Hi Kenneth,
When you stand in front of the facade on a bright day, do you see a mirror image of yourself in the windows? If so, you may assume that all 20% of the reflectance is specular. Diffuse reflection from glass is unusual and would indicate that the glass is frosted, sand blasted, textured, or very dirty.
For data on typical glazing materials, you can check the International Glazing Database for typical glass materials or the Complex Glazing Database for frosted and non-glass materials.