New windows, more condensation

Q: What can I do about water that forms on the inside surface of our new windows? When outside temperatures dropped to -10C or -20C this winter, condensation would form on the lower part of most windows like never before. The relative humidity inside the house is usually between 45%-55%. Is this condensation normal? Is something wrong with the windows?

A: The condensation you see is normal when indoor humidity levels are as high as you have and it gets very cold outside. But 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 stop damaging levels of condensation in most cases, though you might still see some misting along window bottoms. It’s not unusual for condensation to increase when new, better-sealing windows are installed because indoor humidity levels rise. This happens as the unintentional flow of fresh air into a house from drafts decreases, retaining more moisture generated by showering, washing and breathing.