Back to the windows again.
After this history lesson, I wanted to give them an exercise in choosing windows, and I'd spent a long time trying to find realistic costs of windows and U values. I wondered whether something like Heisenberg's uncertainty principle was taking place, and it was impossible to know both the cost and U values of a window at the same time.
We started with the same question: how could you improve the windows in the classroom? And got a fairly good range of answers, which I covered for the rest of the class.
I gave them a short history of double glazing, from the probable invention in Victorian Scotland to commercialisation in 1930s US. Coal was cheap and windows expensive in the UK until the oil shock of the 1970s, which gave birth to the predatory double glazing salesman. I found an Everest TV advert to show them with Ted Molt on top of the Pennines. Nostalgia for me. Alien historical relic for them!
The double glazing wars continued and thicknesses began to increase. At first windows had a 6 mm gap. This went up to 8, bringing up to 50% improvement in window performance and giving the salesmen a number to show customers that their products were better than others. This went up to 12 mm, which showed a slight performance improvement and allowed people to say: "The neighbours have 8 mm double glazing, be we have 12!" They then wanted to go up to 16, but in fact this does not improve the windows, and as they get thicker, the performance starts to get worse. This is because of convection. Air can circulate within the window gap, letting heat from the inside surface rise, then drop down the cold surface on the other side.
Capitalism and competition are not always a bad thing, and in the case of windows they have brought great improvements in performance, which have certainly improved comfort and probably reduced energy use. With extra thickness shut off as an avenue for improvement, the manufacturers had to look to other tricks, such as low-e coatings, triple panes and noble gas fillings.
After this history lesson, I wanted to give them an exercise in choosing windows, and I'd spent a long time trying to find realistic costs of windows and U values. I wondered whether something like Heisenberg's uncertainty principle was taking place, and it was impossible to know both the cost and U values of a window at the same time.
I put "double glass" in Japanese into google shopping and it came up with some rather nice looking Bodum drinking glasses. It certainly looks like insulation is a bigger priority in Japan for beer drinking than house building. There is also a semantic element. Japanese has two words for glass: garasu and gurasu. Gurasu comes from English and refers to drinking glasses. Garasu comes from Dutch, and refers to the material. Even a search for garasu seemed to yeild more drinking glasses than windows.
I had previously spent ages trying to find a U value for the double glazed windows in the classroom, which are basically single-pane frames with two bits of glass forced in there. I started trying to find the worst double glazed windows in the world. Australian wers.com has a useful searchable database. Their worst double-glazed windows have a u-value of 6.4. That's the same as a poor single-glazed window.
I had time in this lesson to properly introduce the surface resistance, which is an extra R value applied on each side of an insulation unit. If you imagine a thin piece of paper in the middle of a wall, it's going to have some effect keeping you warm, just because it's stopping the air from moving. The surface resistance on the outside is 0.04 m2K/W, and on the inside it's 0.13 m2K/W. The surface resistance on the outside is less than on the inside because it's more windy and the heat will be taken away more easily.
I asked them to calculate the U value for a piece of paper by adding up the R values and ignoring the R value of the paper. This gives a U value of 5.9.
Somebody asked how the windows could have a lower U value than a piece of paper.
I know, it's impressive isn't it. That's the power of aluminium!
They did quite a good job comparing the different kinds of windows, working out how much heat each one would lose over a year, translating that into heating costs and working out how many years it would take for the heating savings to pay for the windows.
References
An interesting article here on homes.co.jp (in Japanese). Title roughly translates to "Japanese windows: More interested in convenience for manufacturers than benefits for consumers"
