Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
When EnergyPlus performs the heat balance for the outside face of a surface, it assumes that the emissivity is equal to the absorptance. Since the roof is opaque, transmittance is zero, making reflectance = (1 - absorptance).
$$ \alpha = \epsilon$$ $$ \tau = 0$$ $$ \rho = 1- \alpha$$
If you're modeling a cool roof that has high reflectance, you will want to set the absorptance input fields in EnergyPlus to low values. You can read more information in this post and this post.
