I did it and it works. You can use the rescue blankets from first aid boxes. Be sure to have the sliver part face outside. It makes a real difference, but if the heatwave takes long enough, the walls will heat up and the tinfoil can do nothing against that. It still keeps the sun out, which would make things even worse. Windows wide open at night help. Icepacks from the refrigerator wrapped in a towel around your neck cool without increasing humidity. Definitely avoid anything that increases humidity inside like wet clothing, wet towels etc. Frequent cold showers, but short or humidity will go up and the body will burn even more energy to get warm again if it cools down too much. Turn off any boiler.