Porritt - is there the public appetite to tackle climate change?
With the traditional bad guys of the piece - government and big business - now showing signs of embracing the climate change agenda, we have to ask why there is not a surge of support from the Great British public.
Mr Porritt said that the activities of government and the industry currently filled him with cautious optimism.
"You can't help but notice the productivity going on inside government over the last year to 18 months," he said.
"Only now are we beginning to see a really serious application on the part of politicians to the notion of a sustainable built environment as the cornerstone of a sustainable society."
And there seems to be cross-party agreement on this, he added.
"This agenda is now unstoppably underway," said Mr Porritt.
"There is nothing that's going to happen in terms of any changed balance to the government which will take this agenda backwards - this is now a forward agenda."
And the construction sector, too, has recognised the need for change.
"The construction industry is an industry historically notorious for its lack of innovation," he said.
"I read it differently today I think we can see a lot of things by way of policy interventions already in the pipeline and a lot of new things coming down that pipeline fairly fast.
"There's a serious ferment of new ideas, energy and activity which I just don't think was there before.
"Perhaps most excitingly of all we have to recognise that there is now a sense of the importance of innovation in this whole move for change."
He said there were hurdles that would need to be overcome, questioning the UK's capacity to deliver on these changes.
The industry simply lacks the capacity to retrofit all existing housing stock in a limited timeframe, he argued, and local planning authorities may be less keen than their political masters in Westminster to deliver on this agenda.
However, the biggest challenge is overcoming public inertia, he claimed.
"With now only 25% of the people in the UK now agreeing that climate change is both happening and primarily man made, we have to ask a question about whether we've actually got enough awareness, energy and commitment in the general public to deliver on this agenda," said Mr Porritt.
"That's a question that NGOs don't like to ask because it's often much easier to dump the blame on government, big business and everyone else but actually I suspect the hardest question is why haven't we got a great surge of public opinion coming behind this to drive forward this agenda in a way we now need."
His full speech can be seen here: