yes. The jamb extensions are not fiberglass.

Are they wood or some other product? They never really say in the brochures or online so I thought it was fiberglass too.

Beware of the materials. My only criteria for getting the Essentials was not to have vinyl. i was assured there was no vinyl. They actually have vinyl sash covers on the double hung Essentials , and vinyl on the base of the awning window and in the center of the section that connects two double hung windows (rather rough finish and whiter than the stone white of the fiberglass). Regret buying.