How to address a building envelope leaking from the inside out

The team continues to publish some of the best content related to building science and best practices for sustainable design and construction. The site also offers an online discussion forum that is regularly populated with posts that give insight into the real world issues facing professionals at the cutting edge of high-performance building.

In a recent post at, Scott Gibson draws attention to a thread in the forum’s question and answer section involving a homeowner’s fight with moldy sheathing in the hills above Seattle, Washington. While the owner had suspected water intrusion through the building envelope to be the source of moisture, further investigation and research led to a very different source of moisture – cold sheathing condensation.

In other words, moisture wasn’t coming from the outside, it was coming from the inside. The ground floor room in question contained the home’s HVAC and mechanical equipment, including a couple water heaters and a humidifier attached to the furnace. The heat and moisture produced inside this space reached exterior sheathing due to the lack of a continuous air barrier.

One of’s resident experts, Peter Yost, echoed the recommendations of others for this particular homeowner – establish a complete air barrier. However, he did not agree with suggestions by others to use closed-cell spray polyurethane foam:

While it’s true that cellulose is a lot more airtight than fiberglass batts, it’s not anywhere near tight enough to be the dedicated air barrier. Flash and batt or the airtight drywall approach is the way to go while Brian has the opportunity.

And that same continuous air barrier will improve or eliminate wintertime dryness issues. I never have found a home in which wintertime dryness was not the result of either air leakage or overventilation.

If you are interested in all the building science behind this scenario, check out the entire post at


Image courtesy Erik Söderström