Cold in winter and heat in summer get in through any openings or thinner places in the outer shell of the house. Here’s what my local utility website has to say:
“If your home was built before 1982, it should be checked to see if it has R19 insulation, about six inches deep, across your entire attic. Energy Star® estimates that you can save up to 20 percent on heating and cooling costs (or up to ten percent on the total annual energy bill) by sealing and insulating the “envelope” of your home – its outer walls, ceiling, windows, doors and floors. You can have your attic checked for adequate insulation by scheduling a free Home Energy Survey. If you do need ceiling insulation, FPL can help you pay part of the cost with rebates up to $300.”
Maybe your local utility also offers rebates, information and assistance.
If you live where it snows, you can detect internal heat leaks by checking the roof. If one area thaws faster than others do, then heat is getting into the attic from a specific area. If your whole roof is thawed, and your neighbors’ roofs aren’t, you seriously need some insulation.