It had been a busy week with not enough hours to properly get ready for this lesson, and a few unfortunate decisions. The biggest of which was probably attempting to work out the U values of the windows in our classroom.
This has left me wondering (like many pc users) why windows? My students probably think the same thing, and I hope I haven't lost them!
I started with a story of a concrete breeze block that had been filled with insulation, with the claim that it would reduce heat loss. Actually a true story, although I couldn't locate a picture of the actual product. The problem with this is that the insulation is just in the middle of the breeze block, and the heat will all escape through the concrete around the edges. The insulation won't make a lot of difference. It's like having a really good down jacket with no buttons to keep it together at the front.
I did have an hour or so in the morning preparing for the lesson, but a lot of that seemed to be sucked up by a desire to find some actual U values for Japanese windows, and the internet connection not working very well.
This image came up in my search, which is similar to the insulation-filled breeze block, and a symptom of the idea that just adding insulation will improve performance. In reality, the important point is where you add insulation.
This website zissil.com - efficient on the truth, though probably not energy - states "Standard aluminum, fiberglass and vinyl window frames have hollow channels inside them which offer higher insulating capabilities when they are filled with foam."
This is kind of true. The hollow channels will insulate better if they are filled with foam. But if the frames are made of aluminium, so much heat is going to choose that route rather than embarking on a perilous voyage of conduction and convection through an air gap, that filling with foam will not make a difference. If you eat peanuts with beer, changing the peanuts is not going to make a difference to how drunk you get.
Being worried whether I'd prepared enough for the lesson, the idea of estimating the window U values in class seemed quite attractive. In preparation I'd made some preliminary measurements and found that the width of the glazing was 8mm. That would even be a reasonably poor air gap, but 8mm was the width of the whole glazing. It seems unlikely that the panes are each 4mm thick and there is zero mm of air between. I guess each is 2mm thick, with 4mm of air between.
4mm of air.
Is that some kind of joke? I've been talking about down jackets, but that's like putting on a couple of t-shirts. It's as if they are trying to make it as thin as possible. As if they are trying to fool someone that the windows really only have one pane of glass. Maybe they are.
Surely they could make the gap bigger. It's not as if the air costs anything!
I'm still searching for information on these windows, which have no name, serial number or stars on them, except the YKK logo.
Finding information on these is difficult. Putting in a search for YKK and U values will find their high-quality export windows. I'm not sure whether thermodynamics applies to internet searches, but it seems that information on low-energy windows is conducting much better, while there is high resistance to the U values of poor-quality windows seeing the light of day.
It's as clear as mud.
If their windows were this clear, they would be walls.
There is a YKK database here, but I can't find any 2 mm glass, so these windows probably have 3mm glass with a 2mm air gap. One t-shirt.
For the calculation in class, we assumed the aluminium frames were square boxes, and the aluminium was 1mm thick. The high U values came out even higher because we were ignoring the surface resistance. I had thought about teaching this first, but decided not to as it would have complicated things. Another dodgy decision! So we finished the class with an inconclusive decision on the U values of the windows in class, and without going through in detail what makes good windows good.
We had at least spent a few minutes discussing windows, walking around the well-appointed classroom and comparing those bathing in the sun on the South side with those in the cold to the West.
At the end of the lesson one of the students asked me whether people building houses had to make all these calculations. I told him they usually didn't, but they should!
