“Traditionally, in colder climates vapor barriers are required because the thought was if you were to put a lot of insulation into the wall cavity, then as warm moist air infiltrated the wall cavity it could condense on cold surfaces. The thought was ‘we better put up a vapor retarder because it will prevent the flow of moisture as a vapor.’ And it’s true, they do; the reason they haven’t worked very well is because typically there are not very good air barriers on interior walls,” Easley says.
“Most of the moisture that gets into the wall cavity piggybacks on air currents. When you have air that exfiltrates carrying moisture with it and it comes in contact with a cold surface, it condenses,” he adds. “The concept behind a vapor retarder is to keep warm moist air out of the wall cavity; unfortunately, unless there is a very good air barrier, it doesn’t work.”
Further explaining the science, Easley says there are several ways water and moisture can move. First, water moves as a liquid; second, it moves as a vapor on air currents; and third, it can move by diffusion. Diffusion is the process of a building material picking up moisture from a higher relative humidity environment and then drying to a dryer or lower relative humidity. Moisture always moves to a drier environment, Easley notes.
“Diffusion is critical to drying building materials,” Easley continues. “For example, if you have wet OSB and you have a vapor retarder over the top of it, it’s not going to dry. That’s why code specifications for exterior weather-resistant barriers require they have drying potential.”
Two Different Things
Summing up, Easley says weather-resistant barriers and vapor barriers are two very different things.
“A weather-resistant barrier is designed to keep air and water out. If you install the system right, it acts as a barrier in terms of water but also blocks airflow so you don’t have as much infiltration and exfiltration. On the other hand, a vapor retarder is designed to control moisture flow by diffusion. It’s supposed to stop moisture-laden air from diffusing through drywall and getting into the wall cavity. The reality is not that much moisture moves by diffusion under normal circumstances,” he says, “unless you have a really high humidity home with lots of plants or aquariums.”
The difference between new construction and remodeling is worth noting, particularly when it comes to installing new windows, Easley says. “I think one of the biggest mistakes window installers make when retrofitting windows is they don’t take the windows down to the original sheathing and reinstall them. A lot of leaks happen as a result of just cutting the windows back and putting a new window in the opening. Wherever possible, I think it’s important to strip the window back to the original sheathing and then flash it normally.
“I also think it’s important for the average remodeler to try to air seal from the inside to prevent moisture-laden air from exfiltrating around the openings, especially in colder climates,” he says.