Tips for temperature regulation during a heat wave

8 Jul

As a physicist and cyberneticist, I have always been fascinated by
temperature regulation, because it is such a simple and practical
illustration of basic principles. That’s why I have investigated both the
thermodynamic theory behind it, and its use in keeping the house at a nice
temperature. For those of us living in Western Europe, which is presently
undergoing a heat wave but where people rarely use air conditioning, I
wanted to convey my understanding by formulating some tips on coping with
the heat.

First principle is that our body regulates itself to stay at a stable
temperature of about 37 degree C by compensating any deviations. It
compensates downward deviations by producing sufficient internal heat
through its normal metabolic processes. However, these processes cannot be
completely shut down, and therefore we risk overheating even when the
outside temperature is still below 37 C (as in the present heat wave).
Therefore, the body also needs to compensate upward deviations by a cooling
mechanism, namely transpiration. This requires water which is of course why
you need to drink a lot when it is hot.

The principle here is that the sweat that is formed on our skin evaporates,
and the transformation of the water from liquid to gas requires energy,
which it gets by extracting heat from its surroundings, and thus from the
skin. Therefore, you cool down more effectively if you can increase the rate
of evaporation. That rate depends on the humidity and movement of air.

Humidity is measured as the amount of water vapor in the air relative to
full saturation (100% humidity). If air is saturated with water, it cannot
take up any more, and therefore no evaporation occurs. The lower the
humidity, the quicker sweat evaporates, and therefore the easier it is for
the body to cool down. That is why you can stay about as long in a 90 C
sauna (ca. 10% humidity) as in a 50 C steam bath (ca. 90 % humidity). It
also explains why the higher temperatures in a desert are typically more
bearable than the cooler temperatures in a rain forest (provided you are in
the shade). Unfortunately, apart from air conditioning, there is not an easy
way to control humidity. However, you can at least avoid humidity from
accumulating inside a closed space by sufficient aeration.

Aeration and more generally movement of air dissipates local humidity, and
therefore increases the effectiveness of sweating. Moreover, a draft carries
away not only the vapor of your sweat but also some of the internal heat
produced by your body, thus doubly cooling your body. That is why a simple
ventilator, wind, or a draft created by opening windows can be very
effective in coping with the heat. However, this creates the temptation of
opening all windows in your house during a heat wave, which is not a good
strategy.

In the beginning of a heat wave, the temperature inside a building is
typically much cooler than the temperature outside, because the walls
insulate against the heat. The idea is that you should try to maintain that
cooler state as long as possible. You do that first of all by closing all
shutters or curtains on the windows exposed to the sun, in order to minimize
heat entering by radiation. Opening windows and doors during the hotter
parts of the day, on the other hand, will let all the cool air escape, to be
replaced by hot air from outside, so that is something you should also
avoid.

But what then can you do for effective aeration? The first relevant physical
principle is that hot air is lighter than cold air, so it will move upwards.
That is why the attic of a house will be much hotter than the basement, and
typically at least as hot as the outside temperature. Therefore, you should
if possible open the attic window, or more generally the highest window in
the building, to let that superhot air escape.

But air that goes out must be replaced by air that comes in order to
maintain the same level of air pressure. Therefore, a single opening will
not be very effective for aeration, as air must stream in and out through
the same hole. Better is to open a second window somewhere else, so that air
can come in via the one, and out via the other.

The flow of the air (draft) between the windows will depend on two factors:

1) if the one opening is higher than the other, hot air will stream out
through the higher one, and cooler air will stream in through the lower one.
This creates a “convection flow”. The larger the difference in height, the
faster the air flow: this is called the “chimney or
stack effect”. It explains why
chimneys or smokestacks intended for evacuating hot air or smoke work better
the taller they are.

2) if the two openings are roughly at the same height, the direction and
speed of flow will depend mostly on the outside wind. For example, a
westerly wind will push air in through the west window and out of the east
window. The stronger the wind, the better the aeration, but this doesn’t
help much during the typically windless days of a heat wave…

The most effective air flow you create in this way will be one that sweeps
only through the highest part of the house, so that the cooler layers of air
below do not get mixed up with the hotter air from outside. However, any
heat generated below (e.g. by cooking, or your own body) will tend to move
up and be carried away by the flow–if you of course leave all the internal
doors open so that all parts of the building communicate with the air flow
at the top.

This set-up not only slows down the build-up of heat inside the building, it
can effectively cool the building if it continues through the coolest parts
of the night. When the temperatures drop at night, outside air may become
cooler than inside air at the lower layers of the house. The convection flow
between your two (or more) open windows will now extend downward, as cool
air descends down to the level where the air is the same termperature,
“pushing” out the warmer air it encounters towards the highest,
“chimney-like” window. The larger the temperature difference and the wider
the openings, the faster this convection flow, and the quicker your house
will be cooled down.

Note that such a convection flow is a self-organized, dissipative structure,
which can have a quite complex shape depending on the geometry of walls,
windows and staircases, the temperature gradients, wind direction, etc.
(although it would be difficult to observe that shape, unless you make the
flow visible by mixing smoke or small particles into the air).

In this situation, you can generally accelerate the convection flow by
opening more windows (thus making inflow and outflow easier) and by
increasing the difference in height between the highest and the lowest
window (the chimney effect). For example, this is the moment to open a
window in your bedroom while simultaneously keeping the door open to the
corridor that leads to the other open windows, so that a draft is created
along your bed.

However, remember that opening windows lower in the house is
counterproductive if the temperature in that part of the house is less than
the outside temperature. Therefore, you should again close all windows but
the highest when the outside temperature increases too much.

The general principle is that you should make your house a “sink” or
“attractor” for coolness, which cold air can easily enter, but not leave,
and a “source” or “repulsor” for heat: heat should easily escape, but not be
able to enter. Like that, you can maintain a pleasant inside temperature
even during the hottest periods, without need for air conditioning…

Francis Heylighen
Evolution, Complexity and Cognition group
Free University of Brussels

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