The embodied and operational energy of a home

The embodied energy of a home is the energy consumed by all of the processes associated with the production of a building, from the mining and processing of natural resources to manufacturing, transport and product delivery. Different studies [1.2] show that the embodied energy takes up a maximum of 10% of lifetime energy use for a home. This percentage can increase a lot however as new buildings become more thermally efficient. Although not unimportant it does confirm to pay more attention to reduce the operational use of the building. Since we are building a passive solar house there will be no heating system. Hot water is the next big operational user so we need to look at a very efficient system for heating water. Probably a hot water heat pump.

Higher embodied energy energy is not always a bad thing. When increasing the insulation levels more material is required so the embodied energy of the house goes up but this will be offset by lower energy requirements for heating. Concrete has a higher embodied energy but also has good thermal storage capabilities.

A study from the Victoria University of Wellington [3] showed optimum heating CO2-e reductions are achieved with insulation values around R4-R5; beyond these R values total emissions increase again due to higher embodied energy.

An interesting observation that I read in this article is that thermal storage is not needed if buildings have sufficient thermal insulation and new research is showing that buildings with exposed wood interiors may be much more thermally efficient than previously thought, because of moisture movement in and out of the wood surfaces during heating and cooling [1] (L.Bellamy, pers.com).

In the book “A deeper shade of green” from 2008 a chapter has been written about the embodied energy and operational use of materials and homes in New Zealand. It is based on research by Branz for the coldest climate in New Zealand which Northland where we live luckily is not.

Current build timber homes have an embodied energy of 23.7 MWh and a heating requirement of 4.1 MWh/year. Homes on a concrete slab 29.2 MWh with a heating demand of 3.2 MWh/year.

For a next generation house with higher insulation values these values are respectively 26.5 MWh and 1.6 MWh/year.

Converting these numbers to CO2e emissions they range between 3.3 to 4.4 tonnes CO2e for a home. These numbers seem really low to me but on the other hand timber, which most homes are build of, has a negative footprint and New Zealand has a high percentage of renewable energy.

	Embodied energy	Equivalent in ton CO2e	Operational heating	Equivalent in ton CO2e
Current
Concrete slab	29,2 MWh	4.0 ton CO2e	3.2 MWh/year	0.44ton CO2e/year
Timber	23.7 MWh	3.3 ton CO2e	4.1 MWh/year	0.57 ton CO2e/year
Future
Concrete slab	32.0 MWh	4.4 ton CO2e	1.0 MWh/year	0.14 ton CO2e/year
Timber	26.5 MWh	3.7 ton CO2e	1.6 MWh/year	0.22 ton CO2e/year

 

So how do these numbers compare to our expected operational energy?

According to our architect our predicted energy use (excluding our electric vehicle) would be around 25 kWh/m2. For PassiveHaus certification the energy use needs to be below 15 kWh/m2.

So with our 72 m2 home that would total up to 1800 kWh/year. The electricity mix in New Zealand causes 0.138 kgCO2e/kWh so per year this will cause 248 kg CO2e. Based on a lifetime of 50 years this totals up to 12.4 tonnes CO2e.

A lot of numbers but in the end it comes down to the following expected emissions:

Building the house: around 4 tonnes CO2e

Operational use for 50 years: 12.4 tonnes CO2e

So about 24% of the total emissions of our home is the embodied energy.

Comparing this to the average Kiwi household emissions of 15.6 tonnes /year and our own estimated household emissions of around 8 tonnes/year building a home in New Zealand is actually  pretty good value compared to the emissions it causes.

For 4 tonnes of CO2e emissions you get a house, a return flight to Tonga with the family or 25,000 km in a petrol car. I’ll take the house and hope it would last over 100 years instead of the 50 year lifecycle calculation.

 

Margriet Geesink

 

[1] http://www.civil.canterbury.ac.nz/pubs/Energy-CO2AdvantagesWoodSustainableBuildings.pdf

[2]https://cshub.mit.edu/sites/default/files/documents/LCAforResidentialBuildings.pdf

[3] http://www.lcanz.org.nz/sites/default/files/poster_4_alcorn_a.pdf