A window frame comprising top, bottom, and opposed side frame elements connected together to define a rectangular frame, each element having front and back faces and inner and outer edges whereby the outer edges define the frame perimeter. An inner channel extends along the outer edges of at least the side elements, closer to the back face than to the front face. The frame optionally has another channel extending along the outer edges of at least the side elements, closer to the front face than to the back face. Mounting strips are located in the inner channels, having outer edges outside the perimeter of the frame, forming a flange for mounting the frame to a wall surrounding the window to be framed.