How we changed our roof and cut 1.5 tons of CO2e

In 2018, my wife and I bought an old shoe factory in Paris. We entirely demolished and revamped the space.
We were conscious of the importance of energy insulation. We used hemp-based insulation for walls and a double-glass window for the facade. One thing we did not replace was the roof. We loved the industrial-style "verrière" and it was an expensive thing to do back then.
But soon after, we realized it was a bad decision. Insulating the entire flat except the roof led to severe thermal bridging. During winter, as exterior temperature fell, the glass roof was exposed to cold air from the outside and hot air from the inside leading to condensation.
In July 2021 we finally were able to replace our old single-glass roof. After one year looking for a capable artisan, we installed a double glass ceiling roof with thermal break steel from Jansen. The difference in confort was mind boggling. It was like moving in a new flat.
A question I asked myself was: how much energy and CO2 are we saving?
In February 2019, I acquired a Netatmo thermostat that controls the heating system. The device collects temperature, boiler activity on a 30min step. It has an API we can query. See code source in Python here.
In July 2020, I also got installed Gazpar, the smart gas meter from GRDF (French gas operator). While far from perfect (the data portal is often down) it allows me to get an accurate daily measure of my energy consumption.
Below is a line graph plotting internal temperature, exterior temperature (°C) and energy in Kwh between October 2020 and April 2021. Few notes:
In order to measure how much energy we are saving, we need a model. We could for instance compare our energy usage with the previous year. But since exterior temperature varies or we could have been away for our house, comparing with the previous year is imprecise.
A good model would be like a "twin" flat. It would show how much energy we would have consumed had we kept the old roof.
To do so I run a good ol' linear regression with the following inputs: The model is trained from October 2020 to February 2021 and has a nice fit! We are able to capture 90% of the daily energy variance. The graph below displays the model's predictions (dotted line) and the true energy consumed (solid line). Notice that the model "holds well" after the training period.
We can also look at the coefficients returned by our model in the table below.
coef std err t P>|t| [0.025 0.975]
Intercept -8.5130 16.689 -0.510 0.611 -41.528 24.502
occupied[T.1] 9.7810 4.453 2.196 0.030 0.971 18.591
temperature 8.3755 1.179 7.105 0.000 6.044 10.707
temperature_ext -10.0999 0.366 -27.614 0.000 -10.823 -9.376
The interior and exterior temperature coefficients make sense and can be interpreted as follow:
  • Increasing internal temperature by 1°C correlates with a 8Kwh increase in energy heat.
  • Similarly, when external temperature drop by 1°C, we need 10Kwh of energy to heat the home.
If you need some order of magnitude: 10Kwh of energy is equivalent to roughly 10 cyclists pedaling non stop during 10 hours.
So how much are we saving? The graph below displays our forecast and actuals after the new roof was installed. See the difference between the two lines? That's how much we are saving.
Another way to represent is whether we are above or below the forecast on a given day (thank you Observable team for the inspiration). On average we are saving 20kwh of daily heating energy or a 35% reduction in energy usage.
Since Oct 01, 2021 we have saved 2,300 kwh, equivalent to 720 kg CO2e (assuming a 0.3g of CO2e per Kwh of heating gas). At current trend, we would save 5,200 kwh, equivalent to 1,600 kg CO2e until Apr 15, 2022. To put that number in perspective, a French emits about 9,000 kg of CO2e per year. Not bad for a roof.
I had a lot of fun running that analysis and I am inspired by the impact. Energy used in buildings makes about 18% of Global Grenhouse Gas emissions.
If you liked that post and would like to replicate it for your home, you can find my code on Github. I am also happy to help, reach out on Twitter at @martindaniel4.
Lastly, if you like coding, data and cutting carbon, I am hiring at my startup Carbonfact.