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Best Practice: Fat-Wall Window Bucks

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Construction Practices

Best Practice: Fat-Wall Window Bucks

Carefully installed window bucks make for secure fastening and easy alignment of windows and exterior trim 

By By Dan Morrison and Sal Alfano September 14, 2017
Installing fat-wall window bucks
This article first appeared in the August 2017 issue of Pro Remodeler.

Wrapping a building with rigid insulation creates a tight thermal envelope that minimizes heat loss via conduction through the framing. At windows, it’s important to preserve that thermal break while also providing a solid nail base. David Joyce and his crew at Synergy Construction, in Lancaster, Mass., solved the problem on a recent deep energy retrofit on which they wrapped the building in two layers of 2-inch foam. After the first layer was installed, they framed the perimeter of window openings using 2-by material padded out with ½-inch plywood to match the 2-inch foam thickness. Here’s a step-by-step look at how they did it, adapted from a video at ProTradeCraft.com.

Fat-Wall Window bucks-1This top piece spans a series of three identical windows. Calvin and David (in the hat) have fastened each end and are using a straightedge to make sure the 2-by isn’t bowed in the middle, which could create alignment problems later when the trim is installed.


Fat-Wall Window bucks-2In a bank of windows like this, making sure the wood bucks are straight and level helps with alignment during window installation. Here, David aligns an intermediate vertical buck with a centerline marked on the top piece to ensure even spacing.


Fat-Wall Window bucks-3Damien applies pressure to straighten a side buck while it’s fastened to the frame with a 6-inch screw. Countersinks for the screwheads are created ahead of time using a spade bit.


Fat-Wall Window bucks-4Fastening the bucks with screws can compress the foam and create a wavy surface. Use a straightedge and tighten or backout fasteners as necessary to make sure the bucks are flat.


Fat-Wall Window bucks-5The last step before installing the windows is to flash the opening. In this case, a starter strip for the second layer of foam was run under the windows, then cut on a bevel to promote drainage of any water that might penetrate the opening. The double layer of foam at the sills will be flashed with peeland- stick tape, so the wood bucks are omitted to promote drainage.

Next: Flashing the openings

Get more Jobsite Know-How here

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