I thought I knew all about this topic, but I did not really get a grip on this until a few years ago while preparing to teach the subject. If you’re looking for the ultimate motivator for learning, I highly recommend getting hired to train a room full of experienced contractors so you can stand in front of them and preach, “This is how it is!”
The topic (this time), is moisture. Recall in the past we knew a home had a moisture load (or latent heat load) that needed to be managed, but we also knew our cold AC coil did a remarkable job of sucking the water out of the air. Even if you did not understand the psychrometric chart or read the AC manufacturer’s latent capacity specs, you still learned all about the great moisture removal abilities of an air conditioner the first time you forgot to hook up the condensate pipe. What a mess!
So what have I learned recently that makes me think I deserve your attention for the next few minutes? I now understand the potential water volume in air, and I appreciate the high levels of moisture we have in the Midwest for a large portion of the year.
Don’t stop reading because you have already read about moisture control. You are right, it is a topic frequently talked about today. However, I do not think many in our business are following through and helping the customer understand. It is time for all of us to sign up for a class on moisture. We can no longer ignore our professional responsibility to understand and control the humidity in our customer’s home.
Are you afraid of the psychrometric chart? Well, I am. Maybe not so much afraid as overwhelmed by how much information it reveals. Fortunately, you do not need to know it all. In fact, I do not even “work the chart” in my short class, but let’s remind ourselves on a few of the basics:
- In the summer, in humid climates, we would like to comfort our homes to about 55% Relative Humidity. A dryer 50% RH is even better.
- Outside air is beginning to feel uncomfortably humid at a dew point temperature of 60 degrees.
- Outside air at 60 degree dew point brought inside an air conditioned home at 75 degrees will result in 60% RH. Too high.
- Outside air at 65 degree dew point brought inside an air conditioned home at 75 degrees will result in 70% RH. Much too high. Bad things can start to happen, depending on how much of this moist air you allow inside.
- Outside air at 70 degree dew point? Throw me a life jacket and snorkel!
So what? The air conditioner runs and we’re all fine, right? Two things are different now, compared to the past:
- Home Performance measures for existing homes (or building energy efficient new homes) helps our homes become more energy efficient, but now the AC runs less.
- Sometimes we deliberately suck in outside air that is above 60 degree dew point.
Either of these two actions on their own can cause moisture issues. Having both could be double trouble. Sometimes these measures simply affect the homeowner’s comfort. Sometimes there is so much moisture we cause health and structural problems. Who’s responsible? You. You are the expert. You must understand moisture.
What do you need to do?
The first step is to understand dew point and track it! This is an easy measuring stick for how much water is in the air, and you need to follow it. It’s on your weather app on your phone. Look at this number as frequently as you check the radar. Appreciate that some climates are very humid.
The summer so far in Indiana has been terrible. Wanna see it? Go to WeatherSpark.com. This is my favorite site to view historical Dew Point. The link below is for my city in Indiana, but you can change it to your city. This is a great resource when you learn to navigate through all the adjustments.
Note the dew point levels during the week I wrote this article. Those are serious moisture levels! Now notice the temperatures (Dry bulb) for this period. Not too hot, and especially if your home is energy efficient like mine, then there is not too much load on the AC.
My geothermal heat pump did not run too much that week, and not nearly enough for adequate moisture removal, but I was very comfortable thanks to my whole house dehumidifier. Fortunately, I did not let much of that Indiana soup into my home, other than a few, normal exhaust fan run times, clothes dryer exhaust, and normal door traffic. I condition to a very comfortable 76 degrees and 45% RH. Half my home is over a basement and the other over a sealed crawl space, and both are dry and odor free.
This is not the way it was in the past. I love the outdoors and in the past I kept my home open as much as possible. Odors grew downstairs, and spent portable dehumidifiers piled up in the basement.
Not so anymore! Once I condition my home in terms of temperature, humidity, and even pollens and dust, then I like to keep it that way. Sure, I still keep a lookout for “fresh” air and bring it in when it is available, but for some stretches of time in the Midwest, “fresh” is rare and I do not intentionally allow the Indiana soup to get into my home.
What about mandatory residential ventilation? (Oops, will you look at that. We are almost out of room for this article and we will need to cut it short.) Briefly, until we have more time, yes fresh air makes sense; lots of it for some situations, much less for others. But with what I know about moisture, I think we are blindly rushing into problems with indiscriminate ventilation. Ventilation timing and moisture knowledge is critical. In many areas, ventilation strategies must include a dehumidifier. And by the way: An ERV is not a Dehumidifier. (Related article https://hvac-blog.acca.org/erv/)
Incidentally, I could go for a good dose of fresh air right now. Let’s see, checking the dew point map, I wonder what size duct I might need to bring in 100 cfm of fresh air from Ely, Minnesota to Indianapolis?
- Spoiled Rotten for Summer Comfort - August 12, 2015
- Indiana Soup - July 20, 2015
- When Home Performance Customers Ask About Payback, What Do You Say? - January 6, 2015
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