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How do I estimate airflow at home-ventilation pressures in order to determine the amount of venting needed?

TzJpE9tuZC | Posted in Green Building Techniques on

We are building a new house, and are weighing the relative benefits of having venting windows and operable skylights. We want assure adequate airflow, but not overdesign for it – operable windows and operable skylights are considerably more expensive than fixed ones. Our architect is telling us that *everything* should be operable, but this doesn’t make sense – airflow inlet area and outlet area should be relatively equal, if not the larger area is simply wasted.

As an adjunct question, perhaps operable vents or low-speed fans would be as effective and less expensive than operable skylights?

We live in N. CA, where the number of hot days (days where temp > 90) are quite limited – perhaps 10 in a year. It is almost invariably cool at night.

I’d be grateful for any pointers.

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  1. Riversong | | #1


    Wind-driven air flow through open windows is dependent on wind speed, wind direction, site wind screens (terrain, trees or other buildings), the opening area of each window, and the directionality of the windows (casements can scoop the wind, while double-hungs can only passively receive it).

    In other words, there is no way to quantify passive air flow. But good design requires provision for natural cross-ventilation in every livable space (bathrooms typically exempted), and cross-ventilation is most effective when operable fenestration is located on two adjacent or opposite walls in a room with a flow path that sweeps as much of the room as possible.

    A combination of windows and skylights (or low and high windows) can provide stack effect ventilation on hot days when there is no wind. But overuse or improper placement of skylights often results in glare and overheating (I try to avoid them whenever possible).

    Having approximately equal intake and exhaust openings helps natural ventilation, but it's not as critical as proper location. Since operable windows offer the most options for natural ventilation regardless of wind direction, it's usually sensible to install operable units except where excess glazing is desired for passive solar gain or views. In that case, smaller operable awning windows can be combined with large fixed units to provide both ventilation and the other desired benefits of glazing.

    Finally, casement windows offer the best natural ventilation, both because the entire unit opens (as opposed to half a double-hung) and because they can be oriented to scoop the prevailing winds or block street noises.

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