Experimental investigations about the air flow in the ventilation layer of low pitched roofs

Roof constructions with an inclination of >20 degrees and a ventilation layer beneath the roof tiles are common for housing in Central Europe. This kind of construction is more or less robust against diffusion based moisture damages due to the low diffusion resistance at the exterior cavity side and the outer ventilation layer. However, the air flow in the ventilation layer of low pitched roofs (inclination <= 7 degrees e.g. for industrial buildings) might be too low for sufficient moisture transport, which can lead to moisture damages. To investigate the air flow in the ventilation layer of low pitched roofs measurements at a full scale test stand were conducted. The tracer gas method as well as a thermo-anemometer were used for the measurements. The length and the inclination of the ventilation layer as well as the height of the ventilation gap were varied. Moreover the influence of the air inlet and outlet design and the temperature difference between roof surface and surrounding air were investigated. In this paper, we present the experimental method and compare the air flow velocities of the different roof variations and the different measuring methods. It can be shown by the measurements that the tracer gas method is suitable for measuring the air flow velocity in the ventilation layer. It is also pointed out that the larger the temperature difference between ventilation layer and surrounding air the higher the air flow velocity in the ventilation layer. The measurements clarify that increasing the slope from 3 degrees to 5 degrees does not necessarily result in a measurable higher air flow in the ventilation layer. The effect of increasing the slope depends on the in-/outlet design and the height of the ventilation layer. It can also be shown that the influence of the in-/outlet design is more pronounced by increasing the slope and height of the ventilation layer. Furthermore the measuring results indicate that increasing the height of the ventilation layer can lead to decreased air flow velocity.