If buildings consisted of only foundations, walls, and roofs, then adding water, air, vapor, and thermal control layers would be straightforward. Doors and windows, of course, complicate matters. They significantly interrupt the continuity of those layers. The question is, how do we handle the interruptions to ensure a building’s successful performance?
Windows and their components make up a self-compartmentalized system of control layers, which are installed in a wall assembly’s rough openings. To maintain overall continuity of the assembly, the window unit’s control layers need to be stitched together with those of the wall. The detail shown is of a flanged double-hung window in a 2×6 16 in. o.c. wood-framed wall sheathed in 1-7/16-in. Zip R-6. In detailing the windowsill, the four control layers are taken into account.
With respect to water management, the objective is two-fold. The primary goal is to keep water out of the system with a barrier. Secondly, water needs to be managed should it breach that barrier. We use a “down and out” strategy, meaning if water does find its way into the assembly, it has a managed pathway out. In this case, the strategy is a rainscreen made of 1×3 vertical wood furring strips spaced 16 in. o.c. to align with the framing studs. The rainscreen provides free drainage and ventilation for the wall cladding and the window unit. Connecting the rough opening to the rainscreen system maximizes the opening’s drying potential.
Note the piece of beveled lap siding placed on top of the rough sill to create a slope to direct water toward the exterior. The siding is accompanied by a 1×3 back dam, the top of which is slightly higher than the high point of the beveled siding. The raised edge of the back dam contains any water at the sill. (In areas where beveled siding isn’t available, builders rip their own pitched blocking for this purpose or frame the rough sill with a slope toward the exterior.) A self-adhered sill flashing membrane extends from the exterior sheathing along the top of the sloped sill and over the top of the back dam to keep the framing from absorbing water. The membrane is then turned up the sides of the rough opening framing a minimum of 4 in., providing a self-contained sloped flashing for the rainscreen.
The side and top window flanges are taped with shingle-style lapping to keep water out of the rough opening. It is common for architects to spec taping the bottom window flange as well, but it is critical to avoid this. The point of these details it to let water drain out of the rough opening.
Regarding air tightness, the window unit is typically suspended in the rough opening, which creates a gap between the window and the wall assembly. The window itself is relatively airtight—what’s needed is continuity from the window to the wall, which can be achieved with a continuous bead of expanding foam sealant around the perimeter of the window rough opening. To ensure water management, it is important for the bead to be as close as possible to the interior of the window unit. The gap can also be filled with a backer rod and a bead of sealant, depending on the size of the space between the window unit and rough opening.
With window installations, vapor control is about two things: allowing drying around the rough opening and preventing condensation on the glass. Keeping the bottom flange open for drying is a start; some builders also use vapor-open air-sealing and flashing tapes instead of caulks and expanding spray foam. Preventing condensation on the window is a matter of choosing the right glazing performance—some windows have a condensation resistance rating on their NFRC label—and controlling indoor humidity levels.
The window’s performance rating, in great part, determines thermal control. This detail features a double-glazed low-E window unit, though more aggressive triple-glazed options are available. In most climate zones, International Residential Code (IRC) dictates a minimum U-value for window performance.
To ensure a building’s optimal performance in terms of water, air, vapor, and thermal control, it is critical to integrate the the window units into the wall assembly in such a way that continuity between materials and components is guaranteed.
Alexandra Baczek is an associate at Steven Baczek Architect. She is a graduate with a Master of Architecture from Roger Williams University. Illustration by the author.