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Beginning stages of starting to add window jambs to my basement. Question for those of you familiar...what material have you all found typically works best? MDF? Pine? Poplar? These jambs will more than likely be painted and not stained. Also, is it recommended to leave a small gap between the jamb and the actual window? Is this an area that is then filled with caulk? Silicone? Thanks!

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    Just to be clear, you've got a window in and you need the connection from the window to the trim? In my neck of the woods, that's called a jamb extension. Any wood (pine/poplar) will do. Prime 6 sides before installation. I'd avoid mdf, as any moisture will swell it. Don't gap deliberately. Caulk with a paintable latex. Avoid pure silicone as it's not paintable. Apr 26, 2021 at 3:06
  • Sorry, jamb extension. Thanks!
    – EGrant23
    Apr 26, 2021 at 4:01
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    @AloysiusDefenestrate, that's good for an answer!
    – Jack
    Apr 26, 2021 at 4:43
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    I did some work on the windows in my parents property - the wood was oak and just needed a quick rub down, prime and paint. Over 80 years in all weather and till going strong. Built to last.
    – Solar Mike
    Apr 26, 2021 at 8:13

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For jamb extensions, I'll always use wood (pine/poplar) over mdf, as the tiniest bit of moisture on mdf will swell it like old cardboard. Be sure to prime every side of the wood (including cut ends) before installation.

You shouldn't try to have a gap between the extension and the window. Just butt it as close as you can and caulk with a paintable latex caulking. Avoid pure silicone, as it won't take paint.

In anticipation of casing the window, you want the jamb extension to be perfectly in-plane with the drywall. If the window is set slightly out of plumb, or there's something going on with the framing, that might mean putting a slight taper on the jamb extension before install. It's not a perfect solution, but if you find you need a taper, just mark the top/bottom of the jamb extension to where they'll be flush with the drywall and cut a straight line between the points. Rotate the jamb extension so that the cut edge is against the window and the factory edge meets casing. (You could go nuts trying to scribe the jamb extension exactly to the drywall, but your casing isn't going to conform to small wobbles and you'd have a cut edge that you'd have to tidy up a lot.) All that said, sometimes you just need to take a hammer and mash down a section of drywall that's sticking up. Stay within the boundaries of the casing and nobody but you will know what a terrible thing you did.

If you're fastening into a wood buck or framing, life is pretty easy -- just shims and nails. If it's into concrete, I'd try to build the entire unit with screws at the butt joints and then insert it into the window opening. You'll still need a few attachment points into the concrete, but it'll hold itself together without too much strain.

Be accurate, even, and square with the jamb extensions and the casing will go more easily. Having said that, I will occasionally intone the carpenter's creed: "Caulk and paint make you the carpenter you ain't."

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  • + for not using mdf, I don’t like that stuff at all but especially not around areas that could be damp.
    – Ed Beal
    Apr 26, 2021 at 16:17
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    What an awesome reply! Thank you! I ended up going with finger jointed primed pine. Figured this would work the best - that way I only need to prime cut sides.
    – EGrant23
    Apr 27, 2021 at 17:11

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