When Is the Humidity Low Enough to Open the Windows?
Relative humidity is such a confusing way to talk about moisture in the air. I’ve been hammering on this point for a decade in this blog, covering it in several different ways. (See the related articles below for more.) But here’s an angle that I haven’t covered before. The weather is cooling off here in Georgia, so we don’t have to run our air conditioners all the time now. We still have humid days, though, so many people have the question: When is the humidity low enough to open the windows?
I opened the windows to 90% relative humidity
It’s humid and warm here now, but a week ago we had some nice cool weather. Low temperatures in the low to mid 50s Fahrenheit (10-13° C), highs in the 70s Fahrenheit (25ish Celsius), and sunny. Gorgeous fall weather! About a week ago I opened one of my basement windows and put a fan in it to blow outdoor air into my house. And the outdoor relative humidity at the time was about 90%.
Opening the windows means letting outdoor air into your house. In a humid climate, you don’t want to do that when the outdoor air is more humid than indoor air, right? But the trap many people fall into is looking only at the relative humidity. They see the outdoor relative humidity at 70% and say, “Oh, I should keep the house closed up now because that’s too humid.”
There’s a better metric
And that’s wrong! Well, it might happen to be right accidentally, but you can’t look at the relative humidity by itself. You also need to know what the indoor and outdoor temperatures are. But there’s an even better number to look at, and fortunately, weather apps show this number. It’s the dew point.
To decide whether it’s OK to open the windows or not, the dew point is a much better metric because it tells you which air has a higher concentration of water vapor. Let’s look at the example of when I opened the windows last week. Here were the conditions:
Outdoors: 58° F, 90% relative humidity
Indoors: 72° F, 64% relative humidity
It looks like the outdoor air is more humid. 90% is much higher than 64%, right? Nope. The temperatures are different, so you have to account for that. The dew point tells the truth here.
Outdoor dew point: 55° F
Indoor dew point: 59° F
By bringing in outdoor at 90% relative humidity, I actually dried out the air in my house. When the outdoor air entered and mixed with indoor air, it warmed up. I watched the relative humidity on my indoor data logger drop from 64% down into the 50s in just a short while with the fan running.
So, don’t use relative humidity as your basis to judge when the humidity is low enough to open the windows. Use dew point, and you’ll get it right. A good reference point for dew point, by the way, is 55° F. That’s the dew point for conditioned at 75° F and 50% relative humidity.
If you don’t want to look up the dew point, a good benchmark to use is an air temperature (dry bulb) of 60 °F. With the outdoor relative humidity is 100%, the dew point of the air you bring in will by 60 °F. Most of the time, the outdoor air is less than 100% RH, so the dew point of the air will be less than 60 °F.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
Make Dew Point Your Friend for Humidity
The Problem with Relative Humidity
What Is the Relative Humidity When It’s Raining?
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This Post Has 30 Comments
Another excellent article — this topic is so important and you make it so easy to grasp. It’s not just for the humid south. Many people open up the house as soon as the outdoor temperature drops below the inside temperature and proceed to load the home with moisture. And then have to remove it when they return to close house conditions.
Pat, thanks for mentioning using temperature as a guide. That’s another wrong way to do it, as you stated.
Our AC is turned off and windows are opened when the outdoor dew point drops below 60F.
That’s about what I do, David.
I let my ERV bring fresh air in versus opening the windows
That’s another way to handle it.
My house doesn’t need cooling (mechanical or natural) when it is in the 50’s outside. So why open the windows? Are you after the dust or noise?
Neither, Roy. I want to hear the birds. ;~)
Well, that’s not the main reason. If the temperature stays in the 50s for a while, yeah, our house stays cool enough, too. But when we go from summer weather to fall weather, and the daytime highs are in the 70s, the house can feel a bit stuffy.
Allison, funny you mentioned that. I spent a few days last week at my kid’s house near San Diego where they don’t need heating or cooling most of the year. The windows were open at night and I couldn’t sleep because of the hoot owls screeching and the dogs barking. We even had a rooster crowing in the early morning hours. I guess I just don’t like “nature”.
So if your house is stuffy, doesn’t that mean you need more mechanical ventilation?
Yes, Roy, as a matter of fact, I do need better mechanical ventilation. Currently, we rely on the exhaust fan in the spray foam attic to bring outdoor air into the house and the occasional running of our bath and kitchen exhaust fans. But I have a beautiful new Zehnder ERV sitting in my basement mechanical room waiting to be installed. As soon as I get the book I’m writing done, that’s going to be one of my highest priorities.
Allison: Then you can remove all of your screens, caulk your windows shut, and rely on your HVAC system to provide optimum comfort 😉 You can probably get a cell phone app for the bird noises.
My wife and I “camped out” on our upstairs screen porch this year with the brands. They loved it; we had no idea we had so many dogs in our neighborhood. I think relying on opening windows is a very archaic and inefficient method – unless you live in Southern CA. Even then, the closing-when-you-leave (or closing-at-night if you live in a normal crime area) effort makes it questionable. In the Mid-atlantic (where I am), it is utter nonsense. If you want to experience the outdoor climate (even with a fireplace in Winter) well, that’s what screen porches are for.
I live in SoCal and in Europe before that, where opening windows was considered quite normal albeit oldschool. This article about dewpoints is excellent for when I want to humidify or dehumidify the house, heat it or cool it, or some combination. Closing the windows when we leave is not much more effort than opening them. In a strange way I feel good about opening windows, just like I do about using my car’s horn, or functional buttons on a suit sleeve: They seem to exist for a reason. To me this feels like a feature of the house.
It’s so simple! Just pull up your psychometric app and compare enthalpies between indoor conditions and outdoors. I personally like it when it’s around 30 BTU/lb or lower.
Excellent article Allison! I preach opening windows in shoulder seasons when the temps are lower esp. at night to help flush a home’s extra heat which may occur in passive solar homes when the sun starts to dip further south in winter and below the overhang, but you really sum up the details by discussing dew point. Thanks. I just added a new weather gadget to my birthday present list that prominently shows the dew point (not just temperature and humidity) of both inside and outside so I can see it without having to pull out a computer, tablet or smart phone.
Hey Allison, a long time ago we were teaching HERS class at Southface in early December. It was late afternoon, cold (~40F) and had just finished raining all day (so at least 90%RH). I remember telling everyone that if a cubic foot of outside air leaked into our building right now it would dry out the building. Everyone was incredulous until we proved it using the psych chart.
I often tell that story while teaching moisture. And remind everyone to control things (equipment, window openings, etc.) using dewpoint/grains. Best, -mikeb
I use the link below to calculate indoor RH based on outdoor dew point. I open the windows whenever it is cool and dry enough to dehumidify the house without AC.
@Scott, that’s a useful link as it provides the underlying equations. Here’s another tool that uses sliders instead of fill-in boxes: Dew Point Calculator. Sliders make it easier to evaluate a range of input values.
Great article. I’ve done the same hammering here at an unnamed high energy physics lab for almost 30 years. Once upon a time we heated underground enclosures after unconditioned outdoor air was supplied into them in an attempt to “dry” the space. Needless to say in the early 90s I saw fog in accelerator magnet tunnels. Now the protocol is to dehumidify (sometimes by desiccant) the outdoor air to dewpoints lower than the below grade surface temperatures.
Excellent observation – I’ve certainly found that heating indoor spaces generally raises the dew point, sometimes considerably. I wonder if Joe Lstiburek would call this “accidental humidification”. 🙂
This article reminds me of a situation I came across last week that I don’t understand with indoor/outdoor dewpoints, and @David, that Dew point calc – http://www.dpcalc.org is the source of my confusion. I had assumed MOLD risk was due to humidity or water vapor in the air, based on everything I’ve read. Inside, I was at 71F, 53% RH which the calc said dew point was 53. Outside, was 56F, 78%RH, for a lower dew point of 49. However, it said outside had mold risk, meanwhile inside with the higher dewpoint had “no risk”. Is the calc wrong or what am I missing?
Bill, no, the dew point calculator isn’t wrong. Relative humidity is the quantity that indicates mold risk because it’s what determines how much water porous materials like wood and drywall will accumulate. The higher the RH, the wetter those materials will be and the more likely to grow mold.
Dew point is the best metric for things like I wrote about in this article, though. When you mix two volumes of air (opening the window) or change the temperature of air, dew point gives you a better idea of how much humidity you’re actually dealing with.
I find myself referring to dpcalc.org often! I believe mould risk is also a function of temperature – at very high or very low temperatures it cannot grow, regardless of availability of moisture – but in this case, I agree that it’s the low RH that makes it low risk.
I’d add that indoor dew point can be a useful metric when trying to avoid winter condensation on surfaces such as aluminium window frames with no thermal break (common here in New Zealand). But even if you avoid condensation, the RH near the cold surface can still increase to the point where mould can set up shop. Without ventilation, it can be a challenge to keep the indoor dew point low enough. Of course, if you do have MVHR, the dry outdoor air in winter really works in your favour!
I totally agree with the message in this article. Well done!
However, rather than taking a fan and driving the drier outdoor air into the building, (thus positively pressurizing the wetter indoor air), I would configure a strategy that pumps the wetter indoor air to the outside.
If you positively pressurize the wetter indoor environment… it can be driven into the exterior cavities and in some cases, encounter the cold exterior sheathing and condense. That’s not good.
But under negative pressure… no issues are created.
Good message though!
@Ken, if conditions are appropriate to open windows and operate a fan, I don’t think we have to worry about condensation due to cold sheathing 😉
When talking about year-round ventilation design, supply ventilation (positive pressure) pressure is preferred over exhaust ventilation in warm & mixed humid climates like Allison’s (Georgia) so as to avoid condensation on back of drywall in summer. Conversely, exhaust ventilation is preferred in cold climates so as to avoid condensation on back of exterior sheathing in winter (and also why we want some insulation outboard of sheathing, and why vapor retarder goes on warm side in both warm-humid and cold climates).
I’d also add that negative indoor pressure can be undesirable because it can draw crawl space air (with all the attendant damp, spores, smells, radon, etc.) into the occupied space.
It’s no coincidence that the typical target for supply air temperature when cooling is 55F/13C.
Tom, I tell people all the time that 55° F is a special number for air conditioned spaces in humid climates. That’s the dew point temperature for air at 75° F and 50% relative humidity.
Ok so I’m trying to wrap my head around this – is there a quick rule to answer the windows question? I.e. are we saying:
when indoor dew point > outdoor dew point = open windows
but if indoor dew point < outdoor dew point = close windows
(goal is to decrease moisture inside!)
Thanks for the help – posted from the very wet la niña in eastern Australia!
Emily: A quick way to make the call would be to look at the outdoor dew point temperature, yes. If it’s below about 15 °C, it’s not a problem to open the windows.
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