If the sun has unobstructed access to the inside of your house through lots of window space, and if you have a well-insulated house, the sun can cause overheating in the summer and sometimes even in the winter.
If the south-facing window area of your house is more than 6 per cent of the floor area (basement included), you may have overheating problems. More ventilation during the afternoon or more "thermal mass" in the sunny rooms can even out the radical temperature swings.
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