One wall of our building is a curtain wall. This curtain wall contains 4 different types of glazing, each with a different visible transmittance value, all within one room. We propose using the average visible transmittance value of the four types of glazing to do daylighting calculations for areas adjacent to the curtain wall. Please advise whether this approach is acceptable.
No, the method described of taking the overall average visible transmittance value of all four types of glazing within the curtain wall and using it for all areas adjacent to the curtain wall is not an acceptable means of calculating the daylight factor of the areas in question. However, doing a weighted average of the visible transmittance value (Tvis) of all the glazing classified as daylight glazing and a weighted average of the visible transmittance values of all the glazing classified as vision glazing on a room by room basis would be acceptable. This methodology would effectively take into account the window geometry and associated minimum visible transmittance values. Alternatively, the project team may elect to perform a daylight simulation model to demonstrate credit compliance. See the LEED NC v2.1 EQc8.1 ruling dated 6/21/2004 for guidance on pursuing this approach. Applicable Internationally.