On our jobs we always make plywood jambs. 3/4" cabinet-grade stuff. Shim the boxes, then use minimally-expanding foam to secure. No drafts.
Casey

Ditto the plywwod. You would be surprised how dinked up drywall gets when opening/closing the window as well as setting items on the sill.

Makes sense. Thank you!

I use 1" pine #1 in the width that fits the space.

Ditto what worthy did. Here's one of mine, 1" pine on the jambs, quarter-round on the inner seam:

The point is use solid wood vs.drywall. Dont use mdf or any other particle board based product! Nice trim details, Worthy and Weedy!

Worthy and Weedy. Did you paint or stain the wood jambs? Did you apply sealant after? What about your trim, are they wood as well painted white? Thanks.

My jambs and casings are all primed and painted wood, no sealant.

Ditto.

If you're considering a basement renovation as a way to expand your living space, don't be surprised if your building-codes office has something to say about itÂ- especially if a new bedroom is part of your plan. Without an egress window in case of fire, regulations simply won't allow a basement bedroom. And because this ruling is fairly recent in many parts of the country, most existing basements were not built with egress windows