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Extension Jambs for Replacement Windows

An exterior energy upgrade uses jamb extensions to align new windows with rigid foam-board insulation

By Daniel S. Morrison Feb 11, 2008

This 92-year-old house received an energy upgrade that included insulating the entire exterior with rigid foam. The crew installed two layers of 2-in. foam over drainage housewrap before the wood siding went up. As part of the process, they fastened extension jambs to the replacement windows to align with the exterior’s new bulk. View the slideshow for details on the interplay between the extension jambs and the foam board insulation.

Photos by: Daniel S. Morrison

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2x6s around the windows provide backing for trim boards. This house will have two layers of 2-in. foam, outside the sheathing. The first layer butts the 2x6s and the second layer covers it.

The extra deep extension jambs are fastened to the replacement windows with pocket screws. The hole extends through a dadoed groove that slips into the window jamb.

Caulk the jamb, slide the extensions on, and nail through the jambs into the 2×6 with finish nails. Also nail from under the sill into the side jambs and from on top of the head jamb into the sides.

Tape the seams of the foam and make sure to stagger the joints.

The second layer of foam butts the window jambs and covers the 2×6 backing. Pressure treated 1x4s will be screwed to the surface of the foam to provide solid backing and an air space behind the wood siding.

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