Originally Posted by Xringer
Because, the attic's thermal mass tends to hold on to that nighttime temp...
I looked at your photo, and I really don't see anything that looks very "massy".
I think that the continual heat loss through you living space ceilings would make it appear that you had a thermal mass up there...
This would be pretty easy to prove or disprove, since you now have that dandy WebControl board, you could put a string of 1-wire sensors at about every foot, from the attic peak to the insulation, and establish a baseline of thermal mass effect. Then you could shut off your heat pumps and open the windows overnight and see what your thermal mass curve looked like then.
It is a pity that the WebControl board doesn't have a built in data logger... It has everything else!