I have a project looking to achieve some of the spaces via Natural Ventilation compliance. The windows are installed with a 60% porosity window screen. I found an ASHRAE paper stating that the natural airflow coefficient through a 60% porosity window screen is a coefficient of 0.8. As the windows are adjustable to 100% with an adjustable lock, I need to designate the % of window opening and want to know if I should use the unobstructed area as the window porosity or if I can use the 0.8 (compared to 0.6), since it has been proven by ASHRAE. It seems that the 0.8 makes more sense but theres no additional information in ASHRAE 62 outside of we must take into account the insect screens.

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Roger Chang

Principal, Energy and Engineering Leader

DLR Group | Westlake Reed Leskosky

LEEDuser Expert

267 thumbs up

Sun, 11/25/2012 - 16:22

Daniel, when there are any obstructions, you'll want to use the net free area. If a screen is 60% open, then you'll want to use a factor of 0.6. I think that you may have hit upon a gray area though, because a screen has minimal depth and thus limited resistance to airflow. You may want to email the ASHRAE Standards interpretations chair to get further clarification.

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