The last few days here in Silicon Valley we have had temperatures in the upper 90'sF. Today, it cooled off substantially, but the heat gave me an opportunity to see how our newly re-insulated house performs in hot weather.
What I did was open all the windows up at night and let the house cool down. After the first night, the downstairs cooled down to around 70F, the upstairs to around 72F. After the second night, the downstairs cooled down to around 72F and the upstairs to around 74F. It seems that the thermal mass of the house didn't shed much heat during the night without some moving air (like with exhaust fans) to encourage it.
During the day, I closed all the windows with the exception of the skylight window. The house was not uncomfortable. The upstairs was at 82F the first day and 84F the second. I can say that this performance is much better than we would have seen without having done the reinsulation. The upstairs would have been into the upper 80's.
Today, I tried closing all the windows and running the HRV all day, but it did not help much. The inside of the house was in the 80's, up to 81 upstairs, though the outside temperatures were way down, in the mid to lower 70's. I wonder if that had to do with the fact that the ducting for the HRV's runs through uninsulated attic, which could be expected to heat up substantially?
I hope to find out in the fall, when I am planning to install some wireless sensors to see. If it turns out to be so, I'll look into heavily insulating the HRV ducting.