How Humidity Affects Heating and Cooling

To explain how humidity affects heating and cooling costs, let me start with an analogy.  There’s no shortage of hot air in Washington D.C. these days, so carry a bucket there and fill it with hot air and cover it. Then fill a second bucket with hot water and cover it. Now take the covers off both at the same time. Which bucket cools off fastest? (That’s a rhetorical question)

The reason the water cools more slowly is due to the concept of thermal mass. Water has more thermal mass than air. In other words, it can store more heat.

Humid air has more water in it than dry air, so which do you think requires more energy to cool? (Another rhetorical question)

This same concept applies to heating – it takes more energy to heat humid air than dry air.

How Climate Affects Energy Usage

Looking at the chart above, hot and humid areas use 21.1% of their energy on air conditioning each year, while hot and dry areas only use 9.6% of their energy to air condition. Check out this document prepared for the Department of Energy in 2010 that explains the 5 climate regions. The document lists cities across the country so you can use it to find which region you live in.

There are many other factors affecting energy use besides humidity, but it does play a very large role in how much energy is needed to cool a home, and to a much lesser degree, how much energy is needed to heat a home.

What is Relative Humidity?

It is a measure of the amount of water air can hold at a given temperature. So air at 50% relative humidity contains half the amount of water that it is capable of holding. Relative humidity cannot exceed 100% because water condenses out of the air as small droplets once the air is saturated.

Psychometrics

Psychometrics is the study of the thermodynamic properties of moist air. You don’t need to know all the technical details. Just remember that hot air is able to hold more water than cold air. So air in hot, humid climates is more expensive to cool. In fact, there is so much water in hot, humid air, that as it is cooled water condenses out of it, because, remember cool air cannot hold as much water so the water has to go somewhere. This is the reason air conditioners create condensate as part of the process of cooling air. The same thing happens to a glass of ice water on a hot, humid day – the air near the glass is cooled and can no longer hold as much water so the water condenses out of the air onto the surface of the cold glass, runs down the glass and ruins your grandmother’s antique wood table.

It’s a wonderful thing that water condenses out of the air as it is cooled for 2 reasons. One is because homo sapiens are most comfortable in a relative humidity of 40% to 60% so we often want to reduce the humidity of outside air. The other is because of a four-letter word we don’t want around – mold. Mold needs an environment of greater than 60% relative humidity to live so it is vitally important to keep building environments at or below 60% relative humidity. Cooling air squeezes the water out of it and gives us some control over humidity.

So what about heating humid air? That’s easy – just heat it up. The hotter the air gets the more water it can hold so any water in the air stays there. It does take a bit more energy to heat humid air than dry air because you’re heating more water.  However, since the amount of water vapor in a given volume of air near room temperature is small, the measurable energy difference in heating humid air verses dry air in your home is very small and not a factor that needs to be addressed.

As air is heated, the relative humidity decreases because the air is able to hold more and more water as it increases in temperature. In a controlled environment, like your home, if you heat air at 60% humidity and 40F up to 70F, you decrease the humidity to about 18%. That’s why air is so dry in the house in the winter and also why static electricity is much more prevalent in the winter. You can increase humidity with a humidifier when heating air, but it is expensive and most commonly done only when health conditions call for it, and in buildings with special requirements like museums.

Even though a humidity range of 40% to 60% is most comfortable for people, it is often necessary to maintain humidity below 40% in the winter to keep condensate from forming on cold surfaces like windows.

If you want to get into the geek stuff and find out how I determined how much the humidity would decrease when the air is heated from 40F to 70F, do some Google searches on “Psychometric Chart”. Here is a great pdf book on air conditioning psychometrics.

How to Improve Comfort and Save Money

It’s clear that we want to heat and cool drier air rather than more humid air to save money, but comfort comes into play too. So here are some tips.

• • • Buy a hygrometer. A hygrometer measures relative humidity and is an essential tool in controlling humidity because you cannot manage what you don’t measure. Inexpensive hygrometers are not very accurate, they are typically within +/- 5% relative humidity, but you don’t need exact numbers. You just need to know if your home is in the comfort range, or if it is too dry, or too humid. Very dry is ok and is energy efficient, but it can dry out sinuses and cause excessive static electricity. Very humid is not ok because it invites the growth of mold. Run your air conditioner to condense some water out of the air and lower the humidity whenever your home exceeds 60% humidity for any extended period of time.
    • When air conditioning your home, use the bathroom exhaust fan to remove warm, humid air after a shower. Then turn it off so you don’t continue to exhaust conditioned air. In the winter, you may actually want to leave that warm humid air in the house because humidity can get uncomfortably low during cold weather. Cooking can add heat and humidity to the home and this tip applies in that situation also. Let your hygrometer be your guide.
    • During cold weather, filter and exhaust warm, humid air from your electric clothes dryer into the house. This saves energy and humidifies at the same time. Dryer duct kits are available with a damper that lets you direct air into the house or outdoors.  Check out this kit at Amazon as an example. (Warning: never vent gas dryers into the house because of the risk of carbon monoxide poisoning)
    • Keep doors and windows closed, especially during warm, humid weather. Every bit of humid air you let into the house requires additional energy to cool.
    • Try to achieve a relative humidity of 30% to 40% in the winter and 40% to 50% in the summer. This will keep your home comfortable, free of mold, minimize static electricity, and keep condensate from forming on interior surfaces. It is ok to fall outside the desired range for a day or two occasionally.