Tuesday, 17 November 2015

Lesson 6: A lesson in humidity

A week after my lesson in humility.

Once again I got about half way through my lesson plan by the time the bell went. This time it was a good thing as I reached a fairly neat cut-off point.

The title of the lesson was Air and Water, and after explaining humidity, I had planned to go on to talk about ventilation, but that will wait for another day. I hope nobody is holding their breath!

I started by asking why my glasses steam up when I come in from the cold, why mirrors mist up when you breathe on them, and what this has got to do with low energy buildings. The answer of course is humidity.

I next asked them to estimate how much air was in the room, and how much water was in the room. Their estimates for the amount of air in the room ranged from 150 to 600 cubic metres. I had a tape measure which allowed a more precise calculation, of around 190. Their estimates of the amount of water in the room were just as varied, although one group was also taking into account the human beings in the room, who are 70% water. I managed to steer us onto the water in the air, or more precisely water vapour.

Next I asked what you would do with water if you wanted to dissolve a lot of sugar in it. One of the students had brought to class a thermos flask with sugar water, which provided a nice link to this question.

In just the same was as you heat up water to dissolve more sugar in it, heating up air allows it to hold more water vapour. In fact the amount of water it holds doubles every ten degrees or so. Very roughly a kilogram of air at freezing will hold almost 4 grammes of water. At 10 degrees it will hold almost 8 grammes. At 20 degrees 15 grammes and at 30 degrees 28 grammes.

The trickier part to understand is relative humidity. This is the amount of water in the air as a percentage of the maximum moisture the air can hold. So for a given body of air, as the temperature goes up, the relative humidity will go down. As the temperature goes down, the relative humidity will go up.

I tried to explain this by talking about the class, which had a total of nine students, of whom three were Japanese. So the class was around 30% Japanese. If three of the non-Japanese people left the class, there would still be three Japanese, but they would now be 50% of the class.

Back to the moisture in the air, if the temperature continued to go down, at some point it would become saturated and the water would start precipitating or condensing. That's called the dew point.

Next we considered what would happen if air were able to pass through insulation. In winter it's going to be something like 20 degrees inside and freezing outside. As the air passes through the insulation and the temperature drops, it's going to hit dew point and you'll get condensation forming in the wall.

This left me with my top two suggestions if you want condensation in your house: make it airtight with no insulation, or make it well insulated but not airtight. The moral of the story, in fact the moral of the course so far, a little insulation is a dangerous thing.

Temperature and humidity chart from sustainabilityworkshop.autodesk.com