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Window jamb extensions

Posted by mike13 (My Page) on
Wed, Jun 14, 06 at 12:00

I'm in the process of finishing off an attic space. The gable walls are 3.5" thick (2X4 construction). To increase the insulation R-value I'm making a double wall which will result in a 7" wall thickness. There is an existing single double-hung window which I need to extend the jambs on.

Anyone with recommendations on the best way to accomplish this?



Follow-Up Postings:

RE: Window jamb extensions

I'm not going to claim to be an expert, but I think all you need to do is to rip some 3/4 stock to the required width. You might try attaching to the existing jambs with finishing screws rather than with nails. Predilling for the screws will avoid splits. You don't need many screws because the casings will also hold the extension jambs in place. If you are using stool stock at the bottom you can do something similar. Shims may be helpful underneath. Worst case scenario is to shim and nail the extensions to the framing, but it's nice to avoid this. Another alternative is to drywall right to the existing jamb and corner-bead the corner (trimless look).

RE: Window jamb extensions

I had to do this in my basement and it was fairly easy. I just bought some clear pine and ripped it to the correct width and then cut some some slots on the vertical pieces for the horizontals to fit into.

Because my pieces were about 2" wide I chose to shim between the framing and the extensions so that they would be rigid. Then I used a 2" brad nailer to attach them.


RE: Window jamb extensions

or you can visit a lumber yard and buy jamb extensions from 1/4" to 2 1/2"


RE: Window jamb extensions

We have existing anderson casement windows for 2x4 wall, we are going to a 2x6 wall, our local lumber yard ordered us extension jambs for 2x6 wall from Andersen, we just took out the existing 2x4 jambs, and added the 2x6 jambs.

RE: Window jamb extensions

You'd be better off replacing the old window than doubling the wall thickness.

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