New Windows Are Wet

Q: We recently installed new double-pane, low-e, argon-gas windows in our house and they’re dripping with water. When the outside temperatures drop to -10C or -20C, condensation forms on the lower part of most windows like never before. The relative humidity inside the house is between 45%-55%. Is this condensation normal? Is something wrong with our windows?

A: The condensation you see is to be expected when indoor humidity levels are as high as you’ve got, this isn’t something to be ignored. My rule of thumb is not to worry until condensation gets bad enough to run down the windows and form pools. Then it's time to boost indoor ventilation to drop relative humidity levels to about 35%. This will usually stop damaging levels of condensation, though you might still see some misting along the bottom edges of windows. It’s not unusual for condensation to increase when new, better-sealing windows are installed because indoor humidity levels rise due to the reduction in drafts. This happens as the unintentional flow of fresh air into a house decreases, retaining more moisture generated by showering, washing and breathing.