Ventilation: fresh air without throwing away the heat

Ventilation is the awkward compromise at the centre of a well-insulated home. You need fresh air for health, and every litre of warm air you throw out is heat you paid for. Get the balance wrong in one direction and you have condensation, mould and stuffy bedrooms. Get it wrong the other way and you have a draughty house and high bills.

The good news is that the compromise is largely solvable, and the solutions range from free to expensive with useful options at every point.

Why modern houses need deliberate ventilation

Older houses ventilated themselves, through gaps, chimneys, suspended floors and ill-fitting windows. That was wasteful and it meant air changed constantly whether you wanted it to or not.

Sealing a house — new windows, insulation, draughtproofing, a blocked chimney — removes that accidental ventilation without necessarily replacing it. The moisture a household produces then has nowhere to go, which is why condensation and mould problems frequently appear after energy efficiency work rather than before. This is a well-documented pattern and it catches out a great many retrofits.

So in a reasonably airtight house, ventilation has to be intentional. The question is only how much heat you lose doing it.

The options, cheapest first

Purge ventilation: opening windows

Free, and more effective than people assume when done properly. Opening windows on opposite sides of the house wide for five to ten minutes exchanges the air almost completely while barely cooling the walls, furniture and floors, which hold most of the stored heat. Leaving one window ajar for hours loses far more heat for less air exchange.

So the technique is short, wide, and on both sides, two or three times a day. It is the standard practice across much of continental Europe and it is genuinely good advice.

Background ventilation: trickle vents

Small controllable openings in window frames providing continuous low-rate air exchange. Unfashionable, frequently taped over, and doing an important job — they are the reason a sealed house does not slowly accumulate moisture. Blocking them is the direct cause of a good number of mould complaints.

Extract ventilation: bathroom and kitchen fans

Removing moisture at the point of production is far more efficient than diluting it through the house. Two things make the difference between a fan that works and one that does not: it must vent outside, not into a loft, and it must run long enough — most are switched off far too early, while the room is still humid.

Humidity-triggered running, rather than a fixed timer, is the single best improvement available to a bathroom fan, and it is cheap.

Continuous mechanical extract

A single quiet fan running continuously at a low rate, extracting from wet rooms with air entering through trickle vents. Effective, inexpensive to install, and it loses the heat in the extracted air.

Heat recovery ventilation

The proper answer, covered next.

Mechanical ventilation with heat recovery, explained

MVHR is a system that continuously extracts stale air from wet rooms and supplies fresh air to living rooms and bedrooms, passing both streams through a heat exchanger so the outgoing air warms the incoming air without the two mixing. Good units recover a high proportion of the heat — often quoted above 85 per cent in laboratory conditions, less in practice.

What it gives you: continuous fresh air with modest heat loss, filtered incoming air which helps in a polluted or high-pollen area, no need to open windows in winter, and consistently low humidity and carbon dioxide.

What it demands, and this is the part often glossed over:

An airtight house. In a leaky building the system is largely pointless, because air is arriving and leaving through the gaps regardless. MVHR belongs with a genuinely well-sealed envelope.

Ductwork. Rigid, correctly sized, insulated where it passes through cold spaces, and short-run where possible. Poor duct design ruins more MVHR installations than poor units do — long flexible runs and crushed bends destroy performance and create noise.

Commissioning. The flow rates must be measured and balanced room by room after installation. An uncommissioned system is a guess, and a supply-heavy or extract-heavy imbalance causes problems.

Filter maintenance. Every few months, forever. A neglected filter reduces flow, increases noise and raises running cost, and it is the commonest reason owners end up dissatisfied.

A summer bypass, which lets the system stop recovering heat when you do not want it — essential unless you want warm air recovered into the house in August.

Retrofitting into an existing house is possible and disruptive, and the honest advice is that it suits a deep retrofit or a new build far better than a piecemeal upgrade. In an ordinary older house, continuous extract plus good humidity-triggered fans achieves most of the health benefit for a fraction of the cost and mess.

What to measure

Carbon dioxide is the best single indicator of whether ventilation is keeping up with the people in a room. Under 800 parts per million is comfortable; consistently above 1,200 says the ventilation is being outpaced. Bedrooms overnight are the revealing measurement.

Relative humidity per room, since sustained readings above 65 per cent predict mould.

Indoor and outdoor temperature and humidity together, to compute absolute humidity and decide whether ventilating will dry or dampen the house. Cold outdoor air at high relative humidity holds much less water than warm indoor air at moderate relative humidity, so opening up on a cold damp day genuinely dries a house. Automations built on relative humidity get this backwards.

Fan run time, which tells you whether the bathroom fan is actually running long enough.

Automations that make ventilation work

Humidity-differential extraction. Run the bathroom fan while that room is significantly more humid than the house baseline, and stop when it returns. A relative comparison works year-round where a fixed threshold does not.

CO₂-triggered ventilation in bedrooms and home offices, boosting a mechanical system or prompting a window.

Absolute-humidity purge prompts, telling you when outdoor air is genuinely drier.

Pause heat recovery in summer, or open the bypass, when outdoor air is cooler than indoor.

Boost on cooking, using a hob power reading or a temperature rise, so extraction starts before the room fills rather than after.

Filter reminders, based on run hours rather than a calendar.

Interlock with the alarm: stop mechanical ventilation on a confirmed smoke alarm, so the system is not distributing smoke through the house.

What we would do, by house type

1. Older, leaky house: unblock the trickle vents, fit humidity-triggered extract fans that vent outside and run long enough, purge twice a day, and stop drying laundry indoors. Cheap and usually sufficient.
2. Recently insulated or new-window house with condensation: the same, plus measure CO₂ and humidity per room, because this is the pattern where problems appear.
3. Well-sealed modern house: continuous mechanical extract as a minimum, MVHR if the ductwork can be done properly.
4. Deep retrofit or new build: MVHR, designed with the house rather than added to it, properly commissioned, with a summer bypass and a filter routine.

Whichever applies, measure before and after. Ventilation is invisible, and the only way to know it is working is a carbon dioxide reading that stops climbing and a humidity reading that comes back down.