Why are there puddles inside my window frames?

The other day someone asked me, “Robert, I know Seattle is wet and all, but why do I have puddles inside my windows?”

Unless you have experienced a window and window failure and the water is actually running in and behind the window, the likely cause is condensation on the inside of the window and frames. This is actually very common in older windows with metal frames. With our humid Seattle air, it’s very common on cold, humid days to see water condensing and collecting inside old or poor quality windows.

What happens is that when the weather is cold outside and you have warm, humid air inside your home, the water will condense on the cold edges of the window. This is exactly the same as when you fill a glass with ice water on a hot summer day. The warm, humid air comes into contact with the cold glass and the water condenses into water droplets that run to the bottom of the windows, forming the puddles you are seeing.

In this case, although the condensing water runs down the window, it collects and eventually soaks up the window sill. This can lead to two main problems. The first is the deterioration of the window frames. Over time, repeated moisture can cause frames to rot and disintegrate. The other problem, and one that is extremely common in the Seattle area, is mold growth.

Mold around windows is commonly seen as patches of blackheads around the more humid areas. While you may not think that this small amount of mold is bad for you, consider that this is simply the mold that you can see and that they send out spores at night, usually when we sleep in our bedrooms. Sounds nice doesn’t it?

What can you do about condensation on your windows?

The first thing you can do is lower the relative humidity in your home. I know it can be difficult to do during our wet Seattle winters, but there are a few places to start. Check that the bathrooms are properly ventilated. Showers generate a lot of moisture and it can spread throughout your home.

Inspect your dryer vent and confirm that it is properly connected and working. You can also consider using a dehumidifier to reduce the total amount of moisture in the air.

The most direct way to solve the problem is to stop condensation at the source; the place where the water condenses. Replacing your low-efficiency windows with high-quality windows will usually solve the problem. Older windows let in cooler outside and increase the humidity problem. Newer vinyl windows with insulated frames reduce or eliminate condensation problems. Also look for windows with some kind of “warm edge technology.”

Traditional windows have metal spacers that separate the glass panes. This metal acts as a short circuit that transfers the cold through the glass to the interior and produces condensation around the edge of the window frame.

Whatever you do, I highly recommend that you take some steps to prevent condensation from repeatedly forming on your window sills. Even if you live in Seattle, you shouldn’t have puddles inside your windows.