Hot and humid days are days that contractors hope for. However, from the homeowner’s point of view, hot and humid days are endless days of trying to control the humidity indoors.

With summer around the corner, those hot and humid days that contractors live for will hopefully be here. However, from a homeowner’s standpoint, these are the endless days of trying to control the humidity indoors.

Unfortunately, in many cases, all they have is a simple thermostat, and while turning it down can sometimes help for a short period of time, it can have some adverse side effects.

Let’s start with the basics. As air heats up, it has the ability to hold more moisture. To make it simple, imagine three buckets: a 5-gallon bucket, a 10-gallon bucket, and a 2.5-gallon bucket. If you put 2.5 gallons of water in the 5-gallon bucket, you are 50 percent full.

However, put the same amount of water in the 10-gallon bucket and you are only 25 percent full, as there is just more volume in the bigger bucket.

However, if you put 2.5 gallons in the smaller bucket, you are 100 percent full because it has less volume. The same is true for air. When you cool air down, its ability to hold moisture goes down and the relative humidity (rh) will go up.

As another example: a day that has a 70°F dew point will have 110 grains of moisture in the air. At 90° the rh is only 52 percent. In the attic (approximately 120 degrees), the rh of the air is 22 percent, but in the home where the thermostat is set at 78°, the rh is 76 percent. So, we have to learn how to control humidity.


To a contractor, controlling humidity raises two questions:

1.What is the right amount we can have, and

2.For how long?

According to authorities such as the Environmental Protection Agency (EPA), American Medical Association (AMA), and Consumer Product Safety Commission (CPSC), they all say to keep your rh below 50 percent. If you are allergic to dust mites, then one needs to keep rh below 45 percent - and you must do this over a 24-hour period of time. Now, of course, we will have spikes, but the proper equipment can deal with it.

So how do houses get summertime humidity? Easy: Infiltration. And there are three types: natural, mechanical, and balanced ventilation. Houses breathe; how moisture enters the home depends upon how leaky they are. Depending on temperature and moisture content in the air and how big the hole in the home is, the home will naturally see a rise in rh.

Mechanical ventilation can also affect humidity levels. A dryer, with a clean lint filter, can bring in 150 cfm of humid air into the home. Range hoods can be as little as 100 cfm, up to 800 cfm or more. A recent study found that one powered attic ventilator (PAV) could bring in an average of 237 cfm air.

Another type of mechanical ventilation is duct leakage to the outside. Just 100 cfm of duct leakage in a humid climate can add over 10 gallons of moisture to a home per day. If the home has pressure imbalances caused by closed doors, basements, or bonus rooms, it could be even greater.

Balanced ventilation can also add moisture. Heat recovery ventilators (HRVs) in summer can bring in outside air that is full of humidity. An energy recovery ventilator (ERV) will remove some of the moisture coming in from outside, but it still adds moisture as the air inside is drier than the outside air.

A psychrometric calculator is a great tool for any HVACR professional, to determine, among other things, dry bulb temperature, dew point temperature, relative humidity, and more.


Can an air conditioner control the summertime humidity? Yes, in many cases. A side effect of air conditioning is dehumidification. Some air conditioners do a great job and others not as well. But they all have one thing in common. They must have a sensible load. If the air conditioner is not running and trying to cool the air, then it is not dehumidifying.

So when does an air conditioner work the best for humidity control? Answer: On hot sunny afternoons, especially if the house is tight, there is no duct leakage, the crawlspace is dry and sealed up, and the air conditioner is properly sized. But we can still see a problem at night or when it is raining.

If the homeowner continues to turn down the temperature, in order to compensate for any number of design problems, the system may be trying to cool below the dew point and then we start to see the real problems arise.

So, what is the answer? One needs to understand how the building works as an interactive system. In order to make the equipment work properly, a contractor needs to know how both interact.

So, tighten up homes. Get rid of thermal bypasses and house leakage. Tighten up ducts and get rid of duct leakage. Make sure that there are no pressure imbalances in the home. (I still suggest one should turn off PAVs or remove them.)

Educate the homeowner to the other types of mechanical ventilation that may be running. Do a proper load calculation on the home when replacing equipment and don’t be pressured by peers or homeowners to go up in size.

Use the proper design criteria. Don’t fudge on the outside summertime temperatures in the load calculation. Don’t add unnecessary ventilation into the home, such as an outside air damper or HRV/ERV without knowing if the home needs it.

Don’t be afraid to go down in size on air conditioning units if the load calculation requires it. Air conditioners, to get proper humidity control and energy saving, must run to what we call steady state and this can take 15 minutes or so to get there. So an air conditioner that starts and stops will not remove humidity and will cost more to run.

We can deal with summertime humidity problems, but we need to know what is going on in order to do so.

Publication date:05/26/2008