Homeowners frequently complain: "I just blew 16 inches of fiberglass into my attic, but my house is still drafty and my energy bills barely dropped." The problem is a fundamental misunderstanding of physics. Insulation provides thermal resistance (R-Value), but it does NOT block airflow.
representing R-Value / Insulation
If you wear a thick, heavy wool sweater outside on a freezing winter day, it keeps you warm by trapping your body heat near your skin. This is exactly what blown-in fiberglass does. However, if a strong 30mph gust of wind blows across you, the wind cuts straight through the knitted fibers, and you instantly freeze. The thick wool cannot stop air movement.
representing Air Sealing
If you put a thin nylon windbreaker jacket on over the wool sweater, you create a complete thermal barrier. The windbreaker physically blocks the cutting wind, allowing the thick wool sweater underneath to do its job without interference. In your attic, expanding foam is the windbreaker.
Before a single ounce of fiberglass is blown into an attic, a technician must crawl across the bare drywall deck with a gun of expanding foam, sealing every top-plate gap, wire hole, and light fixture. Only when the house is totally air-sealed (Windbreaker) do we blow the high R-value fiberglass (Wool Sweater) over the top.