How well is your attic insulated? Biggest cooling problem is almost always from the huge heat load on the roof that builds up furnace-like temperatures in an attic. Engineers find most attics “inadequately insulated.”
So your first step should be a thorough attic investigation. As much as six inches of bulk insulation laid over the ceiling or the equivalent in aluminum foil insulation will usually pay for itself.
Most houses today have less than three inches of insulation, if any at all. The gable ends of the attic should also be opened up with large ventilating louvers.
Are large windows shaded from direct sun rays? If not, they should be protected by outside shading devices. Remember that ten times as much sun heat invades a house through glass as through an equal area of insulated wall.
Are the walls insulated? Although it may be more costly to insulate the finished walls of an existing house, it will still pay when measured against cooling savings. Walls should get three inches of insulation and this can usually be blown in.