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We recently bought a 1939 wooden house in California. I don't know if the windows are original but they sure aren't new. We're in a pretty mild climate, so we might get a morning frost from time to time but that is as cold as it gets. We keep the thermostat around 63° (17°) during the day and off entirely at night -- the sellers' thermostat was broken and wouldn't move off 70° (21℃) so I'm guessing they kept the house a lot warmer.

I've noticed that in the morning all of our windows are fogged. That seems to clear over the day and we don't get pooling or drips. So I'm wondering if we need to do more to reduce the humidity?

I read this question, about exterior condensation: How much condensation is okay on storm windows?

But I'm concerned about indoor condensation.

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If the windows are original to the house and are single pane and not insulated glass units, you are going to have a lot of condensation on the inside on cold days. If it only fogs and don't run off, I consider that normal for non-insulated glass.

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Aside from the condition of the windows themselves (wooden ones will tolerate moisture worse than vinyl certainly) the thing to watch for is indoor relative humidity going above 60% which puts it in the range where pathogens and mold can thrive. Choosing no climate control overnight can cause this quite often since a comfortable 50% humidity at a daytime temp of 75F (dewpoint 55F) can quickly become a dangerous 80% RH if the indoor temp is allowed to drop to 62F (same dewpoint, 55F). I would recommend observing with an inexpensive humidity monitor and using either a dehumidifier or simply heating the home to prevent the RH% from reaching a dangerous level. The possibility of condensation in hidden areas (i.e. in the exterior walls) is also quite possible if there are particular areas that allow air to escape (warm moist indoor air will condense on the cold insides of the walls before fully leaving the home). If your home is "Drafty" this can be another huge risk.

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That it is fogging up means that the window pane drops in temperature to below the dew point. Your windows could be good window, they just aren't enough to prevent that outcome in your climate.

The better solution is to add more layers. If the window is a single pane, replace it with a double pane window.

If the window is a double pane, add a storm window to the exterior. Storm windows condensate on the inside side, but drip to the drainage tray of the window. They increase the insulation of the window by a lot by limiting cold air from the outside.

If the window already has a storm window you could upgrade to a three pane window on the inside. Insane expensive yes, but effective.

Of course the cheap solution is to get a window sealing kit, and cover the outside and/or inside with plastic. Temperature wise a layer of plastic works as well as a single pane window. Just more temporary.

Also make sure you are actually using the ventilation fans in the kitchen and bathroom. Properly used ventilation around water sources will keep the humidity in check in most houses.

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