Bathroom windows will almost always "sweat" in the winter

When cold weather sets in, I invariably get a slew of questions from people worried about their windows “sweating.” There’s a perception, propagated by window salespeople, that condensation is a  sure sign that you need to buy new windows for your home. What’s the truth?

The photo above is of a north facing garden window in my master bathroom. In general, I keep the humidity quite moderate in my home but this window definitely shows more condensation than others. In this case, during a spell of temperatures in the low teens, a layer of ice even formed. Should I replace this window?

In fact, this is my least favorite window in my home. It’s a basic, double glazed vinyl window. The “U-value“, a measure of it’s insulation level, is mediocre, at about 0.5 (with U-values, the lower the better). I can feel cold convection currents drop off this window and the wood framing is uninsulated underneath (the installers ripped out all the foam I installed when they remounted it. Grrr.)

But again, the question is “should I replace this window?”

The fact is, most window upgrades aren’t worth it. This particular window would have condensation on it regardless if it were a top of the line unit or this one simply because, given sufficient humidity, condensation will always form on cold surfaces, and windows are always the coldest things in the room. Even though I run the bath fan when I’m in the shower, there’s always enough moisture in the air to cause scenes like this in cold weather.

In the bathroom, condensation is pretty much a fact of life. This vinyl window is probably a good choice because the vinyl won’t rot with repeated wettings. I still recommend wiping the windows down to prevent standing water or you’ll end up with mold, like I did. But with vinyl windows, even the mold isn’t so bad – a quick wipe with a baby-wipe and the mold is gone.

What about the ice?

I don’t worry too much about that too because this window is intentionally cold so I expect to see ice when it gets cold out. Yep, that’s because I have a nice heavy curtain inside the garden window. When that is closed at night, the overall insulation of the window is better than any low-e, Argon filled, wonder-window. So the ice is just showing me that the curtain is doing its job and keeping the bathroom warmth where it belongs.

Condensation on wooden windows is more problematic

This window is in my bedroom. It’s a top-of-the-line, double glazed, Argon-Krypton filled, low-e window from Canada. I’ve also got a triple glazed window on my north wall and it sweats too. But it’s expected even though these are literally among the best windows money can buy.

Bedrooms are naturally higher humidity than other rooms in the house because of plants and respiration. All that heavy breathing fogs the windows! In addition, I have cellular shades on these windows, so, like the bathroom window, the glass gets cold at night when the shades are down.

The problem is that these are wooden windows, and even treated wood is prone to long term moisture damage if allowed to stay wet or if repeatedly wet by condensation. So whenever I see water on the windows, I wipe them down. And if the wood is wet, I’m careful to dry it off. But I occasionally forget, and some of the wood is showing signs of wear and could use a new coat of water sealant.