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Forum discussion

CS-2009 EAp2:Minimum Energy Performance

Calculating "Projection Factor" for curtainwall shading devices

Amit Ahiel

LEEDuser Basic Member

7 thumbs up

May 9, 2013

I'm working on a project where part of the curtain wall has a few random horizontal elements that project from the vision panels from floor to ceiling. These elements provide shading and therefor contribute to the reduction of the SHGC for the glazing. My problem is to calculate the Projection Factor (PF) per ASHRAE's instructions (pg 28 of ASHRAE 90.1 2007): Os = (Ai ⋅ Oi) + (Af ⋅ Of) where Os = percent opacity of the shading device Ai = percent of the area of the shading device that is a partially opaque infill Oi = percent opacity of the infill—for glass Oi = (100% − Ts), where Ts is the solar transmittance as determined in accordance with NFRC 300; for perforated or decorative metal panels Oi = percentage of solid material Af = percent of the area of the shading device that represents the framing members Of = percent opacity of the framing members; if solid, then 100% My English is pretty good but I don't understand this description of the equation. Does anyone have a simpler or even a graphic representation of how to calculate the "Projection Factor"?

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