Window sills water damage

June 11, 2021
The first two causes of stains

There are three common causes of water stains on windows:

1. Leaving the windows open.

The windows get left open, and water pours in through the window during a rain storm. This type of staining will often leave fairly uniform water staining across the window sill.

2. Condensation.

If it’s a wood window, you’ll see stains at the corners of the window sashes and the window sills if the stains are caused by condensation (photo, right). You’ll also find the worst stains on the windows that are most likely to be damp, such as in bathrooms or bedrooms just outside the bathrooms. The windows at the north side of the house will be worse than windows on the south side.

3. Leaking windows.

If water is actually leaking in to the house around the opening for the window at the siding, this will typically show up as staining at the corners of the window sills. The photo below left shows what a window sill may look like with minor water leakage in to the wall; the photo below right shows a window sill with major damage from water leakage. A home inspector could use a moisture meter to help determine if the stains are currently damp.

The first two causes of stains are fairly straightforward and easy to prevent; remember to close the windows before it rains, and lower the humidity in your home. Here are a few tips to lower the humidity in your home:

  • Turn on your bathroom exhaust fan during showers and leave them on for a half hour after every shower. If you don’t have a bathroom exhaust fan, get one. While the building code allows an openable window as a substitute for a fan, I don’t.
  • If you have a crawl space, make sure that a proper vapor barrier is installed on the crawl space floor.

The third cause of staining at windows, leaking water from the exterior, is the one that home buyers should be concerned about. A window can leak from just the slightest defect in flashing at the top, and unfortunately, it’s not easy for home inspectors to know if a window is going to leak just by looking at the siding.

If the flashing above a window is installed properly, all of the water coming down the siding will be diverted around the sides of the window. The windows that will be exposed to the most water are the windows that aren’t protected from rainwater by soffits and gutters – such as the window shown at left.

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