Assisted natural ventilation: stack draught through a duct (2026 site guide)

Understanding the principle of assisted natural ventilation through a duct

Difference between natural ventilation, mechanical ventilation and stack draught through a duct

Natural ventilation relies on wind and air leaks. It varies a lot depending on the weather and the airtightness of the home. Mechanical ventilation (like an MVHR/extract system) uses a fan to extract or supply a stable flow rate, but it needs electricity, ducts and maintenance. Stack draught through a duct relies on a vertical duct that helps stale air escape without a motor, provided air inlets are in place.

How the duct creates the draught: pressure, height and temperature difference

The principle is that of the chimney effect. Warmer indoor air is lighter. It rises in the duct and creates a negative pressure that draws in fresh air through the vents. The taller the duct, the greater the pressure difference. The draught is also strengthened when the indoor-outdoor temperature gap is significant, and sometimes by wind at roof level.

Which homes this ventilation suits in 2026 (renovation, multi-unit building, house)

In 2026, it becomes worthwhile after insulation work and window replacement, when the house becomes airtight and humidity rises. In renovation, it is a good compromise when running ducts is difficult. In multi-unit buildings, it can be considered using existing ducts, with a draught study and a check of back-draught risks. It remains limited for very humid rooms if the draught is weak.

Choosing and sizing the duct for effective ventilation

Height, diameter, routing: the criteria that really make a difference to draught

For a ventilation system that extracts well, the duct must limit losses. The longer it is, the more it slows the flow rate. Keep the routing as straight as possible, with few bends and wide radii. The diameter is chosen based on the flow rate to be evacuated. Undersized, the draught drops and noise increases. Oversized, air can stagnate.

Materials and installation: masonry duct, metal duct, ducting and airtightness

In renovation, favour rigid metal ducts, more consistent and easier to make airtight than flexible ducting. Insulated ducting limits condensation in cold lofts. Treat every joint with an air-rated tape, not improvised tape.

Key locations: air inlets, extract points, roof outlets and recirculation risk

Place extract points in wet rooms and air inlets in living rooms. The roof outlet must stay above turbulence zones, away from openings and other outlets (cooker hood, boiler). Otherwise, extracted air can flow back into the home — a guaranteed boomerang effect. To go further on sizing, see MVHR ducting network: sizing and installation quality.

Carrying out the installation on site without bad surprises

Preparing the substrate and securing penetrations (lofts, floors, roof)

Before running the ventilation ducting, do a simple survey. Routing, access, fixing points, cold zones. In lofts, avoid crushing within the insulation. At floor and roof penetrations, treat the sleeves, seals and protections carefully. Goal: airtightness and continuity of the vapour barrier.

Limiting pressure losses: bends, lengths, narrowings and fittings

Pressure losses are won on the routing. Keep lengths short, ducts at the right diameter, and bends with wide radii. Avoid narrowings and unnecessary fittings. Prefer smooth joints, and plan accessible adjustments for balancing. Less resistance means a more stable flow rate and less noise.

Managing on-site constraints: humidity, odours, noise, back-draught and wind

In humid areas, insulate ducting in unheated spaces and check slopes to limit condensation. For odours and back-draught, secure outlets with a suitable non-return damper. For acoustics, install on anti-vibration mounts and limit transmission through partitions. On the roof, choose a terminal that withstands wind and keep the outlet away from openings.

Checking performance and air quality after the works

Simple tests and useful measurements: flow rate, smoke test, anemometer and check points

After the ventilation is installed, check that air flows from dry rooms to humid ones. A smoke test or a sheet of paper near an extract point shows the direction of suction. To quantify it, measure the air flow rate with an anemometer at each extract point, then compare against the manufacturer's expected flow rates. Also check: unblocked air inlets, undercut doors, no leaks on the ducting.

Adjustments and balancing: air inlets, extract points, dampers and consistency across wet rooms

Adjustments are made calmly, room by room. Adjust air inlets and extract points to avoid an over-extracted kitchen and overly quiet bedrooms. Work with dampers and extract point settings, then redo the measurements to validate the balancing. Wet rooms must remain a priority, without creating excessive negative pressure that draws back odours or air from the garage.

Maintenance and monitoring: duct cleaning, annual checks and warning signs

A good result lasts over time. Regularly clean extract points, filters and, if accessible, the network. Plan an annual check: condition of the unit, tightness, condensate, abnormal noise. Watch for warning signs: persistent condensation, mould, odours, a sense of heavy air. To go further, you can rely on measurements to validate balancing.

Regulatory framework, safety and aid related to ventilation in 2026

Basic rules to follow: air renewal, service rooms and compatibility with heating

In a home, ventilation must ensure general and permanent air renewal. Fresh air enters through main rooms and stale air is extracted from service rooms (kitchen, bathroom, WC). On site, also check compatibility with heating, especially with a combustion appliance. A home under too much negative pressure can increase CO risk.

Fire safety and clearances: penetrations, materials, proximity to flue ducts

Networks are not installed "by feel". At wall penetrations, plan for suitable devices (sleeves, sealing) to preserve fire-resistance performance where required. Choose compliant ducts and fittings, and keep safety clearances from flue ducts. The goal is to avoid overheating, fire spread, and crushed ducting that chokes ventilation.

Aid and requirements depending on 2026 projects: RGE, energy audit and consistency with insulation

On the aid side, ventilation can be included in CEE and MaPrimeRénov' applications, depending on the pathway and the scope of works. In most cases, the company must be RGE-certified in the relevant qualification. For a major renovation, an energy audit and support may be required. And the more you insulate and seal the building envelope, the more ventilation becomes the safety valve protecting comfort and humidity levels.