When this is the right measure
Every serious insulation or airtightness project needs a ventilation design — it rides inside every whole-house retrofit we build. It is also the answer for houses already suffering: persistent condensation on windows, mould in bathroom corners and behind wardrobes, stale bedrooms in winter. And it is essential in the classic pre-retrofit condition we found at Orchard House — a house leaky in the wrong places yet with no deliberate background ventilation at all, condensation already growing mould at the cold window reveals.
How we do it
Engineered, not guessed. Each system is calculated to Part F of the Building Regulations and CIBSE guidance, sized to the actual house. At Church Way in Iffley — a 275 m² conservation-area house — the design set a moisture generation rate of 82.5 l/s and a minimum whole-house rate of 49 l/s, met by two central extract fans (116 l/s combined) at a 60 Pa design pressure, with humidity- and presence-sensing extract units in the kitchen, utility, WC and en-suite.
Demand-controlled by humidity. The system runs continuously at a low background rate and ramps up only when sensors detect moisture or use. At St Swithun's House in Kennington we designed a whole-house Aereco demand-control system: humidity-sensitive air inlets (EMM2) that open as rooms are occupied, extract units with PIR boost (BXC2) in the wet rooms, and a central acoustic fan (V4A Premium, 210 m³/h) running continuously at low power — sized to 247 m² of floor area, targeting 0.5–1 air changes per hour.
Air paths designed through the house. Background air to habitable rooms, transfer paths and door undercuts connecting them to the extract zones. The details are deliberate: at Orchard House the background-air decision was settled before the windows went into production, so inlets were designed into the fabric rather than bored through it afterwards — and wet-room windows carry no trickle vents at all, because their air leaves through the extract terminals, under control. At Church Way, trickle vents are concealed in the window frame heads, invisible on the period elevations.
Sequenced before airtightness. Penetrations are formed before insulation and render close the walls — at St Swithun's, every new wall penetration was installed before the external insulation went on, and redundant extract fans were removed with their openings insulated and sealed airtight.
Commissioned and handed over. Every system is commissioned against its design figures and explained to the owners at handover — a ventilation system nobody understands is a ventilation system somebody switches off.
What it pairs with
Ventilation is one half of a pair with airtightness and membranes; it also shapes windows and doors (trickle vents in or out, decided before manufacture) and threads through internal and external wall insulation sequencing.
Proof
At Church Way, Iffley, the humidity-controlled continuous system was engineered to Part F and CIBSE guidance for the 275 m² house, installed with every penetration sealed airtight, and fully commissioned — inside a single coordinated retrofit contract. Read the full case study.
Questions we're asked
Why does an insulated house need a ventilation system?
Because sealing a leaky house removes the accidental draughts it relied on for fresh air. Insulating and sealing a house without planning its ventilation is how you buy a mould problem. On our projects the ventilation system is designed — and its penetrations formed — before the airtightness works, never after.
What is humidity-controlled ventilation?
A system that ventilates in proportion to how the house is actually lived in. Extract units in kitchens and bathrooms sense moisture and occupancy and ramp up automatically — when showers and cooking raise humidity — then fall back to a quiet background rate. Fresh air on demand, not draughts by accident.
How is a system sized?
By calculation, to Part F of the Building Regulations and CIBSE guidance. For a 275 m² house at Church Way in Iffley the engineered design 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 116 l/s combined capacity at a 60 Pa design pressure. Every system we design carries numbers like these — sized to the house, then commissioned against them.
Is whole-house ventilation noisy?
Designed properly, no. The continuous system specified on St Margaret's Road runs at 18 dBA or less at 3 metres — quieter than a library — while sipping power at a specific fan power of 0.09 W/l/s. You notice the absence of condensation, not the presence of the fans.