This is not a caution against insulating. It is the second half of insulating properly.
The physics in one paragraph
A family produces litres of water vapour every day. In a leaky house, that moisture escapes through the same random gaps that make the house cold. Seal those gaps — new windows, airtightness tapes, continuous plaster — and indoor humidity rises until the vapour condenses on the coolest surfaces it can find: window reveals, cold corners, the wall behind the wardrobe. Condensation feeds mould; mould damages fabric and health. The answer is never to leave a house leaky. It is to give moisture a planned exit and fresh air a planned entrance.
How our whole-house systems work
The systems we design are humidity-controlled continuous extract ventilation — and every element below comes from our real project record, not a catalogue.
Extract where moisture is made. Terminals sit in the wet rooms — kitchen, bathrooms, utility. At Orchard House near Banbury that meant the kitchen, the ground-floor shower room, both first-floor bathrooms and the utility area, all served by a whole-house humidity-controlled extract system.
Continuous, but demand-controlled. A quiet central fan runs constantly at a low background rate; humidity and presence sensors in the extract units ramp it up when showers, cooking or occupancy raise moisture, and let it fall back afterwards. On a 1920s conservation-area house on Church Way in Iffley, the engineered design for the 275 m² house set a moisture generation rate of 82.5 l/s and a minimum whole-house rate of 49 l/s, delivered by two central extract fans with humidity- and presence-sensing units in the wet rooms — designed to Part F of the Building Regulations and CIBSE guidance. For a large Kennington house we designed a similar demand-controlled system around 247 m² of floor area, with humidity-sensitive air inlets that open as rooms are occupied.
Background air to the habitable rooms. Extract is only half the loop: fresh air must arrive in the living rooms and bedrooms, then travel to the wet rooms to be exhausted. Planned inlets, transfer paths and door undercuts connect the two — a detail as mundane and as essential as any in the ventilation drawings.
Quiet enough to forget. Specified properly, these systems sip power and barely whisper — one of our specifications sets the extract system at 18 dBA or less at three metres, quieter than a library. A ventilation system that annoys people gets switched off, and a switched-off system protects nothing.
The trickle-vent decision
Background ventilators — trickle vents — are where whole-house thinking shows most clearly, because the right answer differs room by room and project by project:
- At Orchard House, the wet-room windows carry no trickle vents at all. Their air leaves through the extract terminals, under control — a deliberate design decision, settled before the windows went into production, so background air was designed into the fabric rather than bored through finished frames afterwards.
- At Church Way, the new traditional box-sash windows have concealed vents hidden in the frame heads — background ventilation provided without visible plastic vents on a conservation-area elevation.
- On Howard Street in East Oxford, humidity-sensitive trickle vents were designed into the new window frames — inlets that themselves respond to indoor moisture.
Three projects, three different answers, one principle: the trickle-vent decision belongs to the ventilation design, not to the window supplier’s default tick-box.
Sequencing: ventilation is designed before the house is sealed
On every one of our whole-house programmes, ventilation is planned with the fabric, not after it:
- On Howard Street, the controlled ventilation system went in as a precondition of the fabric works — before the house was made airtight.
- On the Kennington project, redundant extract fans were removed and their openings insulated and sealed airtight, and every new wall penetration was installed before the external insulation went on — no holes cut through finished render.
- At Orchard House, the background-air strategy was fixed before the window order was placed.
Retrofitting ventilation into an already-sealed house means drilling through finished, airtight, insulated fabric — every hole a repair. Designing it first means the system disappears into the building.
Commissioning: the step that makes it real
A ventilation system is a set of measured airflows, not a box on the wall. Commissioning — measuring and balancing the actual flows room by room against the design, then demonstrating the system to the people who live with it — is part of the installation, not an optional extra. At Orchard House the system was commissioned and explained to the owners at handover; the Church Way design specified full commissioning, with every new penetration through the fabric sealed airtight. If nobody measured the flows, nobody knows the system works.
The bottom line
Ventilation is not the tax you pay on insulation — it is what turns an insulated, airtight house into a healthy one. Warm surfaces from the insulation, controlled humidity from the ventilation: that pairing is why a properly retrofitted old house has better air than the draughty version ever did, at a fraction of the heat loss. Plan them together, always.
This article is part of our whole-house retrofit guide. Related reading: airtightness in old houses and the full Orchard House project.