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Carbon emissions – The difference between natural and synthetic insulation

There is often an assumption that when you insulate a building, you instantly start reducing your carbon emissions by reducing your heat loss. Whilst that is true, the carbon cost of creating the insulation used is not often factored in and so the point at which you actually start reducing your overall emissions varies considerably. Given that any carbon we emit now has more of an impact than carbon we produce in the future, we should really count the carbon cost of the materials we use.

To highlight the differences between materials and what impact they have on our carbon emissions overall we’re going to compare external wall insulation on a brick building in the midlands and to make this as transparent as possible, we’ll run though the calculations and assumptions. If this is of less interest to you, feel free to skip to the end!

So, starting with the building, we’ll base our assumptions on a solid 225mm walled brick building with a starting U-value in the walls of around 2.0 W/m2K. We’re going to insulate the building with 120mm of mineral wool slab and 120mm of wood fibre board so that our U-value is down to around 0.29 W/m2K, or a saving of 1.71 W/m2K per m2.

We heat this building from the beginning of October until the end of March using gas or electric heating, both currently produce around 180g of CO2 per kWh of heat energy. During the winter months, we’ll work on two levels of heating, one where the internal temperature is an average of 16C and one with an average of 18C during the winter. This assumes that the house isn’t continuously heated but instead for around 2 hours in the morning and 4 in the afternoon/evening, hence the fairly low average internal temperatures.

The average external temperature in the midlands area for this period is around 5.3C and so we’ll assume an average continuous temperature difference of 10.7C or 12.7C for the two internal temperatures. If we compare the heat loss before and after the insulation has been added, we’re saving 10.7 x 1.71 = 18.30 W/m2 at 16C and 12.7 x 1.71 = 21.72 W/m2 at 18C. Assuming the above emissions for heating, this works out at a saving of either 3.29 gCO2/m2h or 3.91 gCO2/m2h respectively.

Now we look at the embodied energy of the insulation materials and we’ll assume that all components within the systems are the same so that the only comparison is the insulation material itself. A typical 120mm thick mineral wool EWI batt will produce carbon emissions 15.66 kgCO2/m2 (based on EPD data). However, 120mm of typical wood fibre board would emit (actually lock up) around -23.11 kgCO2/m2.

So, if we use the mineral wool option it would take either 4760 hours or 198 days and 4005 hours or 167 days to save the energy required to make the insulation material. This would be more than the entire winter period in our model and so it wouldn’t be until the following winter that you actually reduced your carbon emissions.

If we look at the wood fibre option, rather than taking time to save energy to create the insulation, we’re actually soaking up previous emissions. In this case the wood fibre absorbs CO2 equivalent to us having insulated the walls between 7024 hours or 293 days and 5910 hours or 246 days previously. This is well over the heating season of 6 months we’re assuming and so it would be as if you had installed the insulation more like a year and a half previously.


In summary, it is very important to consider carbon emissions from the materials we use so that the impact of our efforts is immediate. As you can see from materials like wood fibre insulation, we actually have something that can undo some of our past emissions and so the more we can do to use natural fibre insulation, the greater chance we have of steering away from climate disaster.

Hopefully the above makes sense but if you need clarification, please feel free to contact us.

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