Correct window installation is important in preserving the benefits of a concrete building. Particularly the energy efficiency, air tightness, and water tightness of the envelope can be compromised if windows are attached poorly.
The most common window installation combines a window buck for the rough opening and a face-mounted flanged window. Although flanged windows were designed for installation in wood-frame walls, the task can be just as convenient and sound in concrete walls if proper procedure is followed.
First consider installation in a wall with an exterior face of foam.
The first step is to wrap the sill with self-adhering flashing. Cut a piece about 10 inches longer than the rough opening width. The piece should be about 14 inches wide. Position the flashing so that it covers the 6 inches of the top face of the sill nearest the exterior and then subsequently the ends of the flashing run about 5 inches up the jambs on either side. The flashing will form a sort of wide “U” at the bottom of the opening. Cut the flashing along the two corners of the “U” so that the side flaps fold over the buck jambs on either side, and the bottom flap folds over the buck sill below.
Fasten the window to the buck as would be done to a wood frame rough opening, screwing through the face of the window flanges.
Prepare metal flashing to go over the window. It should have a dam on each end and a drip edge along its bottom edge. Its width should match that of the rough opening. Set it temporarily, so that it rests on the top edge of the window and mark the position of the flashing's top edge in the foam. Along this mark, cut a shallow horizontal slot in the foam face and push the top edge of the flashing into the slot.
Next, put self-adhering flashing over the jambs and head. For each jamb cut a strip of flashing about 8 inches wide and 10 inches longer than the rough opening height. Place each piece over the side flange so that it completely covers it, extending about 3 inches above the top of the flange and several inches below the bottom, overlapping the sill flashing. For the head, cut a piece about 8 inches wide and about 20 inches longer than the width of the opening. Use it to overlap the metal flashing, almost completely covering the vertical part. It also will overlap the pieces of self-adhering flashing at the jambs.
The procedure differs slightly for a wall with a concrete surface. In this case, the metal head flashing is not cut into the wall's surface, but simply lies flat against it. Self-adhering flashing overlaps the metal flashing as before.
— Pieter VanderWerf is president of Building Works Inc. (www.buildingworks.com), a consulting firm that helps companies introduce new construction products. John Gajda is principal engineer with CTL Group (www.ctlgroup.com), a construction materials research, engineering, and consulting company.