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Internal Condensation

Condensation and frosting are common complaints about windows. Sometimes the problem is light fogging on some windows; other times there may be persistent and heavy frost covering the glass. Many homeowners buy new windows only to find that the problem becomes worse.

Condensation occurs when water vapour in the air is cooled to the point where it condenses on the cold surface as water droplets or frost. The more humid the air and the cooler the surface, the greater the accumulation of condensation. One solution is to reduce the humidity levels in the house.

Solutions

If the problem is one of high humidity or condensation, the first step is to reduce the humidity level by controlling the amount of water vapour that goes into the air. The following suggestions will all reduce a home's humidity levels.

  • Do not store wood in the house.
  • Avoid hand-drying laundry in the house.
  • Disconnect any humidifiers
  • Cover exposed earth floors in basements or crawl spaces with a moisture barrier
  • Install a sump pump to remove excessive moisture from the soil under the slab.
    Fix all water leaks in the basement.
  • Do not allow any standing water in the house or against the foundation wall.
  • Make sure the ground slopes away from the foundation wall and that there are properly functioning eavestroughs around the house.
  • Ventilate kitchens and bathrooms during use.
  • Adjust your living habits to produce less humidity (cleaning, washing, cooking, etc.).

The following table shows the maximum levels of humidity at which there will be no condensation on double-glazed windows at certain outside temperatures.

Outside Air Temperature (C) Maximum Inside Humidity at 20 C (68F)
-30° or below ............................................... 15 percent
-30° to -24° .................................................. 20 per cent
-24° to -18° .................................................. 25 percent
-18° to -12° ................................................. 35 percent
-12° to 0° .................................................... 40 percent

It can be very difficult to accurately measure and maintain the recommended humidity levels. A simple approach is to let the house become your indicator.

If condensation starts to appear on the indoor face of any double-glazed windows (except those in the kitchen and bathroom), you have found the balance point. Occasional condensation does not pose a problem. Excessive condensation or frosting is an indication that you should reduce moisture production or reduce humidity levels by increasing ventilation.

(Excerpt from Keeping the Heat In, Natural Resources Canada)