EPA: Moisture, Condensation & Humidity in Homes

NOTE: This information is from the EPA Mold website and has been modified for Sunrooms & Replacement Windows by Ben Kripps, Owner of ClearView Sunrooms & Windows. For more information visit www.epa.gov/mold/index.html to learn about moisture in a home and mold.


A home's humidity level is too high if excessive moisture collects on windows. Inefficient old windows, however, may collect moisture even if humidity levels are normal. Excess humidity for a prolonged time can damage walls & wood trim or window frames especially when outdoor air temperatures are very low. Mold can become a health issue if it forms on any home surface including your windows (usually seen on old wood windows or doors). Excess moisture condenses on window glass because the glass is cold and it is hitting the moist warm air of the home.


Sources of excess moisture include overuse of a humidifier (both portable units and permanent furnace units), long or multiple showers times, indoor water features, hot tubs, fish tanks, boiling or steaming water, cooking, plants, washing/drying clothes and a basement seeping moisture/water (caused sometimes by overflowing gutters or improper downspouts). Holidays, especially when more people are in a home, can create excessive moisture levels. Insulated window coverings can form a cold pocket to promote condensation even if humidity levels are normal.


A tight, energy efficient home holds more moisture inside; you may need to run a dehumidifier, kitchen or bath ventilating fan, or open windows briefly if moisture is forming. A cool thermostat setting below 65 degrees in the winter can cause condensation. Try turning the thermostat up a couple degrees if you are experiencing condensation. The warmer the air temperature, the more moisture the air can hold. Four season sunroom temperatures should be set to align or be a couple of degrees warmer than the inside temperature of the home.


Use a humidity indicator to measure the relative humidity in your house. The American Society of Heating and Air Conditioning Engineers (ASHRAE) recommends these maximum indoor humidity levels.


Outdoor Temperature / Recommended Relative Humidity

Outdoor Temperature Recommended Relative Himidity
+ 20 degrees Fahrenheit 35%
+ 10 degrees Fahrenheit 30%
0 degrees Fahrenheit 25%
- 10 degrees Fahrenheit 20%
- 20 degrees Fahrenheit 15%


Source: Anne Field, Extension Specialist, Emeritus, with reference from the Association for Home Appliance Manufacturers (www.aham.org)