Why are the interiors of some houses hottest late at night long after the outside temperature has decreased

"It is probably simply the hot air rising over the day and you notice
it when you go upstairs to bed (presuming the bedrooms are
upstairs).

If it is then try opening some of the windows on the ground floor
and top floor slightly during the day. Opening them wide is not
necessary. This will produce a gentle current of air which will
limit temperature rises. In particular try opening the windows on
the shaded side of the ground floor and sunny side of the top floor.
Obviously leave the internal doors open. This may be difficult if
the house is unoccupied during the day, but opening them this way in
the evening may help and you can experiment at the weekends.

Insulation keeps heat in... But I guess you close the windows at night
even if they are open during the day. This will also trap any heat. Open windows a little low down on the N side and high up on the S. Draw the curtains on the south side during the day."



The summary in quotes is not my own, and I do not claim it as mine.