For more info on window installation, take a look at this series about high-performance window installs:
Windows should be placed in the middle of the insulation layer to help reduce thermal bridging. For more info, check out the “Positioning the Window in the Rough Opening” blog post. If the wall assembly has the majority of the insulation outboard of the sheathing, then it’s appropriate to build a window buck to locate the window unit in-line with the exterior insulation. Window bucks can be site constructed using 2x framing, but many are made using plywood sheathing to help reduce thermal bridging of the window buck framing. Deep sills are often created and need to be waterproofed using the EXTOSEAL ENCORS tape (see “How To Prep Rough Openings” blog post for more info).
Window Buck Header
Sills are waterproofed with the vapor closed tape EXTOSEAL ENCORS because there could be standing water on this horizontal surface. A window buck that extends past the WRB often creates a similar horizontal surface concern at the header of the window. This horizontal header - that could pool water - also needs to be waterproofed with EXTOSEAL ENCORS and like all window headers flashed to the exterior side of the siding. The EXTOSEAL ENCORS should lap 4” - 6” up the vertical face of the WRB at this transition point.
The underside of the header is sealed with a vapor permeable Pro Clima tape (TESCON VANA or CONTEGA SOLIDO IQ) and can be waterproofed with an additional tape connection (TESCON PROFIL or CONTEGA SOLIDO IQ) directly to the window frame.
If the project’s wall assembly includes interior insulation (between the wall framing) and some exterior insulation (but not a higher R-value than the interior insulation) then the window unit can be placed in-line with the wall’s sheathing. No window buck is needed. Here’s an overview of how the window’s waterproofing and airsealing connections might slightly adjust based on the exterior insulation:
Exterior Mineral Wool insulation
Wall assemblies with exterior mineral wool insulation typically have the WRB installed on the sheathing with a vented rainscreen outboard of the mineral wool insulation board. The exterior window waterproof taping needs to be sealed from the window frame to the WRB. Exterior mineral wool boards can get wet without damage so it’s not a concern if there’s water draining between the WRB and mineral wool board.
Exterior Wood Fiberboard Insulation
Wall assemblies with exterior wood fiberboard insulation typically have the windows flashed to the exterior side of the WRB – which can be the exterior face of the wood fiberboard insulation if the board has been treated to be water resistant (such as Gutex MULTITHERM). The wood fiberboard needs to be treated with TESCON PRIMER RP or VISCONN to ensure proper tape adhesion. The wood fiberboard is not an airtight material. There will need to be an additional air sealing tape connection from the window to the wall assembly’s air barrier.
Exterior Foamboard insulation
Exterior foamboard insulation can be a problematic vapor retarder, especially in climate zones 4 and up. See 475’s “Foam Fails” blog series for more info on a variety of concerns with foam insulation. Exterior vapor open fibrous insulation is a much better option. Unfortunately, many projects still rely on foam insulation - it’s an industry standard that should be addressed. Wall assemblies with exterior rigid foamboard insulation typically have the windows taped to the WRB and then there’s an additional sill tape to flash the sills to the exterior side of the rigid foam board. This is done when the exterior foam board has the seams taped or a shiplap/T&G joint connection to act as a WRB. The foam board install is typically not airtight, so there does need to be a separate airtight tape connection to the primary air barrier. Some rigid foam boards are grooved to allow drainage between the WRB and foam board. Drainage in this location is key when using a vapor-closed hydrophobic exterior insulation board. The WRB used in these applications needs to self-seal around fastener penetrations (ex. ADHERO) since there could be standing water on a fastener as it slowly drains in the small cavity. The drainage plane needs to be small enough to prevent venting that’d reduce the effective R-value of the wall assembly.
