Insulation is actually helpul against heat too!

But since modern buildings usually have huge windows, we're letting in about 1kw of heat from sunlight per square meter of window. That's 6kw for a 3x2m bay window, or about the equivalent of 6 electric heaters at full power.

Block that heat ouside of your house with all kind of shade you can get: trees, blinds, adjustable shades, rolling shutters...

Also, close every openings during the day, vent at night.