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For energy efficiency, when should windows be opened for overnight cooling?

We have a 2 story home with air conditioning in the Chicago area. In summer, we've done several things to improve our energy efficiency. Our a/c doesn't need to run during the day, even on the hottest days. The interior temp rises gently up to 76-80F and, if needed, the programmed thermostat causes the a/c to run overnight (to 76F) when electricity is cheaper. If we know tomorrow will be 85F+ we set the overnight set point to 74-75F temporarily. It's always cooler at night but the temp varies. On hot days it may be a low of say 75-80F overnight. On warm days the low may be 65-70F. We rarely have 80+F days and 55-60F overnight lows. It's usually humid in the summers. Naturally when the nights are cool and crisp, we'd like to get some free cooling. Trouble is the outside is usually more humid than the interior because the a/c dehumidifies when it runs. We are satisfied with the ventilation we get so for this discussion, the window settings are only for free cooling.

From experience, I look for an inside/outside temp difference of about 10F. I go outside to still air and assess the humidity. If the humidity is low by feel (we have no sling psychrometer) and the 10F is there, we turn off the a/c and open selected windows. If there's no breeze, we get some stack effect cooling but a breeze is better. We have exhaust fans but they consume electricity so we prefer they not run unless there's a ventilation need. Sometimes I regret the windows open because we wind up running the a/c later just to lower the humidity. By early morning we usually have the windows closed again unless the seasons have changed.

I've looked at the psychometric chart again and read the NOAA tables on relative humidity comfort index but they seem to recommend opening the windows when I already know from experience just boosts the humidity and later forces us to run the a/c. If someone has a proven rule of their own, I'd like to hear it. I know if we lived in a more arid climate, I'd have the windows open more often. I realize personal preference is a factor. Those indifferent to humidity effects already enjoy an energy efficiency advantage.

Asked by user-3549882
Posted Aug 14, 2014 5:42 PM ET


2 Answers

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Look at the dew point forecasts, not just the overnight temperature forecasts (Weatherspark.com has reasonable dew point projections.) If the overnight dew points are going to be in the 60s F opening the windows may result in lower comfort and elevated mold risk.

If the dew points are in the 70s F it means you 'll get almost no sensible cooling and BUCKETS of latent load.

Ventilating with air that has a dew point of 55F or less is pretty healthy & comfortable.

When outdoor dew points are between 55-60F is something of a guessing game as to whether the sensible cooling benefits will be undercut by the additional latent load, but it's not a mold & comfort risk the way 65F+ dew point air is.

I personally hold the line at an outdoor dew point of 55F, but then my sensible loads are quite small- YMMV.

Answered by user-1004076
Posted Aug 15, 2014 9:34 AM ET


i went to college for engineering, I am a pilot of many types of aircraft and have studied meteorology, and much more, have to know my local conditions for para sailing, sail boarding, soaring, flying a drone, RC, skiing, skating, hiking, ultralights, hang gliders and more...


You two are killing me.

I live with no AC. No mold. I open the windows for the summer season, and then operate one or two maybe five or ten more times a year. The doors are left open a few times.

I do NOT own a sling psychrometer. I do own an indoor outdoor temp and humidity monitor. I glance at it before selecting my shorts or pants.

IF you fly, you know that humidity is part of figuring a take off run length. High humidity and the air is less dense so the take off run is longer.

Anyway, I guess for some of you number crunchers it is critical mission to crunch these numbers daily to save a quarter or two daily on AC?


For me, I spend foolishly flying, diving, and playing in the humidity and sleeping in it too oh and building in it even.

We sure are all individuals aren't we.

In the 50's tonight.... here, closed a window............. so nice....

not buying the sling... hey remember when your earth science teacher slung one and splained how they work??

Answered by wjrobinson
Posted Aug 15, 2014 8:43 PM ET

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