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Will your new windows let in more than sunlight? Leakage around windows causes an array of problems, regardless of window type. The problems occur in both new construction and the replacement market. Water can cause rot in any wood-frame house and can supply the moisture needed for mold growth. Home inspectors routinely find that window leaks have rotted the house structure - wall studs, flooring, floor joists, even the sills supporting the house. The primary cause of leakage around windows and doors is improper installation. The problem is so widespread that the American Society for Testing and Materials, now ASTM International, has developed an industry standard for proper installation - one of the few times in its 125-year history that it has developed standards for the installation of building materials.
Using the ASTM standards, the American Architectural Manufacturers Association has developed a training and registration program for window and door installers. After passing a test, they are registered as installation masters. While it would be desirable to have all windows and doors installed by AAMA-certified installers, fewer than 1,000 of the estimated 125,000 door and window installers in the country are trained and certified. How can you tell if your windows are installed properly? Hundreds of window and door manufacturers across the country have developed products with specific uses and installation instructions. In all cases, the manufacturer's instructions must be followed. The AAMA training manual contains five different details for installing replacement windows and 17 variations for windows in new construction. However, there are some general guidelines that are used in most cases. While nearly all siding materials are waterproof in themselves, wind can blow rain past joints, cracks and cut ends.
Therefore, it is essential that a waterproof membrane or weather barrier be installed behind the siding (even brick veneer), so that water penetrating the siding cannot reach the interior of the wall. This membrane can be roofing felt, house wrap or foil-faced sheathing boards with sealed or taped joints. Because windows penetrate this waterproof layer, the joints between their frames and the weather barrier must be protected with a combination of flashings and sealants. Flashings may be rigid or flexible, with or without an adhesive backing. Paper flashing, consisting of two sheets of paper laminated with asphalt and reinforced with fiberglass yarns, is common. Some have a polyethylene coating, and others have a bitumen-type core between sheets of polyethylene or polypropylene.
Any flexible flashing may have an adhesive backing. Rigid flashings may be metal (galvanized steel, aluminum, copper), plastic, vinyl or fiberglass. Regardless of the material used, all top, side and bottom flashings should be a minimum of 9 inches wide. The flashing should be shingled so the lowest piece is overlapped by the section above it. The sill flashing is applied first, extending at least 9 inches beyond the ends of the sill. The side jamb flashings are applied next with the bottom overlapping the sill flashing. The head flashing is applied last, and it overlaps the tops of the jamb flashings and any window mounting flange and is tucked under the weather barrier on the wall surface above. Sill pans made of rigid flashing material are used under some windows.
They should be sloped toward the outside of the structure, and the height of the ends and back of the pans will vary with the type of window or door being installed. The back and ends of the pan should be caulked to the jamb. The bottom of the pan should be caulked to the sill, but at least two breaks in the caulking bead should be provided so that any water getting under the pan will not be trapped. Sealants are used to prevent air and water movement around joints between the opening and the window or door. When they are visible, they also contribute to the finished appearance of the installation. The sealant or caulk should be selected according to the materials being sealed. Acrylic, butyl, urethane and silicone-based caulks are available.
All holes, nails and other penetrations of the flashings should be sealed. Any siding trim around the opening should be sealed to the flashing. Replacement Windows For replacement windows, the original jamb is often left in place and the new window sealed to it. The original jamb must be checked carefully to be sure there is no leakage around it. A new window installed in the original jamb will not eliminate existing leaks. Caulking the trim to the siding is not a solution to the problem. There are several types of replacement windows. Block-frame units may be installed from either the inside or outside of the house and sealed to the original jamb. "Flush-fin" replacement units are usually installed from the outside, and are also sealed to the original jamb. Installing a mounting-flange replacement window requires removing the original window entirely.
It may also require the removal of some siding to flash the new unit to the wall properly. If the cutout in the siding is at least four inches larger than the new window, a self-adhesive flashing can be used to seal the existing weather-resistant barrier to the mounting flange. While complete removal of the existing window will require siding work and repair of the interior finish, it is often more effective because the space between the rough opening and the jamb can be insulated and sealed. Air that seems to be blowing in around the window often comes through this space because it was not insulated and sealed in the original construction. If you are having new windows or doors installed in your house or in new construction, insist that the installation meet the ASTM standard. Henry Spies is a home inspector based in Champaign, Ill.