At Just Housing, designing a durable, feasible, and continuous air barrier is a priority—so much so that we devote an entire sheet in our construction sets to air-barrier assemblies. This sheet of drawings and information aids builders in executing a truly continuous air-control layer. In our previous energy smart blog post, we covered some of the details included on this construction sheet: penetrations through the exterior wall, the exterior-wall-to-ceiling connection, the interior-wall-to-ceiling connection, and penetrations between framing members in the ceiling. In this article, we’ll illustrate and explain our typical details for air-sealing at penetrations through interior partitions, electrical boxes, and dropped ceilings. For more information on air-sealing details, read these Green Building Advisor blog posts:

• Getting Started with Air-Sealing – GreenBuildingAdvisor
• Installing an Interior Air Barrier – GreenBuildingAdvisor
• Ceiling Air-Sealing – GreenBuildingAdvisor

Air-sealing at the ceiling

Our standard roof system is designed with roof trusses, a polyethylene air barrier and vapor retarder attached directly under the roof trusses, and drywall as the ceiling finish. (Editor’s note, the use of polyethylene as an interior air barrier and vapor retarder is more common in cold and very cold climates. Responsive (smart) vapor retarders are a good and recommended alternative to polyethylene sheeting.)  Penetrations in the top plate of an interior partition are a common condition and are often sealed at the bottom (lower) top plate. However, this approach fails the red line test: The sealant (usually caulk or one-part polyurethane foam) must connect to the polyethylene at the top of the top plate.

Air-sealing electrical boxes in the ceiling should follow a similar logic: seal to the polyethylene. In order to keep the air barrier continuous, electrical boxes in the ceiling must be airtight. We recommend using the wide-flanged airtight Lessco boxes at the locations of all ceiling fixtures. As…