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You Need Cooling

As a runner, I’ve had some direct experience with temperature regulation in the human body.  In winter (and we’re having a real one this year in Atlanta), I’m always faced with the choice of how to dress for the conditions I’ll encounter during a run.  It usually comes down to whether I want to uncomfortable at the beginning or at the end.  Put on enough clothing to be OK at the start, and I’ll be stripping down and figuring out how best to carry the extra layers once I get warmed up.  Put on just enough to be comfortable later in the run, and I’ll suffer being too cold at first.

If you’re wondering what this has to do with homes, let me state for the record that building science is really more about people than about buildings.  If you don’t understand how the human body responds to its environment, you can’t understand what we need buildings to do.  And that brings us back to my running in the cold experience and, of course, Led Zeppelin.  They clearly understood human thermoregulation better than most rock bands.  You might say they had a whole lotta love for the subject.  (Yeah, I really did that. Sorry.)  In their words:

You need cooling
Baby I’m not fooling
I’m gonna send ya
Back to schooling

Misconception #1:  Winter cooling

The first misconception about human thermoregulation is that we need to cool off in summer and warm up in winter.  Right?  The language we use to describe keeping our bodies comfortable in winter isn’t accurate.  We don’t need to warm up.  Well, most times we don’t anyway.  If you fall through the ice and are stuck in a freezing cold pond for a few minutes, your body cools too much and you really do need to warm up.  It’s called hypothermia, and it can kill you.

Under normal circumstances, however, our bodies need constant cooling, even in winter.  That’s because the human body is a lot like an internal combustion engine.  It takes in fuel, puts it through chemical transformations, and generates heat.  It’s easy to understand our need for cooling in summer.  In winter, too, we still need to cool.  It’s just that we need to shed less heat than in summer because the environment around us is cooler, providing a kind of hidden cooling.

So we regulate the amount of cooling our bodies do in a number of ways.  As I mentioned at the beginning, one method is with clothing (insulation).  We add or remove layers to adjust the rate of cooling.  Another part of my running example is important, too.  The amount of cooling we need changes with activity level.  If you’re sitting in a chair reading a book, you may need a couple of layers plus a sweater in a cool room.  Go out for a run on chilly day and you may be OK with shorts, a long-sleeve shirt, and no gloves.  Or go jump on the bed for a while, and you’ll be stripping down to your socks.

The human body cools primarily through the skin, and scientists have found that our person reading in a chair generates about 350 BTU per hour of heat.  Scaling that for the average adult, it’s about 18.4 BTU/hr per square foot of skin area.  If you’ve ever used the cardio machines at the gym, you may have noticed that they tell you your output in calories, watts, and mets.  Well, now you know what a met is:  18.4 BTU/hr/sf.

Misconception #2:  Psychology

The other misconception about human thermoregulation and comfort is that it’s an objective factor that can be calculated.  Certainly, we can quantify heat loss from the human body.  Just take a look at chapter 9 in the ASHRAE Handbook of Fundamentals.  You’ll see equations for the energy balance between the body and its environment, heat storage, sensible heat loss from the skin, evaporative heat loss from the skin, respiratory losses, and more.  Oh, we know how to quantify this stuff!

But don’t gloss over that very first sentence in chapter 9:  “A principal purpose of HVAC is to provide conditions for human thermal comfort, ‘that condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.'”  Did you catch that?  Thermal comfort is a “condition of mind” that is “assessed by subjective evaluation.”  That quote within the quote, by the way, is straight from ASHRAE’s thermal comfort standard.

The point here is that comfort is about more than the type of insulation in your walls and the type of heating and air conditioning system you have.  It’s about the human body and the mind.  A good building enclosure and a well-designed mechanical system can go a long way to helping you be comfortable, but in the end, other factors are just as important.

Now, it’s time for me to put on some Led Zeppelin II and go run a few miles.  Let’s see…9° C.  That’s T-shirt and shorts weather.

 

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.

This Post Has 5 Comments

  1. As always, a spot on observation. Getting customers to think about heat loss and gain is hard enough but trying to explain the dynamic of comfort can be a real challenge. Starting the conversation from the standpoint of the body’s needs rather than the building envelope’s relationship with outside conditions may be a an approach worth trying. Speaking of “Back to schooling”… When I tell someone that a high school in eastern PA never experiences the need for heating Monday through Friday but rather requires cooling regardless of what the outside temperature is they are stunned until they integrate the met value thousands of adolescents produce. Unfortunately, the designers of the school’s geothermal system may not have accounted for this. The geo loop temp, in the middle of the winter, is 80+F when it should be closer to 40F. It never gets the opportunity to draw heat from the loop field – it’s continuously rejecting heat to the loop field. They eventually installed chilling towers…

  2. Yet another informative and dare I say, entertaining article (the best kind!)

    As someone who lives near a large, cold lake (Tahoe) I want to observe that folks who fall into the lake’s frigid water, even in summer, seldom die of hypothermia. The shock of encountering frigid water often induces a gasp and aspiration, followed by a shutdown of the extremities and drowning.

    Of course, the end result, for a handful of individuals every year here, is sadly the same…

  3. A magic combination. Running – with Robert Plant wailing away in one ear and Jimmy Page blasting it into the other. Bliss!

    1. You stated: Thermal comfort is a “condition of mind” that is “assessed by subjective evaluation.” That may be true, but it doesn’t change the fact that acceptable thermal comfort conditions are well known and quite consistent for almost all people. If you take metabolic rate and clothing level into account, then most people find that the same combinations of temperature, humidity, radiation, and air velocity are generally acceptable to people regardless of gender, age, race, etc. Ole Fanger did a lot of research to show this. ASHRAE Std. 55 and the Handbook provide those results from Fanger and others so that we can design and control buildings properly. When you go running, it isn’t your state of mind that is changing, it is your metabolic rate. You adjust by changing your clothing level, not your state of mind 😉

      1. Excellent point, Roy. I guess my statement could lead people to believe that comfort is ALL in your mind when that’s certainly not what I intended. You never let me get away with anything!

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