It is true that air temps alone do not tell the whole picture, but my coldest winter days also tend to be clear skies and also have lots of direct sunlight. My

It is true that air temps alone do not tell the whole picture, but my coldest winter days also tend to be clear skies and also have lots of direct sunlight. My summer days are also more overcast than one might expect. It isn't my profession, but I've spent a fair amount of time tweaking building envelope and conditions in HVAC calculations to make decisions in how I build. A delta of 30 vs 100 is not the only factor, but I'd bet in my case it is the largest factor. I still agree with your point of bringin up other factors. Even changing the direction a building faces can change the load on it.