There is an interesting post on the Guardian's Green Living Blog about this. In a study in Scotland, a two bedroomed cottage takes 80 tonnes of CO2e to build, using standard building techniques and materials. If built to building regs energy efficiency levels, this is the equivalent of six years' energy consumption/CO2e emissions living in the cottage. If that cottage were a Passivhaus, and assuming the energy needed to build it was similar to the standard build, it would take 60 years to 'payback' the energy/CO2e. This gives some support to the argument used by natural material proponents that, if you build a very low energy house, the embodied energy in construction becomes proportionately more important in determining the building's lifetime energy consumption. In the post, they have concluded that refurbishing is the best option (compared with either new build or do nothing) because a refurb of the same sized property is only emits 8 tonnes of CO2e.
Here is a quote from the article. The link to the original is here.
"80 tonnes is a lot – equivalent to five brand-new family cars, about six years of living for the average Brit or 24 economy-class trips to Hong Kong from London. But a house may last for a century or more, so the annual carbon cost is much less – and for all the new-build options, the up-front emissions from construction work were paid back by savings from better energy efficiency in 15–20 years.
However, the winning option was to refurbish the old house, because the carbon investment of doing this was just 8 tonnes CO2e, and even the highest-specification newbuild could not catch up this advantage over the 100-year period. Once cost was taken into account, refurbishment became dramatically the most practical and attractive option, too."