But the question I get more than any other on this topic is about whether or not the insulation on the attic floor should be removed when insulating the roof deck in an existing home. As you can tell from the title of this article, my answer is to remove it. Here are my three reasons, in increasing order of importance.
1. To prevent moisture problems
Think about temperature. Think about dew point. If you leave the insulation in the attic floor after insulating the roofline, the attic will be cooler in winter than if you remove the insulation.
But a common reason to put spray foam on the roofline is to avoid having to air-seal the attic floor. Thus, the air in the attic is connected with the air in the house. That means it’s more humid than outdoor air and more humid than vented attic air. Cold air is dry air, you know.
But now the attic isn’t vented to outdoors. The attic is much warmer than outdoors in winter but significantly cooler than the living space if you leave the old insulation in the attic floor. That makes the surface of the spray foam cooler, possibly even below the dew point.
Removing the attic floor insulation will solve this problem. It is not, however, the first solution. As Dr. Joe Lstiburek says, we shouldn’t be calling these things sealed attics or unvented attics. We need to think of them as conditioned attics. Once you deal with the air up there, this problem goes away, with or without the insulation in the attic floor.