in the plan. Insulation, airtight membranes, triple glazing and draught-proofing are all doing exactly what they were designed to do: holding warmth in. But the draughts they eliminated were also, inadvertently, the ventilation system. In a Victorian terrace, air leaks constantly through sash windows, floorboards and chimneys. In a house built to current standards, it barely leaks at all — which means every breath, every shower and every pan of pasta adds moisture to a sealed box.

What airtightness does to the air inside

A human household produces a surprising amount of water vapour. Four people breathing overnight, a shower, a round of laundry, a kettle and a hob add up to several litres of moisture a day. Add cooking smells, cleaning products, and the CO2 that builds up in a closed bedroom, and you have a house that needs a way to exchange air — deliberately, and continuously.

Without that, the symptoms are predictable. Windows stream on cold mornings. Grout goes black in the shower corner. Wardrobes against external walls start to smell. Sleepers wake with headaches in rooms where CO2 has quietly climbed overnight. None of it is dramatic, and all of it is fixable.

What trickle vents can and can't do

Building regulations require background ventilation in new homes, and most of it comes from trickle vents — those slotted openings in window frames. Used properly, they allow a small, controlled trickle of air in and out.

Used realistically, they often aren't. They get closed in winter because they feel draughty. They get sealed by painters, blocked by blinds or forgotten entirely. Even when open, they provide a fixed amount of ventilation regardless of whether anyone's home, how many people are in the room or whether the shower's running. On a windy day they over-ventilate and waste heat; on a still, mild evening with six people in the lounge, they do very little.

That's the gap: fixed ventilation in a home with variable needs.

The options, roughly in order of effort

Better background ventilation. The cheapest improvement is using what you already have, properly. Keep trickle vents open, clear the grilles, and use purge ventilation — a short burst of wide-open windows on opposite sides of the house — when moisture spikes. Ten minutes is usually enough to exchange the air without chilling the walls.

Demand-controlled extract. Mechanical extract ventilation (MEV) uses a central fan to pull air out of wet rooms continuously, with fresh air entering through background vents. Demand-controlled versions ramp up automatically when humidity rises in a bathroom or kitchen. It's a modest step up in cost from individual extractor fans, reasonably straightforward to retrofit in a loft or cupboard, and it solves the "nobody remembers to switch the fan on" problem.

MVHR — mechanical ventilation with heat recovery. This is the full solution: a unit that extracts stale, moist air while drawing fresh air in through a heat exchanger, recovering most of the warmth from the outgoing air. Homes with MVHR feel consistently fresh, glazing stays clear, and heat loss from ventilation drops sharply. The catch is that it needs ductwork, so it's far easier to install during a build or a full renovation than afterwards. Retrofit is possible, particularly in lofts and bungalows, but it's a project rather than an afternoon's work.

The 2021 update to Part F reflected this direction of travel, tightening standards and pushing new homes towards more systematic ventilation. If you're buying a recent build, it's worth asking directly what ventilation system is installed and where the filters and controls are.

The honest version on cost and bills

Ventilation removes warm air, and in winter that warm air was paid for. It's a fair objection. But the comparison isn't between ventilating and keeping all your heat — it's between ventilating deliberately and ventilating accidentally through leaks, which is what older homes do anyway, just less efficiently.

MVHR recovers the majority of the heat that would otherwise leave, so the running cost is largely the fan's electricity plus filter replacements. Demand-controlled extract costs little to run and only works hard when it's needed. Against that, a single forgotten dehumidifier running through winter, or repainting a mouldy wall every few years, tends to eat the savings quickly.

Before you commit to anything

Two things are worth doing first. Buy a hygrometer and watch the readings for a fortnight — you may find the problem is one bathroom fan, not the whole house. And if you're renovating, talk to your builder about ventilation at the design stage, not after the plasterboard goes up.

Airtight homes aren't a mistake. They're just homes that need a plan for the air inside them, rather than assuming a draught will handle it.