Window leaks aren’t uncommon, especially in older buildings. There are many factors to consider when analyzing the cause of a window leak, so before repair can take place, a diagnosis is necessary. Let’s explore some of the main reasons that windows leak and what you can do about them.

In order to do this, find the main source of the leak:

1. Water is coming in around the window.

The leaks could be anywhere around the window – top, sides or bottom. Often, these leaks happen because the caulk or sealant around the window has gotten old and cracked, allowing rain to enter the house. The fix for this situation is simple – peel out the old caulking and replace it with new window caulking.

2. The window is leaking at, or near, the top.

Leaks at the top of the window can be a sign of mistakes made during installation. The flashing might be improperly installed, for instance, or it might be missing altogether. If the house wrap isn’t correctly installed, that could cause the window to leak, too.

The remedy for this one is much more complicated than adding caulk. You may have to remove the siding to check the house wrap and flashing tape around and above the window. The purpose of house wrap is to keep any water that gets behind the siding moving down and out. To do that, the product has to be shingled, with the wrap that’s installed higher on the house overlapping the lower band. Water will run down and over the seam that way. If the lower band overlaps the upper band of house wrap, water will run down to that seam, then into the seam and behind the wrap to the wood framing, where you don’t want water to go.

At the top of the window, the house wrap should be folded up onto the exterior of the house and taped up before the window is installed. Once the window is installed, put flashing tape over the nailing fin, then fold the wrap down over it, completing the shingling process.

You might want to add head flashing as well, depending on how much weather the window is exposed to.

3. The window is leaking at the bottom.

There are a couple of reasons why windows leak at the bottom. The fix for one is easy, the other is not.

If the windows have metal or vinyl frames, they have drain holes near the bottom edge of the frame. These holes are designed to allow the  water to escape the frame, but they can become clogged. Clean out the holes with a wire coat hanger or something similar.

The more serious problems that cause windows to leak near the bottom are related to the sill pan or the slope of the window sill. All windows should have a sill pan that allows water to drain to the exterior of the house, and the sill needs to slope away from the window. If either of these is missing or wrong, you may need to reinstall the window to correct the problem.

4. Water is leaking between the panes of a double-paned window.

The most likely cause of this kind of leak is that the glass seal has deteriorated and rain is working its way between the panes. This situation doesn’t mean that water will eventually make its way into your house. However, it indicates that the gas that’s supposed to be between the panes to provide an insulating factor is gone. You can live with this situation, but installing new windows will increase the home’s energy efficiency.

5. Condensation builds up on the inside of the window.

Condensation isn’t a sign of a leak. Instead, it may indicate there’s too much moisture in the house or that there’s not enough insulation around the window. Run the air conditioner or a dehumidifier to reduce the humidity inside the building.

Repairing a window leak can be a quick fix or a major undertaking, depending on the situation. Spend a little time diagnosing the problem before jumping to conclusions about what fix is needed.