When is it better to retrofit a building as opposed to demolishing and rebuilding it? Life cycle environmental and economic analyses are used to address this question through the study of a typical four bedroom detached house in Toronto. Three vintages of the reference house are used: 1930s solid masonry; 1960s wood frame; and post oil crisis, 1980s wood frame. Retrofit studies considered include insulating the attic and basement walls and air leakage sealing. Over a 40-year life cycle, the rebuild option has lower life cycle energy, global warming potential, and air pollution, which are predominantly associated with building operation. But the retrofit options have lower water pollution, solid waste generation, and weighted resource use, associated with material flows. The retrofit options also have lower life cycle economic costs than rebuilding. In this respect, the preferred options are basement plus air leakage sealing retrofit for the 1930s house, basement retrofit for the 1960s house, and no change for 1980s house. There are ways to overcome the trade-off in negative environmental impacts between retrofitting and rebuilding, such as use of renewable energy sources or re-use and recycling of deconstruction and demolition materials in new construction.Key words: life cycle assessment, life cycle costing, building retrofits, sustainable development.

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