Abstract: An integral window unit comprising a sill member, a header member and a pair of jamb members interconnected to form a generally rectangular window frame. The jamb members have first and second pairs of track means each pair of which slideably receives a prime window sash and third and fourth pairs of track means each pair of which slideably receives a storm window sash. The distance between the track members in the first pair is greater than the distance between the track members in the second pair which in turn is greater than the distance between the third pair which is greater than the distance between the fourth pair. At least one pair of track means is in overlapping relationship with the adjacent pair of track means. A thermal barrier is provided in the window frame between the second and third pair of track members to minimize heat loss therethrough and a fifth pair of track means is provided adjacent the thermal barrier for slideably receiving a screen member.