by Sam Rashkin
Insulation (such as fiberglass, rock wool, or cellulose) need a complete air barrier on all six sides. This ensures the thermal control layer works effectively, since air can flow through it.
Think of the difference outdoors on a cold windy day with a sweater and jacket made of a water repellent fabric: Without the jacket, the cold wind easily passes through the sweater and mitigates its thermal resistance.
In contrast, with the jacket functioning as an air impervious control layer, air flow through the sweater is blocked, resulting in much more effective thermal protection. It’s the same with buildings. Note that some types of ...