During cold winters, there are big temperature differences between inside and out. In a well sealed house, moisture collects and condenses on the glass, causing the condensation you see.
Glass, a notoriously poor insulator, will be responsible for a large amountof the heat lost in your home. You will be able to feel the temperature difference between the glass and interior walls, the glass and surrounding air will be much colder. Since cold air has a lower dew point than warm air, the glass surfaces in your home will be the first to show signs of condensation. That doesn’t mean it’s the only place it exists.
As building technology and energy efficiency improves, our homes become increasingly more airtight. Which is great for our heating bill, but may mean that damp air gets trapped inside more often than it used to.
Condensation is unsightly, but that's not the only problem. Nobody likes window fog blocking the view. But the problem goes deeper than that—if condensation is a chronic occurrence in your home; chances are that you have excessive humidity. If water is accumulating on glass, it may also be accumulating on other harder to see surfaces such as wall and roof cavities. If left uncontrolled, excess moisture can have serious consequences, including:
You can determine how much moisture is in your home with an inexpensive Hygrometer that can be purchased from your local hardware store. Place the Hygrometer in your home for a few days and record the readings each day. Compare your data to the chart below. As outside temperatures drop, the indoor relative humidity level of your home should decrease. For homes with windows equipped with insulating glass, the University of Minnesota Agriculture Extension Service reports that the following humidity levels can be maintained in the home without causing window condensation. If your home’s relative humidity is higher than this chart, please consult Excess Moisture: Causes and Cures for tips on identifying and eliminating sources of excess moisture in your home. Outside Air Temperature with ideal inside relative humidity for 70° F (21° C) indoor air temperature:
- 20° F (-29° C) outside should be 15% to 20% humidity inside
- 10° F (-23° C) outside should be 20% to 25% humidity inside
0° F (-18° C) outside should be 25% to 30% humidity inside
+10° F (-12° C) outside should be 30% to 35% humidity inside
+ 20° F (-7° C) outside should be 35% to 40% humidity inside
Excessive humidity can actually be an issue in newer, energy-efficient homes that don’t “breathe.” An inexpensive hygrometer should be able to tell you if your humidity levels are in check.