Wednesday, 17 January 2018

Talking about Passive House. Lesson 14

This low energy building course is really just propaganda in disguise. The whole course has been framing the question, and the answer is Passivhaus. I also like to talk about differences in building culture, and mention my own experience trying to build a low energy house in Japan. I can, with honesty and some innocence, present my discovery Passivhaus as an epiphany on my road to building a house.

As usual term starts running out with too many lessons left over. In week 13 of 15 I had to get them into groups for their final presentations, and then give them some guidelines of what to do, and what not to do, when designing and delivering their own presentations. I could spend a whole lesson talking about preparing presentations. In this class I probably should, focusing on presentation construction as a piece of architecture.

They had put their own presentation ideas into a forum on the online part of the course, and then chosen their top three choices in an online quiz, so I had most of the data needed to make the groups, but of course a few students had not actually added their top choices for a presentation topic, a few others missed the lesson, and as usual a small number of the topics were very popular and they did not fit neatly into seven topics that were the first choices of exactly four students. In the end over half the lesson was taken up discussing their presentations, and I could only get through half of my beautifully prepared full lesson, with its narrative from building cultural differences between the isles on the East and the West of the Eurasian continent, to my own journey into house building, and discovery and application of the Passivhaus standard.
Energy balance for a Passivhaus

Descriptions of standards can be dry, and there's a maximum of ten minutes I can talk to any class in English before they lose attention, so tasks are needed. I like setting them problems to solve, and also want them to practice real-world calculations where possible.

First I had them brainstorm heat gains and losses in a house. They got most of these, but needed a bit of a hint to remember ventilation.

The next task was to get from the definition of Passivhaus in English to the numerical heating load. This is a fairly straightforward calculation from the floor area per person, the volume of air needed per person, the maximum temperature air can be heated to before it burns, and the heat capacity of air.

Next, I wanted them to work out what U value they would need for the walls of a Passivhaus in Matsumoto. This involves several steps, and I made the mistake of giving them too many of the steps to work out in one go. I don't think the calculation itself is particularly difficult, but I guess I'll find out because I've set that for their homework!

The first step, to make the calculation easier, is to assume that the heating load is equal to the loss of heat through the walls. Remembering the energy balance of a building, you can get to this by assuming that solar gains through windows roughly equal heat losses through windows and internal heat gains roughly equal heat losses through roof, ground, and ventilation.

The next step is to work out the wall area of the house. I gave them the volume, told them it was two-story, and assumed they'd just be able to work out the wall area from that. Half of them are studying
architecture so I think I can be forgiven for my assumption. It turned out to be wrong though. Perhaps there were too many assumptions for them to make: the height of the walls, the squareness of the building footprint, the use of square root to get from an area to one of it's sides, the number of sides on the square... Perhaps they were worried about other things: did they need to subtract the windows and doors? What shape was the roof going to be? Perhaps they were distracted from this question because they were expecting thermodynamics rather than geometry. Anyway, I think have learnt my lesson, and will chop the problem into bite-size chunks for them next time.

I should probably have realised this sooner, and modified the task, but while preparing the lesson I had been more impressed by the result of the calculation: 0.162 W/m2K. This number may not mean a lot to you, but as I scrolled down the slides to the introduction to my own house, I noticed that in fact the U value of my walls is 0.162 W/m2K. Perhaps just a coincidence, but it does show you
the power of rough estimates!

Unfortunately I didn't have time to share this bit of synchronicity with my students as the lesson had somewhat dissolved into scratching heads, spurious scribbling and many over-precise, under-accurate sums. The bell was going to go before I got to the happy ending that is Matsumoto Passive House.