Last week, GBA published a photo of a recently installed window in an new house under the headline, “What’s Wrong With This Picture?”
The photo showed the window from the interior. Some of the flexible flashing material was visible on the rough sill and the rough jamb.
The list of problems outlined below was prepared by James Steacy of IBACOS.
Not quite right
Before a window is installed, the sill of the window should be flashed to help protect the sill and to move any water that seeps behind the window back outside to the water-resistive barrier (usually housewrap or asphalt felt). In this case, the builder attempted to integrate the wall drainage plane with the window and window sill flashing, but did so incorrectly.
This type of sloppy installation incurs risks. Possible problems include water intrusion around windows, mold and rot of window framing and exterior sheathing, staining of drywall or window trim, and drywall decomposition.
Issue 1: The flashing is not integrated with the WRB
Ideally, the flexible flashing visible on the rough jambs and the rough sill would be integrated with the water-resistive barrier (WRB). Since no housewrap is visible on the rough jambs or the rough sill, it’s hard to know whether the flashing was properly integrated and lapped.
Issue 2: The flashing material is not suitable for this application
To protect the rough will, installers have two choices: they can install a manufactured sill pan, or they can make a site-made sill pan.
This installer did not include a manufactured sill pan; yet the flashing product chosen for the site-made pan is the wrong type. The installer used straight peel-and-stick flashing tape instead of a product like DuPont FlexWrap that conforms to three-dimensional corners.
Straight flashing (like the product in the photo) requires a cut on the exterior to enable the flashing to make the bend at the sill-to-jamb location. This open cut and fold typically ends up with a hole in the corner that can be a potential leak point. Products like DuPont FlexWrap, on the other hand, can be pulled and wrapped around corners and don’t require any cutting.
Issue 3: The installer forgot to include an interior back dam at the sill
Without an interior back dam, water that reaches the sill can enter the house.
If you don’t want to install a back dam, the site-made sill pan must include a slope to direct the water outdoors. This can easily be done by installing a length of lap siding (clapboard) on the rough sill before making a site-made sill pan.
The right way to flash a window
Here is the usual installation sequence:
- Install the water-resistive barrier (WRB) on the wall over the window rough opening, with upper layers lapping over lower layers at least 6 inches.
- Cut the WRB to allow access to the window opening. Fold back the flaps on the sides and top of the window to expose the rough opening. Cut the WRB material at the edge of the sill rough opening.
- Install flexible sill flashing material or a preformed (manufactured) sill pan over the WRB on the rough sill and behind the WRB plane on the sides (jambs). Sill flashing should include an interior back dam (or should be sloped outward) and should extend up the sides of the rough opening to form end dams at the jambs, unless a preformed sill pan flashing with integral back and end dams is used.
- Lap the WRB material at the jambs (the vertical sides of the window opening) over the end dam of the sill flashing.