New guidelines for air cavity design of roof

The air cavity beneath the roofing must be ventilated in order to 1) remove heat transferred through the insulated roof structure possible causing snowmelt and subsequent icing at the eaves and gutters, and 2) ensure that excessive moisture is removed from the structure. Existing air cavity design guidelines for Norway is limited to roof length from eaves to ridge of 15 m and minimum roof pitch 10-15°. Roofs with larger roof spans and lower angles must be planned in detail for every building project, which is not very efficient.

A important measure to increase the ventilation of the air gap beneath the roofing is the design and dimensions of the counter- and tile battens. Increased counter batten height as well as use of round-edged tile battens was found to considerably lower the pressure loss inside the air cavity.

Assuming an insulation thickness of 350 mm, an air gap height of 160 mm was found to be sufficient for lengths up to 30 m. The calculations are applicable for pitched wooden roofs with a roof angle down to 1.4°. However, such low-pitched roofs bring along other building physical challenges related to: 1) water tightness of the roofing, 2) increased condense water dripping from the roofing, 3) evacuation of water entering the air cavity as rain, snow and condense water.

 A ventilated lean‐to roof structure with snow on the roof. The snow may reduce the air flow through the opening in the upper air cavity opening. The structure has the following build up from the cold face of the structure: roofing, ventilated air cavity, combined wind barrier and underlayer roof and an insulated load‐bearing roof structure closed by the vapour retarder and the ceiling at the interior face.

A ventilated lean‐to roof structure with snow on the roof. The snow may reduce the air flow through the opening in the upper air cavity opening. The structure has the following build up from the cold face of the structure: roofing, ventilated air cavity, combined wind barrier and underlayer roof and an insulated load‐bearing roof structure closed by the vapour retarder and the ceiling at the interior face.

Scientific basis

  • Gullbrekken, L: Climate adaptation of pitched wooden roofs. Doctoral theses at NTNU, 2018:124, Norwegian University of Science and Technology, Faculty of Engineering, Department of Civil and Environmental Engineering. ISBN: 978-82-326-3038-7