Volume 9, Issue 1 - January 2008
It’s Fairly Simple
Even the best-designed product can fail to prevent excessive water penetration if installed improperly. The solution focuses on the integrity of the “drainage plane” of the exterior wall, which consists of a weather- resistant barrier (WRB) installed behind the exterior cladding and coupled with flashing at the base of each wall. The WRB provides a path for rainwater that penetrates the cladding system to escape.
The essential principle of window installation is that the window units work together to form a fully integrated and effective drainage plane. Flashing (applied correctly) and sealant are needed to integrate the window with the drainage plane and divert water away.
To provide basic guidance on integrating the most common window-mounting configurations with the drainage plane properly in the most common construction situations, AAMA has developed two installation standards for residential construction up to four stories in height. One covers the installation of new windows with mounting flanges in a stud frame wall, while the second covers installation of replacement windows over the residual outer framing of an existing window. Both address the common configuration of windows with integral mounting flanges or fins, and emphasize techniques for fully sealing the window to the WRB.
The window should be placed in the opening immediately after the sealant is applied and should be secured all around the full perimeter of the flange with corrosion-resistant fasteners, whose heads are wide enough to cover any pre-punched holes fully.
Careful inspection of existing conditions assumes a prominent role in ensuring weather-resistant installation of replacement windows. Sufficient sealant should be used to ensure a proper seal between the mounting surfaces of the old frame and the flush fin of the new window, taking care not to seal up existing weep holes at the sill. Screws used to attach the new window should penetrate both the new and old windows’ head and jambs and anchor into the surrounding wall framing. A secondary seal should be applied between the fin and the adjacent wall materials around the perimeter of the flush fin.
Finally, expanding aerosol polyurethane foam should be used at the interior to seal between the window frame and the rough opening. It is important to select sealants rated for this application; otherwise, the pressure exerted by the expanding foam while curing could distort the window frame and compromise smooth operation.
The System Viewpoint
John Lewis serves as technical director for AAMA. He may be reached at firstname.lastname@example.org. Mr. Lewis’ opinions are solely his own and do not necessarily reflect those of this magazine.