Montreal homes are the most sustainable in the country, and Edmonton's the least, according to a new University of British Columbia study that compares average household greenhouse gas emissions in major cities across Canada.Using census data over a 12-year span, researchers ranked cities on how much carbon dioxide the average Canadian family (two to three people) with an annual income of $81,000 in each city produced in a year from the combined use of electricity, gasoline and natural gas.The average Montreal household produces five tonnes of GHG per family per year, while the average Edmonton household emits 20 tonnes per family per year.