This is pretty consistent with the results I got when I did a spray on barrier in my last house. The temperature in the attic stayed pretty flat until mid morning, peaked mid afternoon, then fell quickly as the sun set, and was at/near ambient before midnight. The peak temp was about 20-25º lower than before the spray on barrier.
In the same 100º weather and sun conditions before spraying, the attic temp climbed quickly shortly after sunrise, stayed in the 150º range from around 1pm until the sun actually started to set, then cooled slowly. It would still be rather warm in the attic at midnight.
I actually sprayed the barrier on in the middle of July a few years ago when we had a long string of virtually identical cloudless 100º days, and data logged the attic temperature for 2 weeks before and after spraying.
Unfortunately the logs died with my computers hard drive not long after recording.
No changes to ventilation were made when the spray on barrier was put in.
I have all these plans for recording with multiple sensor location and types on my current house, but so far I've been way to lazy to work on it, lol.
Every time I get up in my attic to do much of anything, my spirit is quickly broken by the fact that this house is built with an annoying truss system, and has a lowish pitch roof.