After the window’s rough opening has been waterproofed (see our companion blog post on prepping the rough opening) and the EXTOSEAL ENCORS has been installed to protect the sills the location is now ready to receive the window unit.

The connection from the glass to the window frame and from the window frame to the rough opening are the thermally weak locations for the unit. Ideally the window unit located in the middle of the insulation to best prevent any thermal bridging.

THERM modeling showing thermal performance based on window unit location

If the wall assembly has only exterior insulation then it’d be best to build a window buck or use a high density EPS to be able to locate the unit within the middle of the exterior insulation. This approach can create a horizontal lip at the header that needs water and vapor protection with EXTOSEAL ENCORS since water can pool at this location.

This project held back the ADHERO 3000 WRB from the rough opening and then installed a high density EPS to locate the window units in line with the exterior insulation

The ADHERO 3000 is overlapping and covering the high density EPS so that the ADHERO 3000 turns into the rough opening

The ADHERO 3000 has turned into the rough openings, but left long at the headers to allow a direct airseal to the window unit's header

This project used plywood as a temporary airseal for preliminary blower door testing

The window units have been installed onto the high density EPS. Exterior wood fiberboard insulation will but up against the EPS later on in the project timeline

You can see the EXTOSEAL ENCORS sill protection tape running down the ADHERO 3000 about an inch and the Contega SOLIDO IQ tape used to airseal the jamb connection

Overlap the Window Frame with Insulation

Sometimes aesthetic design choices will force the window placement to be in a less than ideal location from a thermal standpoint. If the unit is located on the interior side of the framing then it’s possible to use a thin layer of board insulation to overlap the frame on the exterior. If the unit is located on the exterior side of the framing then it’s possible to use a thin board insulation to overlap the frame from the interior.

Left image shows rigid foam insulation used to overlap the interior side of the window frame. The rigid foam can have vapor permeability concerns since it tends to be a vapor retarder.

Right image shows mineral wool insulation used to overlaps the interior side of the window frame. The mineral wool insulation is vapor permeable.

For these options the rough opening will need to be larger than the window unit to give the space for the insulation overlap. Tapered rigid foam boards are commonly used for the exterior insulation frame overlap because it creates a pitch to promote drainage and offers compressive strength for installing a finish material.