Multifamily building has been one of the fastest growing market sectors over the past five years, and outlooks predict this trend will continue. Nationwide, authorities having jurisdiction have been adopting the 2015 International Energy Conservation Code, which eliminates the air barrier exception for climate zones 1, 2, and 3. This code change effects multifamily construction across much of the southern United States from Florida to California. Some design professionals do not know how to develop air barrier details, and some contractors do not know how to build air barrier details. This research provides the multifamily construction sector with air and water penetration knowledge for flanged window-to-wall integration detailing. Low-rise multifamily construction typically is multistory, wood-framed construction with vinyl-flanged windows. High-rise condo and apartment building construction typically utilizes metal framing, gypsum sheathing, and aluminum-flanged windows. Various water-resistive barriers (WRBs) as well as details from WRB manufacturers also are utilized. Few data indicate which detailing and installation methods are better for air and water penetration. Building wraps, sheathing systems with integral coatings, and fluid-applied WRBs can be applied according to ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors and Skylights, Methods A, A1, B, and B1. These methods can be used with low-pressure foam at the interior perimeter or backer rod and sealant at the interior perimeter. Various options for flange sealant can be considered. We built six 4′ × 8′ mockups, each with the same simulated flanged window, according to a distinct yet common multifamily windowsill detail. We tested the mockups according to ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors, and Curtain Walls, by Uniform or Cyclic Static Air Pressure Difference, and ASTM E783, Standard Test Method for Field Measurement of Air Leakage Through Installed Exterior Windows and Doors.