I thought I was wrong once, but I was mistaken

For the past couple of years I've been looking at the windows at work and thinking how thick they are.

Not so much as a question, but more as an exclamation. Last year I wanted to estimate their thermal performance as an activity for a lesson on windows, so I measured the thickness of the glass, and it came to 8 mm. This was the thickness for two panes, plus glass, which leaves precious little air between, and it is the air between that is important since it is around fifty times better at insulation than the glass is. I guessed the 8 mm was made up of 2 mm panes with a 4 mm air gap, although usually glass is 3 mm, which would leave a 2 mm air gap. 

A year later, I was still in disbelief at either a 2 mm air gap, or 2 mm panes of glass, and I was still wondering how I could actually measure the thickness of panes and air gap when I remembered that glass was transparent, and its surfaces reflect light. I have no idea why I didn't think of that before. Using a spoon on a sunny day, I got these pictures, and it looks like the ratio of air thickness to glass thickness is around two to one.

I used a smartphone to take these pictures since looking directly into the sun can be very dangerous, even it if is reflected by a spoon and a window. 

Having found no evidence of 2 mm glass window panes, I decided to measure the thickness again, subtracting the inside and outside relief of glass to frame from the width of the frame. It was 11 or 12 mm. I measured this several times to check there was no mistake, but it was definitely not 8 mm, as I had believed for the past year. The windows are likely the industry minimum standard of 3-6-3. Three mm glass panes with a 6 mm air gap.

I have no idea why I didn't take another measurement a year ago when I came up with the highly improbable thickness of 8 mm. It is a useful reminder that everyone is wrong sometimes, and that doesn't stop us from being sure of our beliefs.

Using these values, we calculated the U value of the glass to be around 2.3 W/m²K. We also estimated the U value of the frames, which are non-thermally improved aluminium, and derive most of their benefit from the surface resistance. This means around 5.6 W/m²K.

The relative area of glazing to frame was 77:23, so the windows as a whole had a U value around 3 W/m²K. Not terribly good, and illegal in many countries, but not quite as bad as I estimated last year. 

Cloud-tracking cameras to tackle dips in solar power output

This is a nice bit of technology. Compared to conventional power generators, solar panels present something of a challenge. It's fine on a clear sunny day, when you get a predictable output, peaking sometime around noon. If it's overcast you get a predictably low output. The problem is on partially cloudy days when the output will fluctuate each time the sun goes behind a cloud. 

So according to this article in The Guardian they have installed camera technology on a solar installation in Western Australia that will give a fifteen minute prediction of clouds to come, which can help switching on back up systems when needed, and can give a better idea of prices.

Lesson 5: How do you feel?

When I started planning this lesson, it was going to be about cooling, since it's starting to get a bit cold and would be nice to talk about the summer. And I wanted a break from calculation-heavy thermodynamics. Of course, I have made it clear that if we are going to save the planet, we have to do some mathematics.

Also I wanted to try to integrate the class a bit. After initial success in my language policy of dividing the class into people who want to speak English and people who want to speak Japanese, it was rapidly turning into segregation on national lines, where all the Japanese students were on one side of the class and all the non-Japanese on the other. I thought it would be good to get mixed groups of tropical and temperate-dwellers, along with my permanent goal of getting my students to talk to as many other students as possible. While I try to promote classroom democracy and learner autonomy, as the teacher it is in my power to go around the class and tell students where to sit, so I did!

The title of the lesson was originally cooling, then cooling and heating, but in fact the best title is my first question to the class: How do you feel? I wanted them to choose an answer between Hot and Cold, which I then converted to a number between +3 and -3. 

Hot -3

Warm -2

A bit warm -1

Neutral 0

A bit cool 1

Cool 2

Cold 3

I took the average, which is called the predicted mean vote, after research by Fanger in the 1970s. The average was 0.9. There is a relationship between the predicted mean vote, and the predicted percentage dissatisfied, so if the mean is zero, you can expect fewer than 5% to feel uncomfortable. There will always be some people who are dissatisfied—an important lesson for life! If the mean vote is between plus and minus 0.5 you can expect 90% to be satisfied. At plus or minus one, 28% will be dissatisfied, so our number looks like a quarter of the class were feeling uncomfortable. This is not a terribly good figure, since uncomfortable students are less likely to learn anything, but my immediate goal is in educating about thermodynamic. Solving the effects of thermodynamics on education will take a little longer!

The next question, inevitably, was why they felt hot or cold. I wanted them to brainstorm for a list, and I also gave them the big question: heating makes us feel hotter; true or false?

They wrote down all of my prepared answers, like air temperature and clothing, and a few others such as being with people who you like, eating, and drinking hot drinks. They also mentioned snow, which actually I think makes you feel warmer, but I need to do some more research on this for a definitive answer.

There were four groups and I got one of them to explain each of radiation, humidity, air movement and activity.  

The group talking about activity had some theory about blood going through the veins more quickly and creating more friction. I asked politely whether it might not be because we are burning sugar when we exercise and if we exercise more we burn more sugar and create more heat. I even drew the chemical formula on the board: 

Sugar + O₂ -> H₂O + CO₂

And further elaborated sugar to C_xH_yO_z, which impressed upon them my chemistry inability. 

The group discussing air movement correctly observed that moving air will take away more heat from our bodies, and the humidity group noted that we lose heat when moisture evaporates from our skin, and more humidity means less evaporation so we lose less heat. I later told them that 10% extra humidity feels about 1 degree celsius warmer. 

The radiation group was talking about radiation making the air around us warmer, which I also had to question. They did appreciate that heat was being radiated from many things around us. I later showed them the relation between air temperatures, surrounding surface temperatures and the temperature we feel. For example if the air temperature and surface temperatures are both 20 degrees, it will feel as hot as the surface temperatures being 17 degrees and the air temperature 23 degrees. 

I also told them about radial symmetry, and how it will feel uncomfortable if there are different temperatures between head and feet, or from different directions. 

We all began to agree that heating does not make us hotter. We are warm-blooded, and we make ourselves hot, in fact putting out something like 100 watts. In fact, heating makes us lose less heat.  

So what about the summer? 

Does insulation make buildings hotter in the summer? 

Does insulation work the same for keeping buildings cool in hot climates?

Are there some buildings that don't need insulation?

To which my answers are no, no and probably not. 

Insulation does not make anything hotter, it just stops heat moving so fast. If it's hotter outside, then insulation will stop heat getting into a building. There are two important differences between the way insulation works for hot and cold climates.

First, many things in buildings produce heat, including people, electrical appliances, cooking and hot water. In a cold climate, these are all helping to keep the house warm. In a hot climate, these are extra things we must fight to keel the house cool.

Second, cold places are a lot colder than hot places are hot. For example Yakutsk in Russia has average winter temperatures of minus 34. Around that temperature it doesn't make much difference whether you are talking about fahrenheit or centigrade. Meanwhile the average summer temperature in Kuwait is 38 degrees C. These look strangely symmetrical around the freezing point of water. Or perhaps not so strange considering our environment is as dependent upon the temperature stabilising effect of water phase change as a glass of gin and tonic. However, when we remember that humans favour temperatures between twenty and twenty five degrees, we can see that the temperature difference we are working against is four times more in the cold climate. 

I gave the example of Australia, considered to be a hot country, where more people die of cold weather than hot weather. Additionally, more people die of cold in Australia than they do in Sweden, which is most definitely a cold country. Why is this? Insulation of course. 

There may be somewhere in the world where insulation is unnecessary, but in cold climates it will help to keep you warm, in hot climates it will help to keep you cool, and in many temperate climates it will do both at different times of year. 

If you don't like reading this blog, you probably won't want to listen to this podcast

Or maybe you will like it. It depends what you don't like about the blog.

If you think there is a shortage of practical commentary on actual projects here, then you will like House Planning Podcast.

If there isn't enough of a focus here on the UK, then there certainly is on this podcast. And if you　don't really have time to read but are looking for something to listen to, then this podcast really is for you.

I particularly enjoyed This one about the balance between renewables and a fabric-first approach with Alex Baines.

And episode 126 which looked at whether to demolish or rebuild, and talked about the carbon cycle.