Monday, 18 December 2017

Carbon payback in a month not three days: Check those facts!

It looks like a simple arithmetic mistake has struck again.

In preparation for my lesson, I noticed my slides proudly announcing that five jerry cans of paraffin will put out two hundred times more carbon emissions than ten square metres of glass wool insulation. I had the carbon emissions for paraffin at a quarter of a tonne, which at first seems like a lot, but it's pretty much all carbon, and each one of those atoms is going to bond with a couple of oxygens from the atmosphere as they set of heating up the planet in their cosy little threesomes. You have to remember that fossil fuels are worth more than their weight in carbon dioxide emissions! Adding this to the extra density of the paraffin, a factor of two hundred is reasonable.

To give a little extra support for my students who are good with numbers but less good with foreign languages, I wanted to add a more precise weight of carbon equivalent to the glass wool. I could have just divided the quarter tonnes by two hundred, which would have given me one and a bit, but I wanted to get a more precise figure.

My first port of call for fact checking, as usual, was google. I assumed I could just ask it how much embodied carbon was in glass wool, and it would tell me.

I quickly found this, which doesn't have any actual numbers, but has a few graphs. Very sensibly, it starts with different thicknesses of insulation to reach a respectable wall U value of 0.15 W/m2K, which would be about 23cm for glass wool. Then it has the embodied carbon value for a square metre of wall. If my I've read the graphs right, and my sums are correct, this gives 25 kg of carbon dioxide equivalent for my ten square metres of glass wool. Not two hundred times less than the paraffin, but ten times less.

I started looking for my own workings or references, but didn't find any. Usually I add a reference somewhere nearby, in the last few slides of a presentation or as a footnote of a blog post. At least it's good practice to do that, and I always expect it from my students!

So back to google again for a second opinion. I found a carbon footprint of 1.35 kgCO2/kg for glasswool in table 4.3 on page 118 of Sustainable Construction Processes: A Resource Text, by Steve Goodhew, which is the same as the University of Bath figure I wrote about before. The density is 25 kg/m3 here on engineering toolbox, which gives a slightly higher figure. But if I go with the spec on google shopping of 10kg/m3, I get to about 13 kg of carbon dioxide equivalent for the roll. Twenty times less, not two hundred times less.

There's a factor of ten error somewhere, but since I didn't keep my original workings, I can't see exactly where it is. I've checked a few times, and I'm pretty certain that the roll of glass wool is 10 square metres, and since it's 100 mm thick, it's going to have a volume of one cubic metre. I can well imagine a factor of ten error sneaking in somewhere around there.

Anyway, in terms of the return on carbon investment, instead of a three day carbon emissions payback for switching from paraffin to insulation, it's actually a month. Still seems like a pretty good idea!

This does go to show that it's always a good idea to double check calculations.

When I prepared the lesson two years ago, I was comparing an 11-metre roll of 910-mm wide, 100-mm thick glass wool with five 18-litre cans of paraffin, both of which cost 6000 yen. I'm not sure if it's a trend, or there is some fluctuation, but now the glass wool is a thousand yen cheaper, and the paraffin a thousand yen more expensive.