Glazing values not following manufacturer values

[jchia199]

Dear all

I'm trying to create a new glass material and single glazing based on manufacturer's values as well as values from the software WINDOW 7 by LBNL. So far, even though the details for the glass material such as the:

1) thickness
2) solar transmittance, internal and external reflectance
3) visible transmittance, internal and external reflectance
4) internal and external emissivity

were keyed in, the calculated u-value and the SC value are completely off. My understanding is that the u-value and the SC value calculated by WINDOW is based on the ISO standard and a comparison of the u-value calculated by IES using EN-ISO has a difference of 2W/m^2K.

I know that changing the resistance values will allow me to meet the right u-value and SC (possibly). However, I'm trying to understand why the u-value and SC value calculated is so off. Please help me understand. Also, which environmental conditions are used in the calculations?

Regards
Joshua