Replacement Window in Brick Openings
A replacement window in brick openings is best done by not installing the window right inside the brick even though it may look the cleanest from the exterior, it may create a set of problems. One reason is the edge of the bricks are not uniform and it means the windows will have to jut out quite a bit in order to cover the edge of the bricks which will result in the window being placed on the outer edge of your wall framing. This will leave very little space for the window to be properly screwed.
Caulking Is Important
Brick isn't air tight and the windows will need to be caulked to the bricks. Subsequently the window will be rendered useless if air is moving through it from the one inch gap created between the sheathing and the brick facade.
Brick is not water tight and a replacement window in brick openings should always be sealed behind building paper. The building paper is installed on your sheathing and by installing a window outside onto the brick façade and moving the windows out past the sheathing or building paper will create a problem if there is ever a water leak from above the window. Your windows would need to be sealed to the brick with caulk and the caulking usually lasts a temporary time.
A better way of a replacement window in brick opening would be to add the framing instead. One can use two methods to accomplish this job.
By mounting a sufficient sized frame to install a window and by a nailing a fin into the brick opening would be one good option for a replacement window in brick openings. However, this will reduce the size of the opening by three inches on either side of the window. Subsequently, your trim will not cover your old paint lines unless you get wider trim or by covering the face of the added framing with an extension jamb.
Adding the wood is one of the best methods but be very careful and choose a frame of suitable size for the replacement window in brick openings.
A third option would be to possibly install a replacement window in brick openings inside the metal frame after one removes the casement sashes. However, metal frames are not energy efficient and its best to get rid of them. Moreover, this option would also allow you to leave much of the frame and trim intact.
