Ventilation

Preventing Interstitial Condensation

The BBA certificate for SureCav states that the cavity must be ventilated. Under the section “Interstitial Condensation”:

3.3.2 It is essential that the cavity behind the panel is drained and well-ventilated to the outside with openings equivalent to 1500 mm2·m–1. This ventilation can be provided by air bricks, air vents, open brick perpends and weepholes at cavity trays. The panels must be cut accordingly to accommodate these openings, but care must be taken to minimise the risk of rain ingress. For timber-frame walls, the openings must be below the lowest timber. Timber-frame walls must also include an air vapour control layer (AVCL) and a breather membrane. Sheathing to a timber or steel-frame must also include a breather membrane.

This equates to ventilation being provided at a min. rate of 1500mm2 per metre run and can be achieved by air brick/perp vents at low level, high level and at lintels. The Cavity Trays ‘Cavibrick’ has a free airflow 7,500mm2, so would need to be placed approx. every 5 metres. Ventilation can also be achieved by perp vents at appropriate centres based on the free airflow of the vent. We would suggest perp vents at DPC level and at high level at 500mm centres horizontally. Ensure a slot is cut in the SureCav sheet to correspond to the aperture of the vent.

Download drawings here

As an example, see here to see a perpend vent of 320mm2, with an extender to 200mm here