Friday, 16 February 2018

Teaching Low Energy Building: Final Answers

Here are the answers to the final questions of my low energy building class.

1. The top priority for a low energy building is insulation.

Not solar panels, the latest electronic equipment, increasing the number of windows or planting grass on the roof. All my students got the right answer. They were 100% successful. In educational assessment terms, this question was 0% successful in discriminating between students. But I'm not so interested in discrimination. Just happy that all of my students got the main idea of the course, which is that insulation is the top priority in low energy building.

I could probably have put some tougher distractors in there, like mechanical ventilation with heat recovery, air tightness, good form factor or avoiding thermal bridging. Perhaps I should make a more difficult question next year.

2. Half the students got the next question completely right; eleven out of twenty-two taking the test.

This question did a much better job at discriminating!

This was a real-world low-energy building question getting them to choose the amount of insulation needed depending on the windows they were using. It assumed an energy budget for a small house of given surface area and floor area, and a fixed requirement of window area.

The question was made more tricky since they had to choose insulation thicknesses rounded to the nearest five or ten centimetres, as you tend to get in the real world. Also, in the real world, you need to round up rather than round down when you're trying to meet this kind of target. This may have thrown a couple of them.

Even worse is question 4  
As a language teacher, I usually despair at closed question, especially multiple choice questions where language is polarised into one correct answer and three incorrect ones. In the case of insulation, there are genuine discrete choices since the insulation comes in standard sizes. You can't buy 17.4 mm thick sheets, however much the calculations tell you that's what you need, although you could blow-fill a cavity of any thickness you like. The choices I gave in my test—15, 20, 30 or 40 cm of nano-porous super insulation—are almost a factor of ten thicker than the options given for Neomafoam by Asahi Kaisei, so I guess the choice would be something like two sheets, three sheets or four sheets thick.

Also, this was a matching question, with four different U values of window and five suitable insulation thicknesses to choose from. Obviously the eleven people who got the correct answer all gave the same answer, but the other eleven were each wrong in a different way.

One piece of low-hanging fruit was that with single-pane aluminium-framed windows, it was impossible to make walls thick enough to stay within the energy budget, and 19 out of 22 students got this bit.

At a conceptual level, the better the windows, the less insulation is needed in the walls, so the lower the window U values, the thinner the walls can be, and 16 of them got this in their overall answers, although two of them missed the answer for the single pane windows. A couple of them were choosing progressively thinner walls for higher U values, but both of them got the right answer for the single panes.

As for the other six students, it's difficult to be sure what they were thinking. They may have just been looking at the materials and assumed that wooden windows were better than PVC. They may have miscalcalated and not been thinking of the answers with top-down reasoning.

Anyway, I think the correct answers are:

  • Two times thinner (around 40cm) for U 1.7 Double, low e, argon, wood frames;
  • Three times thinner (around 30 cm) for 1.3 Triple, low e argon, PVC frames:
  • Four times thinner (around 20 cm) for U 0.8 Triple, krypton, insulated wood frames;
  • You can't make walls thick enough for the single pane windows (U 6).

3. I told you the coffee maker question before.

A one kW coffee maker in a teachers room, left on for 90 minutes, twice a day, five days a week, with a possible replacement for 10,000 yen with a thermos flask pot. How many weeks till the new pot pays for itself in electricity savings at 25 yen per kWh?

Fifteen of them got the right answer. One gave the precise answer of 26.7 weeks, but I was pleased to see most of them rounding it to the nearest week. Seven people rounded up and seven rounded down. Strictly speaking the ones who rounded down were wrong, both because rounding up is closer, and because you still haven't paid for the new pot yet. One person got half marks for giving 30 weeks. In a way, that's a better answer than the more precise 26.7. 

A couple gave 40 weeks, the shortest answer was 3 weeks, and the longest 250,000 weeks, which will take us to the year 6825. I'm not sure whether people will still be drinking coffee then.