The ‘Passive’ vent provides an opening on a section of wall normal to the airflow direction with an effective area well in excess of the minimum requirement.
Airflow through the opening is controlled automatically by the intervening mineral wool slab and thereby avoids undue draughts.
It is important to realise that the release of moisture vapour pressure within a dwelling to avoid condensation does not require a specific ventilation rate.
The moisture vapour pressure within a dwelling is always higher than the outside pressure therefore there will always be an escape of moisture vapour by the diffusion process through the Passive vent.
The diffusion process is slow compared with mechanical ventilation and would not cope with the copious amount of water vapour produced in a short time scale without the combined use of mechanical ventilation in kitchens and bathrooms.
Its main use is in habitable rooms where it operates continuously without noise and over a 24-hour period can extract a maximum of 2.5 litres of water/day even at low vapour pressures.
If the relative humidity of the room were 70% at 20°C then each Kg of air would hold 0.0104 Kg of water.
At a recommended maximum velocity of 0.9 m/sec (maximum velocity recommended by Local Authorities) through the Passive, it would transmit 0.0024 M3/sec (i.e. 2.42 Kg per day) of air that contains 2.5 litres of water.
Benefits and features
- Reduces draughty air flow and heat loss
- No running costs
- Works at peak condensation times
- Size of a standard 9″ x 6″ (225mm x 150mm) airbrick
- No night time disturbance from compressors or fans
- Transparent to water vapour
- No moving parts
- Works passively 24 hours a day
- Makes no noise
- Low installation cost