Your Carbon Budget - Understanding Embodied Energy


A carbon tax has been in introduced in ROI on everything from domestic heating oil to the diesel that powers our cars and machinery. So what can you do to reduce the emissions arising from building your own home? Start budgeting! Most people who have gone through a self-build will tell you that costs have a nasty way of spiralling out of control. For a project to be successful, costs simply need constant attention and monitoring. Much in the same manner, carbon emissions can also creep up at an alarming rate, and the answer to keeping them in check is the same: budget, measure progress every step of the way to minimise them when and where necessary.

So where do you start? There are different stages to consider when it comes to measuring how much your home contributes to greenhouse gas (GHG) emissions: birth, life and death (often referred to as 'cradle to grave' or as 'cradle to cradle' in the case of recycling and repurposing/reusing). The so called 'cradle' of your building starts at the mine in which the copper was extracted for your plumbing, it starts at the quarry, in the forest and in the oil field. Your building blocks all come from somewhere and the energy expended to extract, manufacture and transport them to their final destination (your house) is referred to as embodied energy.

According to the Sustainable Energy Agency of Ireland (SEAl), the embodied energy of a house is typically over five times that of its annual energy consumption. This means approximately 5-10% of the total energy consumption expended during the life of the house relates to its construction. But as we move towards passive housing standards (homes whose energy consumption is kept close to zero) this percentage is likely to increase considerably. Indeed, as we consume less during the lifetime of the house the impact of construction on your build's lifecycle emissions is likely to gain in stature.

At present, the lifetime of the building is what really makes a difference in terms of carbon emissions. The occupants' habits will have a huge impact on overall energy use, a point highlighted by the fact that appliances and lighting are the biggest energy consumers. Efficiency is also a consideration: for instance, older appliances normally tend to guzzle more energy than newer ones.

In terms of materials, there are a few things to consider over the building's lifetime, namely its thermal mass (how well the materials retain and release heat, with exposed concrete rating high in this sense), the amount of volatile organic compounds emitted (plastic emits VOCs throughout its lifetime), water consumption, eco­ sourcing (such as sustainable forestry), the environmental impact of quarrying, etc. Furthermore, the building's end-of-life is also responsible for emissions, in the form of demolition, recycling (goes back to the factory to be processed) and waste disposal (Iandfills, burning, etc.).

Number, numbers everywhere

As we're dealing with the building element here, this article will focus on the emissions that arise right before you move in. Here, you have two components to take into account: the embodied energy of your materials and the processes involved in the construction of your home.

The embodied energy pertains to how much energy (and corresponding greenhouse gases) is generated along the entire supply chain - extraction, manufacture, transportation. There are also emissions associated to reclaiming, reusing, recycling or disposing of various building components on site. Adding up the embodied energy of the materials you plan to use will help you make a decision on which products to choose in conjunction with the rest of your decision-making considerations (e.g. personal preferences, thermal performance, etc.).

To pickle matters even further, there isn't a universal 'embodied energy catalogue' one can turn to - not only is this due to the plethora of products out there (timber forested in Ireland will have lower embodied energy than one forested in South America) but also to methodologies. For instance, some will take into consideration the replanting of trees and timber's absorptive qualities, which leads to timber possibly scoring negative values. As you will see in the figures quoted below, extracted from the Concrete Association and the Forestry Commission in Scotland, different methodologies are used by different interest groups. The total embodied energy of a build will depend on how the building has been constructed - two timber frame houses can have very different embodied energy profiles, depending on their size, building methods, etc. This is where the services of an independent consultant can be particularly helpful in putting together unbiased data that will compare the different building options out there for your specific design requirements.

The processes include how the building is constructed (off-site is less energy intensive than on-site construction, larger buildings require more materials and therefore more energy to produce them, etc.) and by what means. So-called "management carbon" looks at the impact arising from administrating the build which would not have arisen had the build not proceeded, e.g. electricity used during construction, going to and from the site, etc. Here a number of mitigation measures can be put into place, namely issues pertaining to transport and consumption on site. Other considerations such as health and safety must also be taken into account, e.g. lighting the site at night where appropriate, etc.

Embodied energy

Different companies will engage in different processes, some more energy intensive than others. However for standard materials, such as plywood for example, most companies follow similar manufacturing processes. In this context, the work done at the University of Bath is particularly helpful in comparing different materials' embodied energy. A group of engineers have compiled statistics on the embodied energy and associated carbon emissions of building materials in the UK (see below).

Here's where the bean counting comes in: each component must be added up cumulatively and be literally weighed. You can of course get a professional to help you out here or use an online calculator, such as the UK's Environment Agency's Carbon Calculator for Construction Activities

Embodied energy of common building materials

Concrete (general)                 0.130 kgCO² per kg

Steel (Average)                       1.77 kgCO² per kg

Stone (General)                       0.056 kgCO² per kg

Timber (General)                    0.46 kgCO² per kg

Lime                                        0.74 kgCO² per kg

Limestone                               0.017 kgCO² per kg

Slate                                        0.006 to 0.056 kgCO² per kg

Single Brick (2.8kg)                 0.62 kgCO² per brick

Concrete Block (8MPa)           0.061 kgCO² per kg

Mineral wool insulation          1.20 kgCO² per kg

Fibreglass insulation               1.35 kgCO² per kg

Plywood                                   0.81 kgCO² per kg

Hardboard                               0.86 kgCO² per kg

Plasterboard                           0.38 kgCO² per kg

MDF                                        0.59 kgCO² per kg

Ceramic tiles                           0.59 kgCO² per kg

Copper                                    2.19 to 3.83 kgCO² per kg

Linoleum                                 1.21 kgCO² per kg

Vinyl Flooring                          2.29 kgCO² per kg

Rubber (general)                    3.18 kgCO² per kg

Standard carpet                      3.89 kgCO² per kg

Wool carpet                            5.48 kgCO² per kg

Paint (general)                        3.56 kgCO² per kg

PVC                                          2.41 to 2.60 kgCO² per kg

Glass (general)                        0.85 kgCO² per kg

1.2mx1.2m Single Glazed Timber Framed Unit     14.66 kgCO²

Timber Framed Unit                                                           12 to 25 kgCO²

Aluminium Framed Unit                                                       279 kgCO²

PVC Framed Unit                                                                110 to 126 kgCO²

For Krypton filled instead of air or argon filled add              26 kgCO²

For Xenon filled instead of air or argon filled add                 229 kgCO²

Source: University of Bath, Inventory of Carbon and Energy, 2008. uk/mech-eng/sert/embodied/

( business/sectors/37543.aspx). The values refer to the amount of embodied C0² in every kilogram of material/ product. Timber is lighter than concrete, hence the seemingly lower value for concrete per kilogram, while plastic is light but it also has a high embodied energy, hence the high value. Also the amount of concrete used on a build is high: on average you can count 30 tonnes for your foundations.

Some companies and interest groups may be tempted to quote figures that put their products in a better light so preferably do your own bean-counting or hire an energy assessor

Remember that in Ireland today, about 90% of the build's emissions correspond to what happens after completion. The above calculations will therefore have to be weighed against the lifetime performance of the materials. For example, the embodied energy of a highly insulated window will be much higher than that of a single-glazed window but this is by far outweighed by the lower emissions clocked up over the lifetime of the house. In the case of concrete, the Concrete Association conducted a study which showed that while a typical concrete and masonry house with a medium level of thermal mass had around 4% more embodied C0² than an equivalent lightweight frame construction, this could be offset in as little as 11 years due to the energy savings provided by its thermal mass. That is, assuming that the concrete is left exposed - otherwise the thermal mass benefits are minimal.


Translating the energy used on your build into its C0² equivalent is less straightforward than you might think. In the case of oil for example, it will depend on the machinery used: different model cars are more or less efficient while heavy equipment such as diggers will use considerably more fuel. In the case of electricity, each country has a different "fuel mix" and despite efforts to introduce more wind power, Ireland still largely relies on fossil fuels and therefore emits a considerable amount of C0² for each unit of electricity produced. Also, the more efficient the machine/technology that transforms the fuel into heat or electricity, the less the emissions.

It's worth noting that greenhouse gas emissions are most commonly measured with C0² emissions as a benchmark equivalent. On a macro scale this is measured in tonnes of C0² equivalent (on a micro scale, it's measured in kilograms of C0² eq) much in the same way that tonnes of oil equivalent (toe) are used as a unit to compare various fuel sources for the amount of energy they release.

Typical conversion factors for management carbon, measured in KgC0²/kg

ROINICarbon Tax (ROI)*

1kWh electricity 0.5380.544-

1kWh natural gas 0.2060.184 €0.003

1kg peat (briquettes) 1.834 €0.0275

1km petrol car** 0.170.182 €0.0026

1km diesel car** 0.1550.153 €0.0023

1passenger km heavy rail 0.0443

Sources: ROI emission factors: Sustainable Energy Ireland (2008), Commission for Energy Regulation (2007) and Irish Rail as extracted from Change CMT Calculator. http://cmt.epa. ie/Global/CMT/emission_factor_sources.pdf

NI emission factors: DEFRA, extracted from the

* The electricity sector already pays for the carbon it emits under the UN's Emissions Trading Scheme and therefore is exempt from paying a carbon tax. To put the numbers in perspective in ROI an average household consumes about 13.750 kWh in natural gas every year. which equates to less than €50 a year in carbon tax. The impact of the carbon tax is therefore currently minimal, especially in the context of one-off projects.

** ROI refers to engine size between 1.51 to 1.7 litres and NI refers to up to 1.4 litre engine size for petrol and up to 1.7 litres for diesel