How to repair a bathroom window with wood rot — and preventing it in the first place

When I looked closely at the photograph, I could see a depressed part of the sash profile in the lower right corner. Water vapor from hot showers no doubt condensed on the glass pane, it then rolled down the window, and finally found a tiny crack to enter the wood at the sash corner.

I would guess that the problem has been going on for some time, and the weight of the glass pushing down on the lower, horizontal sash frame caused the two pieces of wood to separate at the corner. As the crack got bigger, more water entered faster and deeper, accelerating the rot.

Fortunately, this sash can be salvaged with a small amount of effort. Jeff may prefer to wait for warmer weather to do the job, but he can start a repair in the winter if he has the skills to cut a piece of plywood the same size as the sash so he can pull out the damaged sash to work on it. In warm weather, I’d just use a piece of thick cardboard to close up the opening while repairing the sash.

The first thing that needs to be done is to clean the mildew off the sash; a fan should be used to dry the wood at the corner. Once the wood is fairly dry, I’d use a long squeeze clamp to see if a moderate amount of pressure will close up the gap between the two pieces of wood that make up the corner.

Just in case the corner can be drawn tight with the clamp, I’d squirt some yellow carpenter’s glue in the crack and use a Popsicle stick or other thin piece of wood to spread it around in the crack. I’d then start to tighten the clamp hoping the corner draws up tight. If it does, I’d leave the clamp in place for a few hours.

I’d place the bottom pad of the clamp about 1 inch from the outside corner of the sash. I’d want to be able to drill a pilot hole in the bottom of the sash about ½-inch from the outside corner so I could install a 3-inch-long stainless-steel wood screw up through the horizontal sash frame member into the vertical frame member while the clamp is in place. Carefully drill the pilot hole so the screw stays centered in the wood frame.

If the corner can’t be drawn tight with the clamp, the crack can be filled with wood epoxy. I’ve got two videos at AsktheBuilder.com that show how to mix, apply and sand this wonderful material.

Another option is to replace the window, but this is likely to be costly. A year ago I broke the glass in one of my own up-down sash wood windows while cleaning it. I discovered it would be cheaper to buy a new sash from Andersen Windows than to go to all the hassle of ordering a custom insulated glass pane, removing the broken glass, installing the new glass, and then trying to match the custom exterior color.

Nevertheless, much to my surprise, the simple small window sash I needed cost in excess of $350. (That was a year ago; thanks to the raging inflation in the U.S. economy, it’s likely to cost more now.) I decided to wait until the glass starts to fog up before I replace it.

This rot problem could have been avoided. If you’re in a similar situation, all you have to do is place an old towel in contact with the bottom of the window glass each time you shower. The towel will capture the dripping condensate before it could get to the wood.

After getting dressed, use the towel to wipe off the glass pane to ensure it’s dry. Any other visible liquid water on the window should also be dried off with the towel.

The towel should then be hung up to dry. The door to the bathroom should be left open to lower the overall humidity in that room as rapidly as possible. The reason some bathrooms have plentiful mildew growth is simple: There’s a great chance that condensate fog is forming on all the walls and surfaces, just as you see on the mirror.

The mirror in the bathroom is probably the same temperature as the walls and ceiling. Condensate fog forms on all the surfaces that are the same temperature, or cooler, than the mirror. You can’t see the fog because the walls and ceiling are opaque, but trust me, it’s there. This condensate is the water mildew needs to grow and thrive.