A window assembly according to various embodiments includes a frame, a sash, an insulating unit (e.g., one or more glass panels), a glass stop, an elongated frame insulating element slidably disposed on an inner surface of the frame, an elongated sash insulating element slidably disposed on an outer surface of the sash, and an elongated inner sash insulating element slidably disposed on an inner surface of the sash. The elongated frame insulating element, the elongated sash insulating element, and the elongated inner sash insulating element each define a plurality of chambers that extend substantially parallel to a longitudinal axis of each element. These chambers reduce the size of the space between the frame and sash and the size of the space between the sash, the insulating unit, and the glass stop, which results in smaller convection currents within these spaces and a more thermally efficient window assembly.