If you’re like most homeowners, you’ve probably heard that adding insulation to your attic is one of the best ways to lower your energy bills and keep your home comfortable year-round. And that’s absolutely true. But here’s something many people don’t realize: stuffing more insulation into your attic without first stopping the air leaks is kind of like putting a winter coat on over a shirt that’s full of holes. You’re still going to feel the cold. And worse, you might even cause some serious problems behind the scenes.

Air sealing is the step that comes before insulation, and it makes all the difference between an attic that just looks insulated and one that actually works the way it should. The idea is simple: you find every little crack, gap, and hole that connects your living space to the attic, and you seal them up. Think of your attic as the top of your home’s thermal envelope. If that envelope has leaks, your heated air in winter and your cooled air in summer will escape right into the attic, making your HVAC system work harder and your energy bill climb higher.

So where do these leaks hide? A few places are notorious. The biggest offenders are usually around plumbing vents and electrical wires that run up from the walls into the attic. Those small gaps around a pipe or a wire might not look like much, but when you add them up across your whole house, they can create a hole the size of a basketball. Other common spots include the tops of interior walls, where the drywall meets the attic floor, and around recessed lighting fixtures, especially older cans that aren’t rated for direct contact with insulation. Chimney chases, bathroom fan housings, and attic hatches are also prime locations for air sneaking through.

The materials you use for air sealing are simple and affordable. For larger gaps, like where two pieces of drywall don’t quite meet, a can of spray foam insulation works wonders. It expands to fill the space and hardens into a solid seal. For smaller cracks and seams, a tube of high-quality caulk and a caulking gun will do the job. Just run a bead of caulk along the gap and smooth it with your finger or a tool. For areas around masonry or metal chimneys, you need a high-temperature silicone caulk or a special fire-rated sealant, because regular foam can be a fire hazard. Also, never use spray foam directly against a metal chimney flue—it can trap heat and cause trouble.

Once you’ve sealed those air leaks, then—and only then—should you add your insulation. The combination is what really gives you the energy savings. In fact, the U.S. Department of Energy says air sealing and adding insulation can save you an average of fifteen percent on heating and cooling costs. That’s real money, year after year.

But there’s another reason air sealing is critical: moisture. When warm, humid air from your living space leaks into a cold attic, that moisture can condense on the underside of your roof deck. Over time, that leads to mold growth, rotting wood, and even ice dams in winter. By sealing those leaks, you keep the moisture where it belongs—inside your home—where your ventilation and HVAC system can handle it. Your attic stays drier, your roof lasts longer, and your family breathes healthier air.

Now, I can hear some of you thinking, “But my attic already has insulation. Can I air seal it now?” Yes, you can, but it’s a bit more work. You’ll have to gently move the insulation aside, do your sealing, and then put the insulation back. Wear a dust mask and gloves, and be careful not to compress the insulation when you replace it—compression reduces its R-value. If your insulation is old, dirty, or full of mouse droppings, it might be worth hiring a pro to do the job safely.

One more thing: don’t forget the attic hatch or pull-down stairs. That flimsy piece of plywood or fiberglass is basically a giant hole in your ceiling. You can buy an insulated attic tent or simply glue rigid foam board to the back of the hatch and weatherstrip the edges. It makes a huge difference.

So if you’re planning an attic insulation project this season, take the time to air seal first. It’s a weekend job with a big payoff. Your energy bill will thank you, your furnace will run less often, and your attic will stay dry and healthy. And the best part? You’ll feel that cozy, even temperature throughout your house, without any mysterious drafts. That’s what good home repair is all about—doing the small things right so the big things work better.