But , now that we are in the middle of a heatwave , reverse those arrows and you will see how the heat is getting in . Yes , the same way it gets out.
Don't forget the insulation works both ways. If you make your house cool , it won't stay that way if the heat outside can get in and warm up the air you prepared for your comfort .
Think of your house like that cooler you bring on picnics . Why does a cooler keep your goodies cool ?...because it's insulated .
It can be an expensive proposition to insulate a house but it can be done a little at a time. Start with that space under the roof . Add an extra layer there with ventilation to suck out the heat/moisture . You will appreciate the comfort as well as a reduction in the cost of cooling during hot weather and heating during the cold months.
That cooler you use on picnics in summer can be used to port hot foods in winter...because it's insulated .