Air Sealing vs Insulation: Why Your Home Needs Both
When people think about making a house more energy efficient, they usually think about adding insulation. The thought is, more R-value, more inches, bigger number on the bag, must be better. It makes sense, but it leaves out half the story. Insulation slows heat moving through solid materials. Air sealing stops unwanted air moving through cracks and holes in the building envelope. Those are two different jobs. Imagine wearing a thick winter coat with the zipper wide open. The coat itself might be excellent, but the wind is still getting in and ruining your afternoon. That's roughly what happens when a house has plenty of insulation but leaks air around it. The insulation isn't useless, but the building is making its job harder than it needs to be.
So where does the air go? In many homes, the biggest leaks are hiding in the attic. Plumbing and electrical penetrations, open cavities, gaps around top plates, attic hatches, recessed fixtures where applicable, and duct penetrations can connect the living space to the attic. Windows, doors, rim joists, and other transitions can leak too. A tiny hole can move a disproportionate amount of air when there is a pressure difference across it. Wind, the stack effect, and mechanical equipment can all create pressure differences. And one little gap might not seem like much, but multiply it by dozens or hundreds of gaps and you've accidentally built a ventilation system with no controls and no warranty. Air leakage can also move moisture. In cold weather, warm indoor air leaking into cold parts of the building can deposit moisture where you really don't want it. That's one reason air sealing is about more than comfort and utility bills. it's about protecting your home.
Which should you do first? For many attic upgrades, air sealing should happen before new insulation is installed. Otherwise, you can bury the leaks under a foot of cellulose and congratulate yourself on hiding the problem. That a very easy thing to do, not very useful. Air sealing doesn't mean making a house dangerously airtight and forgetting ventilation. A tighter home may need intentional ventilation and moisture control, especially if combustion appliances are involved. The real goal is controlled air movement, not random air movement.
The short version, insulation slows heat, air sealing controls leakage, and ventilation provides fresh air on purpose. If you treat them as separate parts of the same system and your house has a much better chance of performing the way you expected.