Humidity-controlled vents

Humidity-controlled vents are exhaust vents (and sometimes intake vents on the facade for "B" systems) that automatically adjust the ventilation rate according to the indoor relative humidity. Widely used in humidity-controlled mechanical ventilation systems (apartment buildings, student residences, hotels), they increase the airflow when humidity rises (showering, cooking, high occupancy) and reduce it when the air is dry, ensuring good indoor air quality while saving energy.

Humidity-controlled vents: principle, types and installation

The principle is based on a humidity sensor: the most common is a polyamide strip (mechanical, without battery) which expands with humidity and gradually opens the mouth light; there are also electronic versions (sensor + micro-motor, batteries/mains) with boost functions (cord, presence), clock or adjustable thresholds.

Two humidity-controlled mechanical ventilation (VMC hygro) architectures:

1. Hygro A : self-regulating air inlets on facades (fixed flow rate) and humidity-controlled extraction vents (variable flow rate).
   
   
2. Hygro B : humidity-controlled air inlets and humidity-controlled outlets → modulation at the inlet and outlet (enhanced energy efficiency).

Installation : vents in damp rooms (bathroom, toilet, kitchen), connected via ductwork to an extractor fan in the attic/roof. Include attenuators if needed, a backflow preventer, duct slope (to prevent runoff), accessibility for maintenance, and air intakes on facades located away from sources of pollution. In public buildings/light commercial buildings, they are found in ancillary spaces (restrooms, shower rooms), sometimes coupled with occupancy sensors.

Advantages, limitations and points of attention in project/operation

Interests

• Usage-driven IAQ: high flow when needed, low otherwise → Controlled humidity, limited mold.
• Energy efficiency: reduced flow rates during dry periods → kWh of fans and heat loss through fresh air are reduced.
• Simplicity: mechanical versions without energy, quick installation and adjustments.
• Acoustics: models with attenuation and anti-whistling profiles.

Boundaries

• HR dependency: does not treat pollutants not correlated with humidity (VOCs, CO₂ in rooms/offices with low humidity).
• Fouling: grease/dust distorts the opening law (kitchens, workshops).
• Insufficient draft if undersized networks or obstructed air inlets.

Points to consider

• System choice: Hygro B is more economical but requires careful air inlets; A is simpler for renovation.
• Air balancing: check available pressures at the box, equivalent lengths and leaks; aim for a low SFP.
• Acoustics: select target NR, install silent blocks/attenuators, limit speeds in ducts.
• Maintenance: periodic dusting/degreasing of vents, checking of sensors (HR), cleaning of air inlets.
• Building sealing: ensure airflow (door undercuts), seal any unwanted air inlets.
• Climate: in very humid areas, plan for boost mode/schedules and anti-blocking of the shutter.

Anecdote — “The blackened wall in Saint-Nazaire is gone”

In Saint-Nazaire, a condominium building was experiencing condensation and mold growth in the bathrooms. The old self-regulating vents were running at full capacity… even when dry, and not enough after showers. The renovation switched to a Hygro B system: new humidity-controlled air inlets, kitchen/bathroom vents with boost function, and cleaned ductwork. Three months later: peak humidity levels lowered, blackening walls gone, odors reduced, and extraction consumption down 25% thanks to lower flow rates during dry periods. The building manager made the decision: "We ventilate when needed, not all the time."