[Bldg-sim] open window shading

[Chris Yates via Bldg-sim]

Dear learned friends

This is a query with respect to natural ventilation assessment.

Depending on the model/ implementation of opening windows an opening may,
or may not, continue to shade its zone. However, this may not capture the
'real world'. 'Real world' examples:

   1. a sash window will present a percentage 'unshaded' in proportion to
   its opening.
   2. A side hung casement opening will also present a percentage unshaded
   but the relationship will be a bit more complicated.
   3. A top hung awning style opening will effectively continue to shade,
   even when fully open.

A brief check of IES (correct me if I'm wrong) indicates that openings
behave as per 'style 3'. That is, they remain fully present as a shading
element even as their opening area increases.

Taking 'style 1', a sash window should have it's shading degraded in
proportion to its opening. So, a window SC of 0.4 should be modelled as 0.8
when the window is fully open at 50% free area. SHGC or g-value is slightly
more complex than SC due to retransmitted portions and depends on how the
glazing model is implemented at run time.

   - Is this a valid observation that could have significant effects?
   - What different models are out there in the main stream systems? Are
   there any models that capture the effect?
   - If the observation is valid, what parameter could be implemented to
   describe this effect (e.g. preserve-shading-open-window, boolean, fraction,
   ...)?

Best regards

Chris Yates
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