Choosing the right type of mechanical ventilation for the home and the works
Simple-flow, hygro-controlled or heat-recovery: when to choose what
An autoregulated simple-flow ventilation system is often enough as a replacement, when you keep the existing ductwork and the budget is tight. Hygro-controlled ventilation is a good compromise in renovation: it adjusts airflow rates to humidity and limits heat losses. Heat-recovery (double-flow) ventilation makes sense during a full renovation, with reinforced insulation, new ductwork and space for a unit and ducts.
Identifying site constraints: attics, false ceilings, occupied renovation
Before deciding, look at where the ducts will run. Accessible attics and false ceilings make for a clean installation, especially with heat-recovery systems. In an occupied renovation, favour a solution with few penetrations and short interventions, for example a hygro-controlled system on the existing ductwork. Also think about noise, access for maintenance, and the exhaust outlet on the roof or facade.
Checking compatibility with airtightness and insulation
The more airtight and insulated your home, the more the ventilation needs to be well controlled. A hygro-controlled or heat-recovery ventilation system helps avoid overly dry air and heat losses. Take care with air inlets, balancing the vents and duct airtightness, otherwise you risk condensation and mould despite good insulation. To go further on sizing extraction and air-inlet points, see air inlets and balancing the vents.
Sizing the mechanical ventilation ductwork: airflow rates, sections and lengths
Determining airflow rates room by room and matching vents
Start by setting the regulatory airflow rates and the actual needs. A mechanical ventilation system extracts mainly in the kitchen, bathroom and WC. Choose vents consistent with these airflow rates (autoregulated or hygro-controlled). Also keep suitable air inlets in living rooms to avoid a system that "draws" poorly.
Choosing the duct diameter to limit pressure losses
Diameter is sized according to the target airflow rate and length. Too small, and you accumulate pressure losses, noise and a drop in airflow. Diameters of 80, 125 or 160 mm are common, but always validate against the manufacturer's specifications and standard practice. Favour smooth ducts, and insulate in cold volumes to prevent condensation.
Anticipating lengths, bends and branch connections for a balanced network
Before installing, sketch out a simple routing. Every bend and branch connection "costs" pressure and can create imbalance. Aim for similar lengths between vents, limit tight bends, and plan for adjustments to achieve a balanced network. At the end of the job, an airflow check allows for fine-tuning.
Routing and organising the ductwork: a simple, accessible path
Limiting bends and shortening runs: the common-sense rules that change everything
For an efficient mechanical ventilation system, the ductwork must stay simple. Aim for short, direct runs, with as few bends as possible. Every change of direction slows the air and increases noise. When a turn is unavoidable, favour wide curves rather than a sharp angle. And keep the ducts taut, without crushing or "sagging" that traps dust.
Manifold, unit, branch connections: organising to avoid imbalances
Start by positioning the unit and, if you have one, the manifold as close as possible to the heart of the home. Then distribute the branch connections logically. One simple principle helps a lot: the closer the duct lengths are between vents, the easier the airflow rates are to balance. Label each line (room, diameter, direction) to limit errors during installation.
Planning maintenance access: filters, unit, hatches and inspection points
A durable installation also depends on access. Leave a passage to open the unit, remove the filters (heat-recovery systems) and check the connections. Plan a hatch near the key points, especially where ducts run through attics. A quick visual check is often the best prevention against airflow loss and clogging. To go further on this point, see our article on maintenance access.
Quality installation of mechanical ventilation ducts: airtightness, insulation and durable fixing
Ensuring airtight connections: clamps, sleeves and suitable tapes
On a mechanical ventilation system, air losses often come from the connections. Use sleeves matched to the diameter, then tighten with clamps. Finish with tape designed for ventilation, or aluminium or butyl tape according to the manufacturer's specifications. Avoid "DIY" tape that peels off over time. To go further, see our article on duct airtightness in mechanical ventilation.
Insulating ducts in cold zones to prevent condensation and losses
In unheated attics, garages or crawl spaces, switch to insulated ducting or add an insulating sleeve. The goal is twofold: limit condensation and preserve usable airflow. Take care with the continuity of insulation at the connections, and plan a slight slope towards the condensate drain when the instructions call for it.
Fixings and supports: avoiding crushing, vibration and noise
Suspend the ducts without pinching them. Use wide clamps, sound-isolating supports and regular spacing according to the instructions. Respect a good bend radius and avoid tight bends. Less strain means a duct that lasts longer and quieter ventilation.
End-of-job checks and points of attention in 2026
Measuring and adjusting airflow rates: simple methods and common mistakes
At the end of installation, measure the airflow rates at the vents with a flow cone or an anemometer, then adjust to get measured airflow rates consistent room by room. Also check the airtightness of the branch connections and joints. Classic mistakes: inverted vents, adjustment left at maximum speed, flexible ductwork crushed.
Condensate handling and drainage: avoiding water backflow
On a heat-recovery ventilation system, check for a continuous slope towards the drain, the presence of a trap, and the absence of any counter-slope. A pinched pipe or a leaky connection is enough to cause water backflow and odours. Quick test: pour a glass of water into the tray and watch how it drains.
Documents to hand over to the client: network diagram, maintenance and useful references
Hand over a diagram of the ductwork and the location of the adjustment components, hatches and filter access. Add a maintenance guide, with the cleaning frequency for the vents and the filter replacement schedule. A signed record of the airflow rates secures the handover and follow-up.
Key figures
4 m/s
Max air speed
125 mm
Kitchen diameter
6 m per vent
Max duct length
Frequently asked questions
Refer to the amended decree of 24 March 1982: in housing, the kitchen is generally set at 75 m³/h at peak (45 m³/h at base rate depending on configuration), the bathroom around 30 m³/h, and the WC 15 to 30 m³/h. With humidity-controlled (hygro) systems, the modules adjust, but you must still guarantee the minimums and consistency between air inlets and extraction points. When in doubt, start from the regulatory table and check the adjustment ranges of the vents.
Louis Meneteau
CPO of Argile
