These simple rules can help you save money on air conditioning bills and stay comfortable in the summer:
- Is it hot inside?
- Is the outdoor temperature lower than the indoor temperature?
- Is the outdoor humidity lower than the indoor humidity?
Open the windows and let the fresh air in!
Sounds simple? It’s actually more complicated than it sounds. Temperature and humidity affect our level of comfort, so it’s important that both are factored into a decision to open the windows! The rate of body heat loss or gain is affected by convection and evaporation. Convection is the natural transfer of heat via air movement between your body and its surroundings based on the difference in temperatures. Cooling by evaporation occurs your body as perspiration evaporates from your skin. Humidity affects the rate that the body loses heat by evaporation. A higher the humidity level makes it more difficult to evaporate perspiration into the air. Therefore, for comfort a balance between air temperature and humidity is required.
How do you follow the rule?
- Is it hot inside? If you feel too hot the answer is yes! Is the outdoor temperature lower than the indoor temperature? This can be measure with an indoor and outdoor thermometer. Several manufacturers sell a combination thermostat that displays both temperatures which simplifies the comparison.
- Is the outdoor humidity lower than the indoor humidity? This rule is a little more challenging but equally important. Most common home humidity sensors measure relative humidity which cannot be used to compare humidity levels at different temperatures. The correct solution is to compare the absolute humidity of the two areas. The most common measure that is used is a dew point measurement. For effective cooling the outdoor dew point must be below the indoor dew point. Several manufacturers now make an indoor/outdoor weather station that will display both temperature and dew point simplifying the decision process. See
What is the Mother Nature challenge?
The key issue on opening windows is to manage the timing based on daily temperature and humidity changes. In the spring and fall the challenge is to know when to open the windows to cool the house down and then to close them before the house gets cold and uncomfortable from the low nighttime temperatures. During the summer, the goal is to maximize the night time cooling opportunity and avoid bringing in any warm or humid air. Open the windows when it cools down at night. Then, before the temperature climbs in the morning, close the windows. The challenge is that Mother Nature is fickle and changes the weather pattern almost daily. As an example, let’s look at the Atlanta weather patterns in July, 2009 and assume the air conditioner is set at 76 0. On July 18th the temperature dropped to 74 0 at 8pm and stayed below 76 0 until 11am the next morning so fresh air cooling was available for 14 hours. On July 26th the temperature didn’t drop to 74 0 until 5am and only stayed below 76 0 until 8am providing a 3 hour cooling opportunity. High humidity was an issue for 8 of the days even though the temperature was at 74 0 or lower. Free outside cooling is available in the southern city of Atlanta even in the month of July. The challenge is synching the opening and closing of windows with Mother Nature’s weather variations.
Is there a better way to cool with outside air?
Yes, there is a new alternative. Nature’s Cooling Solutions has developed an intelligent fan that continuously monitors outdoor and indoor temperature and humidity. When free cooling is needed and available it automatically opens louvers and brings in the fresh cool outside air and exhausts the hot indoor air. When fresh air cooling is not needed the louvers close and seal out the outdoor air. It will optimize the use of fresh cooling all the time whether you are away from home or sleeping the night away. See https://naturescoolingsolutions.com
