A: The gap you’re worried about is supposed to be left open, without caulk. If water has been getting inside or behind the wood siding, some other detail of the installation is to blame. The most probable cause: flashing above the window that slopes toward the house rather than away from it.
Windows need to be installed so water runs down and out when wind pushes rain between pieces of siding or around the window frame. Construction details vary depending on whether windows or housewrap are installed first, or, in retrofit situations, what the existing conditions are. And different window manufacturers recommend different installation details. You can see illustrations for some of the variations by doing a Web search for “window flashing details.”
But although some of the details vary, the essential strategies are always the same: Start at the bottom and overlap each higher layer, just as when shingling a house. And avoid trapping water next to the sheathing (the plywood or other material nailed directly to the studs).
So the bottom of the window opening always needs to be covered with a waterproof sill pan, often made of peel-and-stick rubberized membrane. This barrier needs to extend inward for the depth of the window opening and, on the outside, wrap down a few inches from the outside edge as well as up several inches on the sides. More self-stick membrane goes up the sides, overlapping the waterproof barrier at the bottom. There are sometimes two layers of this: one before the window goes in, then a second that covers the nails holding the window in place. Across the top, there is more membrane, which overlaps the tops of the membrane pieces up the sides. The top membrane has to be long enough to extend past the window trim, so it can stop any water that gets through the joints. Installers often stick this top membrane directly to the sheathing with a flap of the housewrap overlapping it.
Then they should install rigid cap flashing — the part at the bottom of the gap you see. This is often called Z flashing, because, from an end, it looks similar to how that letter looks if tipped sideways. One flap of the flashing extends upward and is mostly out of sight, behind the siding. The main section runs almost horizontally over the top edge of the window trim, and the final bend hangs down from the top of the trim. Ideally, the top flap fits under the housewrap and the siding, so any water running down the wall will hit the flashing and flow out, away from the window. Having membrane sealed to the sheathing over the window is extra insurance, because it makes any water that gets past the housewrap also flow down and out.
But to make the flashing function as intended, there needs to be a gap between the bottom edge of the siding and the mostly horizontal part of the flashing. Caulking that gap, which is typically ¼ - to ½ -inch wide, traps water against the wall.
If water was getting into your house or behind the siding before you added caulk, it could be because the flashing wasn’t installed properly. If the flashing was installed over the housewrap, for example, it could be trapping water that runs down the wrap. The part of the flashing that extends over the window trim also could be sloped incorrectly. This part of the flashing should angle slightly away from the house, by 10 to 15 degrees. If it’s sloped toward the house, it could be channeling water into the siding.
If your windows are installed incorrectly, or if you aren’t certain whether they are, call a home inspector to check with a moisture meter whether water is getting into the wall and, if so, how far that extends. The inspector should also poke into nearby siding to determine whether it’s rotting and check whether the flashing slopes correctly away from the house.
If an inspection confirms there is a problem, you might also want to check with the manufacturer of the windows and ask for the installation instructions. Armed with this information, call the contractor who did the work again and, if necessary, the store. If they don’t respond, you could file a complaint with the Virginia Department of Professional and Occupational Regulation or get help from a lawyer. But for something like this, you’re probably better off calling in another contractor to fix the problem before it leads to further damage — assuming, of course, that the damage isn’t already extensive.
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