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Tom Shafer

Wood extension jambs for vinyl windows?

March 08, 2012

When new-construction vinyl windows are installed, there is a space from the outer edge of the interior drywall to the window frame. Why? The wall is usually 4 ½ to 4 ¾-inches deep. Vinyl windows are usually 2 ½ to 2 ¾-inches deep, leaving a couple of inches of space from the window to the inside drywall. This space does not exist with wood windows, which are 4 9/16-inches thick and reach the outer edge of the drywall inside the room.

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Covering up this space from the drywall to your vinyl windows can be done one of two ways: with dry wall from the edge of the inside wall up to the window frame or with the use of extension jambs. Extension jambs are usually wood, painted or stained to give the appearance of an actual wood window. This technique is very convincing. I once had a customer whose wife thought for two years after moving into their house that the windows were vinyl and not wood!

Options for extension jambs

In order to install these wood extension jambs, finish carpenters cut a 1x (1-inch-by-6-inch) piece of wood lengthwise to a ¾-inch thickness -- the size of the gap. Then they cut it to the size of the window frame and the opening and shim them square with the frame before attaching them. This is both time-consuming and expensive, especially if the home has many windows.

Most window companies offer optional wood extension jambs for about 10 percent of the cost of the window. The ¾-inch-wide wood is stapled to strips of vinyl, which are, in turn, stapled to the vinyl windows. Wood extension jambs are typically made from poplar, oak or pine. They represent a substantial savings over custom-built jambs, both in terms of time and price. You don't have to wait for the carpenter to come out to put the finishing touches on your window installation.

iVinyl windows require wood extension jambs to fit

Other materials besides wood have been used to creatively fill in the gap; for example, pieces of 4-foot-by-8-foot pre-finished paneling or molding the exact size to fit between the window frame and the outer edge of the drywall. Some of these solutions work well; others, not so much.

Just keep in mind when you are shopping for windows for a new home, or for replacement windows, vinyl windows are a great fit even if the frames are not as thick as wood windows. Extensions get you out of that jam.

About the Author

Tom Shafer has decades of experience in window sales, marketing and product development. He's worked closely with window design engineers in testing, design, and building code interpretation. Past employers include United Windows and Doors and Norandex, MI Windows. He currently works at a home improvement retailer.


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