This is a 14 gauge two piece leaded, telescopic window frame that will adjust for walls from 3 1/2″ to 7 1/4″ thick. It can be installed after the drywall is in place though the rough opening hole and it is therefore much easier for the drywall installer to insure the integrity of the shielding. This frame is more expensive than the one piece unit at nearly twice the cost. Rough opening size 1/2″ larger than glass size.