One of the sticky details when installing exterior rigid-foam insulation is how to fasten flanged windows. Many window manufacturers frown on installing the window directly on top of the foam layer. A popular workaround is to install wood bucks between the framing and the window flange, but this creates a thermal bridge, which partially defeats the point of the foam. The best solution I’ve seen is the ThermalBuck. The high-density plastic-coated EPS-foam backer breaks the thermal bridge and won’t compress like typical foam insulation. The 8-ft. lengths can be integrated into rough openings, mitered at corners, and joined with Dap Dynaflex 800 sealant. Available in widths from 1 in. thick to 4 in. thick in increments that match common insulation thicknesses, they are sold with and without a drainage gap. The sill is sloped for drainage, and the interior flange is 1⁄2 in. thick. Price depends on thickness, but runs between $3.80 and $6 per ft.
—Patrick McCombe, senior editor
From Fine Homebuilding #285