How important is embodied energy?

A common debate among green designers is which is more environmentally appropriate: to renovate or re-skin an existing building or to build a new building. Aside from the heritage or cultural aspects, the main environmental advantage cited for building reuse is the reduction in embodied energy. This however is dwarfed by the operating energy expended over that building’s lifetime.

There are two types of embodied energy: initial and recurring. Initial embodied energy is the energy consumed to harvest/manufacture, transport, and assemble building materials to construct a building. Recurring embodied energy is that which is expended to maintain or repair building materials or systems (separate from the ongoing operating energy needs). Reusing an existing building can reduce initial embodied energy, however reusing an existing building does not eliminate embodied energy since the building will have to be renovated to improve energy efficiency (new windows, added insulation) and adapted to its new use (new interior walls, carpet ceiling tile).

The third energy use of a building is operating energy: the gas and electricity used to heat, cool, and light buildings. But how do these three energy values compare? Research indicates that, at most, embodied energy is as high as 10 to 15 per cent of the energy used in the 100-year lifespan of a building, with this percentage decreasing as the building ages. A CMHC study of a social housing project in Ottawa found that the energy use breakdown was 74 per cent operating energy, 16 per cent initial embodied energy and 10 per cent recurring embodied energy over a 40-year life.

Despite the appeal of reusing a building shell to minimize embodied energy use, new buildings tend to operate more energy efficiently than renovated buildings. New buildings can better orient to minimize heating and cooling loads, be designed with a narrow floorplate for better daylighting, and the ceiling heights can be adjusted to accommodate larger ductwork. Similarly, it is often difficult to add additional insulation to an existing building without changing the dew point in the walls, and this can create durability issues.

Of the many building renovation projects Enermodal has worked on, the best we’ve been able to achieve is 50 per cent energy savings relative to the MNECB. In comparison, the new Enermodal office building built on an abandoned site but orientated along a true east-west axis is designed to achieve 80 per cent energy savings. While this is but one example it does show that new buildings can achieve much higher energy savings than reused buildings. In comparing these two cases, after eight years, the new building will have lower total energy (embodied plus operating energy) and will be way ahead of a reused building over the building life.

The LEED rating system attempts to balance the need to reduce both embodied energy and operating energy of buildStephen ings. LEED 2009 provides 19 points for the operating energy efficiency of a building compared with four points available for building reuse. Whether a new or renovated building, LEED encourages reduction in embodied energy through the use of salvaged, recycled, and local materials as well as providing points for designing building durability.

Certain building materials require less energy to harvest or manufacture than others, such as wood that has lower embodied energy than concrete. However, the comparison is not simple, as concrete provides more thermal mass than wood and this can reduce operating energy. The embodied energy associated with various building materials is available using the ATHENA Impact Estimator for Buildings software. However, Enermodal’s experience during the design of a dozen building reuse projects is that it is difficult to reduce the overall energy use of a building through alternative material selection.

A renovation of an existing building or the reuse of a significant portion of a demolition project obviously has lower embodied energy than a new build. However, to say that a renovated building is more environmentally friendly than a new building as a result of this fact is incorrect. Of course the embodied energy of a building is important and design teams should consider how to minimize this environmental impact. However operating energy, which is expended year after year for upwards of 50 years, should receive the most attention by design teams.

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