If your house gets too warm during the summer and your energy bills are too high, don’t guess why. Hire a pro to figure it out before you invest a lot of money in updating your air conditioning unit or replacing your drafty windows.

Those big-ticket items could be contributing to the problem, but they’re probably not the core of the problem.

In fact, say my friends in the energy-efficiency business, the insulation in your attic is the first thing you should replace when your home starts feeling uncomfortable during warm weather.

Once you get the insulation properly installed, then you can look at other things that also can help lower your energy bill, like compact fluorescent light bulbs, a higher-efficiency air conditioning unit and even solar panels.

Studying the home’s “envelope”—the walls, ceiling and floors--using digital pressure gauges, huge blowers and an infrared camera enables an energy auditor to literally see the tiny holes and crevices that are letting the sweltering summer air sneak into your air conditioned home.

Every home has some leaks, usually around windows and doors, at the point where plumbing, cables and electrical wires enter the house, and in the attic or crawl space. Builders typically use insulation to create a barrier that prevents air from entering the home through those leaks.

But insulation doesn’t last forever. An older home that once had four or five inches of attic insulation might only have one or two left. A 15-year-old home might have enough insulation to comply with the building code of that time, but not with today’s stricter code, which calls for thicker insulation. And a 50-year-old home might not have any at all if the builder relied on shading and wall ventilation rather than insulation to keep it cool.

Plus, not every builder installs insulation correctly. Insulation that is stuffed into cavities in a home’s framing, hung near a leak or simply draped over one won’t perform properly. Instead the insulation must touch the surface it’s meant to protect. If it doesn’t, it will filter the hot air as it comes into the house, but it won’t keep it out. And insulation with gaps in it will let heat in between those gaps.

This common problem is fairly inexpensive to remedy. And once your home’s insulation is working as it should, you might find that your home feels more comfortable and your energy bills are lower.

Then you can concentrate on replacing your aging air conditioner and kitchen appliances with more efficient models. Then you can think about trading those old, drafty windows with double- or triple-pane varieties designed to keep the heat out of the house.

Insulating your home’s envelope first will save your energy-efficient new air conditioner and windows from having to compete with unsealed leaks that let hot air into the home.

Think strategically about energy efficiency. Before you try to solve the problem of high energy bills and rooms that are too warm, find out what’s causing the problem. An energy evaluation costs a few hundred dollars, but it could tell you that your problem is cheaper and easier to fix than you ever would have guessed.