## External Wall Insulation: How well is it working?

How well is my External Wall Insulation (EWI) working?

I am so glad you asked. The EWI installation by Be Constructive was completed in November and at this point in the winter, it appears to have reduced gas consumption by “about 50%.”

In this article I will show you the results of my measurements so far and explain how I made this estimate.

You can find previous articles on this topic listed at the end of this article.

In my house, we use a gas boiler for hot water, room-heating via radiators, and for cooking. I have been reading the gas meter weekly for the last two years or so (see graph below) and the strong seasonal variation is associated almost entirely with heating the house in winter.

Gas consumption in KWh per day for the last two years. The data are averaged over 5 weeks to smooth out the noise. The pink boxes show the dates of key interventions which I think affected gas consumption Click for a larger version.

It is pretty clear that this winter I am using considerably less gas than in previous winters. Also we can see a decline in gas consumption after day 660 when the installation of the External Wall Insulation (EWI) began.

To put the scale in context, using 24 kWh per day is equivalent to having a 1 kW heater on for 24 h. So the peak demand of just over 100 kWh/day is equivalent to having a 4.2 kW heater running all day.

But perhaps the lower gas consumption is due to milder weather?

To check for this I can plot the temperature ‘demand’ alongside the gas consumption. The demand is shown on the right-hand axis.

In case you haven’t seen these two curves plotted together before, I will just note how strong the correlation is.

With my wife’s consent, I have kept the thermostat location and setting (19 °C) the same for this period. So I plot how many degrees below 19 °C the external weekly temperature falls.

Gas consumption in KWh per day for the last two years as shown above. and average temperature ‘demand’ shown against the right-hand axis. The data are averaged over 5 weeks to smooth out the noise. The pink boxes show the dates of key interventions which I think affected gas consumption Click for a larger version.

This winter the temperature ‘demand’ so far appears to be similar to last winter with average temperatures around 8 °C i.e. 19 – 8 = 11 °C of demand.

But instead of 70 kWh per day of gas, I am using just under 40 kWh/day. So gas consumption appears to be about 43% lower.

However we use around 5 kWh of gas on cooking and water heating even in summer – so the space heating performance appears to be improved from 65 kWh per day to 35 kWh/day, i.e. the gas used for heating directly appears to be about 47% lower.

But the uncertainties on this figure are sufficient that I think “about half” covers it for now. I really need a whole winter of performance to get a better figure.

Thermal Model

Finally I can make a model (– – –) that predicts the gas consumption in terms of the weather, and a parameter that describes the house insulation.

Gas consumption in KWh per day as shown in the first graph, and a model (– – –) which tries to predict gas consumption based on the average temperature ‘demand’ shown in the second graph. The constant of proportionality for the model is changed to allow the model to match the gas consumption in the winters of 2018/19 and 2019/20. The constant for the current winter is based on what I had been hoping for.  Click for a larger version.

The model (– – –) assumes that the gas consumption is composed of two parts.

• A year-round consumption of 5 kWh per day (equivalent to a continuous 208 W) on cooking and hot-water heating.
• A weather-dependent part that is proportional to how far below 19 °C the external temperature falls.

The weather-dependent part has a constant of proportionality which describes how much gas power is used for each degree Celsius that the external temperature falls below 19 °C.

In the graph above I have changed the constant of proportionality around Day 250 – when most windows were triple-glazed – and around Day 660 – when the EWI commenced.

• In the winter of 2018/2019 it took 280 W of continuous power for each 1 °C above the external temperature.
• This corresponds to 6.7 kWh/day for each 1 °C above the external temperature.
• In the winter of 2019/2020 it took 240 W of continuous power for each 1 °C above the external temperature.
• This corresponds to 5.7 kWh/day for each 1 °C above the external temperature.
• In this winter of 2020/2021 I hoped the EWI would mean I needed only 134 W of continuous power for each 1 °C above the external temperature.
• This corresponds to 3.2 kWh/day for each 1 °C above the external temperature.

Looking at this winter’s data so far, the actual gas consumption is below the model (– – –) suggesting that the insulation is performing better than expected. The constant of proportionality is probably close to 120 W of continuous power for each 1 °C above the external temperature (or 3.2 kWh/day for each 1 °C above the external temperature).

So how well is my External Wall Insulation (EWI) working?

• It’s performing roughly how I anticipated.

And so as the year ticks over I will add this project to the small pile of ‘Good things that happened in 2020’.

But I have – unwisely perhaps – been making more measurements – recording temperature and gas consumption day-by-day. And these more detailed measurements have been making me think I might not have understood things fully.

But all that is material for another article.

For now I wish anyone who has read this far, a Happy Christmas and a much improved 2021.

Previous articles on this topic

2020

2019

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### 4 Responses to “External Wall Insulation: How well is it working?”

1. Hsien Hung Huang Says:

Happy Christmas

2. abc Says:

I like the statement “With my wife’s consent”. 🙂
Merry Christmas, Michael!

• protonsforbreakfast Says:

Thank you. And a Merry Christmas to you also. It’s operating with this consent that has allowed me to stay married for the last 28 years!