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EN 16798-7:2017 considers that windows on roofs that have a pitch below 60° are not included on the windward side whatever their orientation. It means that roof windows are accounted for, but only on the leeward side when using the existing standard for calculation of air flows, EN 16798-7.
Therefore, in the specific case of a room only equipped with roof windows (e.g. an attic) and aeraulically independent from the rest of the building, whatever the orientation of the roof windows, only the simplified “singlesided” calculation method of EN 16798-7:2017 is applicable.
However, literature (Liddament 1996) shows that roof windows can have in some cases a positive wind pressure coefficient (Cp), when the wind is attacking straight onto the windows.
This purpose of this note is to:
- develop a more precise calculation method adapted to roof windows to take into account cross-ventilation that may occur through them (even when there are no facade windows in the zone)
- compare results obtained using this “adapted” method and the existing single-sided method from EN 16798-7.
This study has shown that, for a building with low buildings surrounding it, the simplified single-sided method from EN 16798-7 was indeed underestimating the airflow rate by up to 77%.
More information on the calculation method proposed in EN 16798-7:2017, including the validation of simplified formula with a pressure code, can be found in (Leprince, Valerie; Carrie, Francois-Remi, 2016) and (Larsen et al. 2018).