In this podcast, Tom Harris discusses moisture control layers and misconceptions and unrealistic expectations around spray foam insulation. In the “Spray Foam Reality” series, Harris discusses a number of the repeated issues I see and hear about.
“There are two types of spray foam in use today: Open cell foam and closed cell foam. Open cell foam is very light-weight and each cell is connected. In closed cell, these windows are not broken and we can replace the air that’s inside the cells with a different gas—one that resists heat transfer. That’s what gives closed cell foams the high R-value.
Think of moisture vapor as the water that is carried in the air. It’s what is referred to as the “relative humidity” of air. If you have a high relative humidity, you have lots of moisture in the air. If you have a low relative humidity, the air doesn’t have much moisture.
“Typically, our homes have higher relative humidity in the summer months than in the winter months,” Harris says. “The purpose of a vapor barrier or vapor retarder is to protect the insulation from high levels of humidity. It’s the moisture in the air that has the potential to condense within the insulation or on the cold side of the insulation causing all kinds of moisture-related issues in framing members and sheathing materials. Wet insulation isn’t insulation, so as the moisture in the air condenses it turns to water and creates a problem. The code dictates the need for a vapor retarder depending on the region of the country in relationship to the environment that you build in. Typically colder environments demand the use of a vapor retarder when we have permeable insulation – like fiberglass or open cell foams.