Humidity.
The assessment of how much water the air holds.
When the weather person announces a “humidity” associated with a temperature, he or she is referencing relative humidity. That is, how much moisture is in the air compared to the maximum amount that the air can hold at that specific temperature.
You see, warm air can hold more moisture than cooler air. As an aside (we don’t want to fall into a rabbit hole), that is why we are seeing record-breaking storms and rainfall associated with the storms – even if the average air temperature is only 1-2 degrees higher than the long-term average, the air can hold a tremendous amount of moisture in excess of what it could only 1-2 degrees lower.
Many of us in the construction and building science disciplines feel that reporting relative humidity isn’t a very useful metric. In fact, if we wanted to explore and scrutinize a psychrometric chart, we would learn how much water it takes to saturate air at any given temperature.
A better metric is dew point. Dew point is defined as the temperature at which the air is 100% saturated with moisture; it can hold no more.
It follows that the higher the dew point, the more moisture that the air can hold. Other pertinent variables include dry bulb and wet bulb temperatures, pressure, and enthalpy. For the purposes of this article, I don’t want to get that nerdy.
The nerd line has been drawn.
According to the National Weather Service, “The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%”.
What if the weather person says that the current air conditions are 75ºF and 60% relative humidity? How do we find the Dew Point? Using the psychrometric chart, we can find that the Dew Point is approximately 60ºF. That means if the outdoor air cools from 75º to 60ºF, it will become saturated and dew will form on outdoor surfaces.
Q: Is it possible that the dew point is higher than the ambient temperature?
A: Yes – at night, when the earth radiates heat away from itself and the air cools, the water in it condenses as dew. The air can’t hold any more water. It is already saturated and any extra moisture from any source will condense on a surface such as grass or your car.
Q: What about really high dew points?
A: I was pushing a lawn mower on a vacant piece of land that I own in the summer of 2020. (I have since wised up and hired it done.) The air was thick and oppressive. The weather person said that the dew point was in the high 70s (Fahrenheit), which was one of the highest dew points ever recorded in my climate zone. I remember feeling like I barely had enough energy to push the lawn mower.
The outside temperature wasn’t much higher than the dew point temperature. Probably was in the mid-80s. The radiant heat of the sun bearing down on me didn’t help at all.
The relative humidity was near 100%.
I’m so glad I don’t cut any grass in 2021.
Q: What in the heck does this have to do with construction or real estate investing?
A: Hang on.
The human body is an incredibly complex machine. Yes, machine. Physiologically, we are wired to sweat in response to a cooling demand. Our internal thermostat calls for it.
The cooling demand in question may be a high temperature, a moderate temp combined with a high humidity (which suppresses the ability of our sweat to evaporate from our skin, making the cycle continue), simply how we are wired (that’s me), or if we are not in optimal physical condition.
Then there is sensible versus latent heat. Sensible heat is temperature-based. Latent heat is heat that is added (such as increasing the humidity inside a home by cooking or taking a shower) without actually changing the temperature of the air.
It turns out that temperature and humidity/dew point are simply raw numbers, and how our body responds to them physiologically is another matter entirely.
I moved my daughter into a college dorm recently. After moving a heavy couch and several boxes and being in a small room with no cooling capacity other than a fan moving warm, moist air around, I felt hot. Then again, my ideal indoor temperature is in the 60F range. I still haven’t found another human that agrees with me in that respect.
In the HVAC world, “Manual J” is the term given to the section of the mechanical code that specifies how to do a heat gain or heat loss calculation for a structure. This is important so that the heating and cooling equipment can be sized correctly.
Q: Let’s err on the side of “over-engineering”. If in doubt, I want a bigger and better AC condensing unit installed so I can always feel cool.
A. Big mistake. If the AC unit only has to run a minute or two in order to satisfy the temperature of the thermostat (Yes, even in 2021, thermostats call for and measure conditioned air via set points based on temperature – not humidity, not dew point, not anything else) – then your AC will shut off before it has the opportunity to remove much humidity from your air.
And you will feel uncomfortable.
And you will think that your HVAC people are idiots.
And you will think that they undersized your unit when in fact it was oversized.
Q: What is the solution here?
A: Make sure your HVAC people run a complete manual J analysis. Make your home and office (my goodness, we spend as much time at work as we do at home) as comfortable as possible – productivity (at work) increases with better thermal comfort, and your ability to relax and be comfortable (think good sleep) at home increases with better thermal comfort.
If you are contemplating a new build or a remodel, think airtight and high insulation levels. More insulation and airtightness is better (to a point – the curve does flatten at ridiculous amounts).
But I’ve seldom seen “ridiculous amounts.”
Side note – I am currently designing a home with no ductwork at all. The thermal heating needs will be satisfied with radiant supply, and supplemental heat as well as cooling needs will be met with strategically located air source heat pumps. (Yes, these are also called “mini-splits”, but the correct term is air source heat pump).
Q: You can have a comfortable home with no ductwork?
A: Come over sometime (in 2022 or later) and see for yourself!
As you think about that, always respect the need for appropriate thermal comfort in your design for your home, office, and investment/rental real estate. It’s easier to design than it is to retrofit. And it’s easier to run (think monthly bills) than if you incorporated this analysis as an afterthought.
{Ask me how I know.}
Until next time,
Dr. Lee Newton
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