building science

Humidity by Trey Farmer

Our local Passive House Chapter likes to throw big geeky parties. Every two years we put on the Humid Climate Conference in Austin. Why not call it the Southern Building Science Conference, or the Hot Climate Conference? Because heat is relatively easy to deal with, humidity on the otherhand, is a bit trickier.

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As buildings get better insulated and more air tight (more energy efficient) they become more comfortable, better for the planet and healthier for their occupants - if they are done right. That is a pretty big caveat for comfort and health especially in a humid climate. When those things happen, the heating and cooling loads get smaller and fresh air (ventilation) becomes more important.

We are living in a 2 year old triplex, the middle unit of three, while we work on Theresa Passive. That means our neighbors help to heat and cool our unit, and the amount of energy loss through our walls and windows is pretty small compared to an older home (or even a free standing new home). About 3 months after moving in late-spring hit. Austin gets hot and humid: lots of rain and 95 degree days in May. Our home was regularly reaching 70-75% relative humidity (way too high). This meant that not only were we uncomfortable, we were also at risk of mold growth, dust mites, mildew and elevated VOCs. At one point our salt lamp night light began to melt. It hadn’t been plugged in yet and was dripping salt solution down the bookshelf.

So why was this new home so humid, why do we care, and how can we do better?

First some really basic physics (sorry). Air has water in it. The amount of water a volume of air can hold is dependent on its temperature. Warmer air can hold a lot more water than the same volume of cold air. Relative humidity is the percentage of total possible moisture in the air at a given temperature. What this means is that when it rains in Austin and is 97 degrees the next day, the outdoor air has a lot of moisture in it. When that air comes inside and we then cool it down to 70 degrees, the absolute amount of water stays the same (measured in grains or 1/7000 of a pound) but the relative amount goes way up, because 70 degree air cannot hold as much as 97 degree air. If you want to really get into this and learn how to do grains calculations come to the next Building Science Philosophical Society Meeting.

A more energy efficient home (like one whose neighbors do a lot of the cooling for them), has to use the air conditioner a lot less. The system will turn on less often and run for shorter durations. The home may also use a smaller system. This means that the HVAC system is removing a lot less latent heat (aka humidity, kinda) than one that is running longer and more often in a less efficient and/or larger home. So in our case we had just as much moisture coming into the home from outdoors, but insufficient capacity to remove it. Our house was plenty cool, but wayyy too humid.

Higher humidity makes it harder for our bodies to cool themselves through evaporative cooling so we ended up turning down our thermostat lower than normal to not feel too hot, but then we felt too cold and clammy. It was not fun, but our discomfort was the least of our worries. Take a look at the chart below:

The Goldilocks spot for relative humidity is between 40% and 60%. Go too low and it feels too dry, the mucos membrane has trouble, nosebleeds and cracked skin start to be problems, the list goes on. Anyone who has lived in a really cold climate knows all about these issues. Wintertime is dry (cold air can’t hold much moisture)!

High humidity is even worse. Humid climates might not have the same respiratory infection rates, but they have mold, mites and VOCs. This photo is from the register in our bedroom where warm air in the house hit the cold surfaces from the AC system. The warm air gets colder and drops below dew point and all that extra moisture has to go somewhere. That somewhere in this case is the drywall and metal register, and that steady source of water on an edible surface (the paper face of drywall) and the right temperature is a happy habitat for mold.

Not cool.

Not cool.


So, what to do?

Build an air tight envelope and make sure your HVAC system has the capacity to handle the latent loads whenever they might be an issue. In Theresa and pretty much all of our new construction we like to bring on a high quality mechanical designer to study the project and layout an appropriate system that will deliver comfortable, healthy air to the spaces we live in. In Austin that means putting in a dedicated dehumidifier like the UltraAire 70h (the 70 refers to how many pints of it can remove in 24 hours) . It supplements the VRF heat pump system and removes moisture incredibly efficiently. This is especially important during the long shoulder seasons in Austin when the AC really doesn’t run very much but our humidity is very high.

Theresa Ave by Trey Farmer

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We have lived in a beautiful old (for Austin) house in an amazing (and terrible) location for the past 7 years. Much of this blog will be dedicated to a major renovation/addition that is about to begin, and the why’s, how’s and what’s of all that goes into such an endeavor. We will get into many aspects of this project in the hopes that it might help others avoid some of the mistakes we have made, and learn some of the things that it took us a while to learn, and to give some insight into our process and priorities. Some of this will be specific to Austin, some specific to historic remodels, some to humid climates, some to high nosed architects, some to city living, and some to homeowners.