What makes water condense on the interior of the windows at night when it's cold? [duplicate]

Question

While staying with some relatives, I noticed that all their windows experience terrible interior condensation at night and am wondering about what they can do to prevent it.

The house is kept to 70 degrees with gas-powered hot water baseboard heaters, and the windows are vinyl double-paned units. The house is 2x4 frame construction with interior drywall, exterior stucco, and I'm being told that the walls are uninsulated(!!!).

They live in a colder part of the high desert southwest where the humidity is very low during the day but rises at night, and where the temperature gets down into the single digits at night during the winter. During this season, the interior of their windows become entirely covered with condensation at around 8 or 9 PM every night without fail. It's kinda weird. What's causing this, and what can they do about it?

Answer 1

There are two things working together to make water condense on the windows. The house is humid, and the windows are cold (even well-insulated windows will usually be the coldest thing in the house because of the low R-value compared to walls and ceilings). To prevent the condensation, you can remove the humidity or make the windows warmer.

Removing Humidity

You have several options for removing humidity.

1. Ventilation system

   Running exhaust fans in areas that produce humidity (like the bathroom while showering) will lower the overall humidity in the house.

2. Forced air heating

   Forced air heating systems wick moisture from the air. If you have a forced air system at your house, and your relatives have radiated heat from hot-water baseboards, this may help explain why they have condensation on their windows and you do not.

3. Dehumidifier

   Dehumidifiers live up to their name. This one claims it can remove 70 pints of water from a 1400sqft house every day. That's almost 9 gallons of water taken out of the air each day. Just remember to put it near a drain.

Making the Windows Warmer

The other option is to make the windows warm enough that the water does not condense.

1. Plastic

   Placing a sheet of plastic over the inside of the windows will insulate the windows better. If the plastic seals all the way around the window frame, it also prevents the humid air from reaching the window in the first place.

2. Space Heater

   If there are only a couple of windows with serious condensation issues, a space heater directed toward those windows should heat the windows enough to lessen or eliminate the condensation.

Answer 2

Not to be rude but just head over to wikipedia and checkout condensation, dew point etc.

Two things they can do:

Lower their humidifier. I live in CO and keep my house around 35% RH in winter with a humidifier. Without a humidifier it would drop to around 15%RH.

Get better windows. Triple pane windows are not uncommon in mountain/very cold climates. Perhaps they have single pane or old windows. I have double pane and do get some condensation if I don't lower the humidifier when it gets super cold. Wood casement windows are the best insulated windows, also most expensive but there are other factors.

BTW, you can get a humidity sensor at HD or amazon for $10. Highly recommend.

Answer 3

When warm moist air comes in contact with a cold surface condensation forms.

Knew someone with this problem. The problem they have is most likely the windows are too thin. Double pane windows come with different sized gaps.

Lots of info on different ways condensation can form on the Wikipedia page http://en.wikipedia.org/wiki/Insulated_glazing