The Window System

With windows such a common component of remodeling projects, and water intrusion and mold such a common source of lawsuits, brushing up on installation techniques is more important than ever. Taking a standard rectangular double-hung window unit as an example, we'll review key steps to ensuring window performance. Specifics vary among manufacturers; follow each firm's instructions for warranties to be valid.

Preparing the rough opening
The opening must allow the window unit to finish out square, plumb, level, straight and true. Each manufacturer suggests rough opening sizes; generally the rough opening should be about 1/2 inch larger all around than the window dimensions.

The following framing problems can affect window or flashing performance: Undersized headers may sag or deflect to distort the window flashings or tear flashings from fasteners. Wood shrinkage or warping can force framing out of alignment. Severe framing distortion can affect window operation and leak resistance and distress flashings and sealants.

Flashing, part one
After installing the house wrap, cut window openings in the wrap. The best pattern to use is an I or an inverted Y. Do not cut an X, which allows moisture to penetrate at the top. Cuts made from bottom corners toward the center and from top corners toward the wall’s top should extend 6 to 9 inches at 45-degree angles. Fold the side and bottom flaps into the opening and staple to the inside wall; fold the top flap up and temporarily fasten it to the outside wall.

Properly installed, flashing allows moisture to flow away from and over joints. Flashing should be at least 9 inches wide but preferably 12. Start with the sill flashing, fastening the top edge to the framing. The ends should extend about 12 to 16 inches past the window sides. Jamb flashings can be installed before or after the windows. Run this flashing beyond the sill flashing and above where the head flashing will intersect.

Shimming, fastening
Most aluminum, vinyl and clad windows have integral mounting flanges or nailing fins. In replacement installations, you might need to remove flanges with a knife and use jamb clips instead. If not, apply a continuous bead of caulk or sealant to the interior of the flange or to the perimeter of the opening. With a wood window, run the sealant along the sill horn, jambs and head jamb.

Insert the window into the rough opening from the exterior. Shim and adjust the window as necessary to achieve a plumb, square and level condition. Shims should be placed at equal distance opposite one another, top and bottom, and side to side.

Secure windows at the full perimeter with the equivalent of 6d fasteners on a maximum 16-inch center (16d is better for nailing through the casing of wood windows). Hinged and pivoted windows might require additional fasteners near hinge and pivot points. In each direction from all corners there must be a fastener from 3 to 10 inches away to prevent frame distortion or fracture of joint seals. For mechanically joined frames, apply seal along the seam at corners where mounting flanges meet. Apply a continuous seal at the head mounting flange.

Flashing, part two
To install the head flashing, embed the bottom of it over the sealant and the mounting flange. The flashing should extend beyond each jamb flashing. Fasten in place. Now detach the top flap of the house wrap and pull it down over the head flashing and flange. Pieces of flashing, at least 1 inch longer than the diagonal cuts in the house wrap, should be applied to the cuts. Be sure to cover the corners.

Interior, exterior seals
Spaces between the window unit and adjacent materials must be closed with a sealant, trim members or both to prevent the passage of air, dust and water around the frame. Inside, the space between the rough opening and the window frame should be filled with fiberglass or low-expansion foam insulation. On the exterior, leave enough space between the siding or other finish and the window for backer rods and sealants.