The average floor area of New Zealand houses almost doubled from 1974 to 2011, yet over the same period the occupancy rate decreased. So what impact does this have on the environment?

A study by Victoria University of Wellington’s Iman Khajehzadeh questioned whether building these bigger houses is a waste of energy.

His investigation found that regardless of house size people spend the majority of their time in a few core spaces of the house and many rooms remain unused.

“My results indicate New Zealanders on average spend 16 hours a day indoors at home, and house size does not affect this. On average 55 percent of their time is spent in their own bedrooms and 30 percent in the living room, dining room and kitchen — the ‘core’ house.

“Houses with more space than the appropriate ‘core’ are considered to be at the level of large housing, based on the number of additional rooms on top of the core house.

“Features of large houses include extra bedrooms, specialised rooms like a study or media room, more than one living space, several bathrooms including ensuites, and double or triple garages.”

Having a large house means using more furniture, appliances and tools to fill additional rooms, more resources for construction and higher operating energy over the life cycle of the house, says Khajehzadeh.

“On average New Zealanders who live in owner-occupied houses use double the energy compared with what is required for living in that ‘core’ house. For example, a couple with two children living in a house with three extra rooms will use 66 percent more energy over 100 years.”

Results of the study show that house size significantly impacts the amount of furniture, appliances and tools people keep in their houses. These items generally have short lives, meaning they will be replaced several times thereby increasing energy consumption in large houses.

In addition, Khajehzadeh compared the impact of new and second-hand furniture items and appliances on housing energy use. His survey also looked at the proportion of new and second hand furniture items, appliances and tools owner occupier households had.

“Having all new furniture, appliances and tool items in a house accounts for 60 percent of the total embodied energy of a house in the first year,” he says.

“This amount decreases to 47 percent when second-hand items are considered, at the level found in the survey. Using second-hand items and extending the useful life of an item can lead to lower housing energy use.

“Appliances have the highest energy impact — therefore generally the kitchen is the most energy-resourced room, although in terms of usage New Zealanders spend limited time in their kitchens.”

Khajehzadeh, who will graduate from Victoria University next week with a PhD in Architecture, carried out several studies, including a questionnaire survey and analysis of 287 floor plans.

His inspiration for the topic came from the book Time to eat the dog? The real guide to sustainable living — written by his supervisor, Brenda Vale, and her husband, Robert Vale, both professors in Victoria’s School of Architecture.

Published in 2009, the book investigates ways to modify behaviour to save energy. In particular, the authors assessed the carbon emissions created by popular pets, taking into account the ingredients of pet food and the land needed to create them.

It suggests the eco-pawprint of a medium-sized pet dog is twice that of a 4.6-litre Land Cruiser driven 10,000 kilometres a year.

“To reduce impact on the environment, pet owners could swap cats and dogs for animals they can eat, such as chickens or rabbits,” says Brenda Vale.

“The title of the book was deliberately controversial since the impact of pets was a surprising result. We are not saying people should eat dogs and cats but that they should recognise the environmental impact of these furry friends.”

The book discusses the potential environmental impacts of our food, transport, possessions, leisure activities — and houses, suggesting living in smaller houses could be much better for the environment. This was the starting point for Iman’s study.

House size is not just a matter of the additional initial cost for buying a house, says Khajehzadeh, but has more significant future consequences.

“Having a big house is a dream for many people. While decisions in terms of house size selection seem personal, they can have significant impacts on resource use as well as the environment.

“More public awareness is needed regarding the role of house size in achieving sustainable architecture, and more consideration put into other options, such as co‐housing, so that little used spaces, and pets, can be shared by more people.”

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