Air-permeable insulation such as fiberglass and blown cellulose does little to stop air moving through it, which is why you need an air barrier. An air barrier is an air-impermeable layer that prevents conditioned air from mixing with outdoor air. Insulation such as closed-cell spray foam and rigid foam is its own air barrier.
A home’s air barrier should be durable and continuous. To reinforce this point, building science experts often say you should be able to trace the air barrier on a set of house plans without lifting your pencil. Plywood, OSB, rigid insulation, gypsum sheathing, and interior drywall can be excellent air barriers, but the devil is in the details.
For an air barrier to be effective, seams and holes must fully be sealed. Otherwise, when these holes are affected by a pressure difference created by the stack effect, wind, or HVAC equipment, you will get air leaks. A hole plus a pressure difference equals an air leak. Air leaks are one of the biggest factors in home comfort and energy efficiency.
If you don’t plug the air leaks before adding insulation, your new insulation will be much less effective and you’ll waste money on heating a cooling. You often can see the result of air leaks in existing fiberglass insulation as a buildup of dust and dirt in the fibers, which have acted as a filter. You can plug leaks with caulk and spray foam or cover them with air-sealing tape.
Larger holes can be sealed with rigid insulation and spray foam. When prioritizing air-barrier improvements on an existing building, it often makes the most sense to start with the ceiling on the top floor. This is where the stack effect contributes the most to heat loss in cold climates. In hot climates, the top-floor ceiling separates boiling hot attics from the conditioned living space.
As you can see, finding and filling air leaks is a very important way to keep your home efficient and cost effective. Many of these methods will help through building sciences to keep your home safe. For more information on building sciences, visit Fine Homebuilding.