People seem to consider heating/cooling efficiency to be the main determining factor of sustainability - an they are very important. But shouldn't the lifespan of a building matter, too? Doesn't matter how efficient a building is, if it's got to be replaced in 15-20 years, is it really that sustainable? Takes a lot of energy to replace a building every 20 years! (Whereas older buildings seem to be able to last a lot longer for some reason.)
How long should the lifespan of our buildings be? What should we be aiming for?