Insulation Does NOT Stop Infiltration
Speaking of things that drive me crazy (we were, weren't we?), here's another one - people confusing the roles of insulation and air barrier. The people I'm talking about are not homeowners, though. I'm talking about contractors, builders, and journalists who should know better but don't.
This is a simple article because it's such a simple misconception. See that photo above? The insulation is dirty precisely because it does NOT stop the movement of air. That's why they make filters out of fiberglass.
A house is made mainly of structure and control layers. There's the framing that holds everything together and keeps it from falling down - the structure. Then there are the control layers, meant to control the flow of heat, air, and moisture across the building envelope. This is Building Science 101. The two control layers we're concerned about here are:
Most of the time, houses are insulated with materials that do NOT stop air leakage. Fiberglass and cellulose are the two most commonly used insulation materials, and they do not qualify as air barriers under the standards of the Air Barrier Association of America. Cellulose will slow it down a lot more than fiberglass, but you still can't think of it as an air barrier.
The cause of this rant was my watching a home improvement show recently (always a dangerous thing to do). During the walkthrough, they said that the homeowners had complaints about drafts, so they needed more insulation. Really?! You're supposed to be pros telling people how to fix their homes properly, and you don't know that more fiberglass in the attic won't help them here?
It's pretty simple really. Reduce the air flow. Reduce the heat flow. Two separate processes. Most of the time, it's done with two separate materials.