12 Dec 2013

Does embodied energy really matter?

We are hearing more and more about embodied energy and the role it plays in the carbon story. But what exactly is embodied energy and should we pay attention to it at all? Good questions both and the answers are not straightforward.

Firstly, what defines embodied energy? Essentially, it's the energy used to manufacture a product or construct a building. Thus building a new house involves assembling a long list of diverse materials and each of these materials had to be made from other materials and all this had to be transported from A to B to C to wherever your building site is. All this manufacturing and transportation involves expending energy and so, long before your house is ever lived in and the heating is turned on, the TV and computers get fired up and the lights switched on, you have used a great deal of energy — embodied energy.

My rule of thumb calculations show that a typical detached new house, built using brick and block and timber windows, uses around 40tonnes of CO2 in its construction. Now what does that represent? Let's have some comparators.
• The UK releases around 10 tonnes of CO2 per head of population per annum
• A car driving 12,000 miles a year uses around 4 tonnes of CO2
• A family of four flying to Spain and back uses around 2.5 tonnes
• A typical uninsulated 1970s house burns as much as 15 tonnes of CO2 per annum
• A building regs standard new house will burn around 4 tonnes of CO2 per annum
• Same house built to Passivhaus standard will burn around 1.5 - 2 tonnes of CO2 per annum

So given all that, how does the 40 tonnes of CO2 used to construct a new house stack up? One interesting observation is that if you replace a 15-tonne 1970s house with a 1.5-tonne Passivhaus, you will have achieve a net saving of CO2 within three years. In such an instance, you might feel you could afford to ignore the embodied energy total of 40 tonnes.

But of course, not many new homes are replacement dwellings, so they are in fact adding to the overall load of CO2 which Britain as a whole emits. And very, very few new homes are being built to Passivhaus standards or similar, so more typically they will be adding something like ten years of operational energy use in the construction phase alone.

This balance between embodied energy and operational energy is critical. And complex. You have to make some heroic assumptions, such as how long will a house last? And what will be the carbon intensity of energy in years to come? Does the embodied energy level go up significantly if you add energy saving features such as triple glazing to the design? What about features that may need replacing in the house — such as triple glazed windows — long before the design life of the house is through? There are no straightforward answers here. Everything rests on the assumptions made.

And what about reducing the embodied energy levels in a new house? Well, it can be done. The bulk of the embodied energy in new UK housing goes into the concrete and masonry elements because they are both energy intensive to manufacture and extremely heavy relative to volume. In the model house in the Housebuilders Bible 10, masonry and concrete materials make up 27 tonnes CO2 out of a total of 36 tonnes (that's 75%).

Interestingly, over half of this 27 tonnes of CO2 is made up of bricks and mortar, rather than concrete products. There are ample opportunities to switch to materials with lower embodied energy, such as timber and natural materials like stone and slate. Plastics and foam insulation have minimal impact overall (partly because they are so light) so that switching over to alternative materials such as natural insulation isn't really going to make any significant difference. Nor will switching to lime-based products.

And, of course, there is the question of cost. There are similar calculations to be made about construction costs versus running costs and maintenance costs and in some ways financial costs are a proxy for energy loadings. As people often point out, if carbon was correctly priced from an environmental point of view (i.e. if there was a carbon tax applied across the board), then there would be no need to carry out embodied energy calculations separate to cost calculations.

Which begs the question, should we be worried about embodied energy at all, or is it just an obscure branch of environmentalism, pursued by geeks and nerds, which is ultimately of no consequence. By the time we get to 2050, we will either have sorted out our power supplies so that they are all low carbon and thus talk of embodied energy will be irrelevant, or we will be all heading to hell in a handcart, in which case embodied energy will also be irrelevant.

I'm not sure there is a clearcut answer to any of this but energy wonks (I count myself as one) find it all rather fascinating. If you want to know more @CraigJonesUK is the man. Craig has created the invaluable (and freely available) ICE database which will tell you more about embodied energy than you are ever likely to want to know.

1 comment:

  1. Mark have you considered the carbon spike issue of building lots of new homes?
    This well described by Tom Wolley - hopefully this link will work: