Our houses are polluting the atmosphere. Focus on the controversy over fracking, or even the laudable plastic bag ban, and you miss arguably the greenest achievement of this government so far - plugging the leak from the places that we live.
Over a quarter of the UK's total carbon emissions come from housing, and the vast majority of that is unnecessarily wasted on inefficiently heating space or water. This isn't a problem confined to stone cottages in the Lakes. It's seeping out of relatively modern homes, keeping energy bills high. The average home built to current standards leaks out almost twice as much CO2 as the average car emits every year (1.5 tonnes).
This is unacceptable, and avoidable. In Sweden, a partly-arctic country, housing generates barely a twentieth of the country's carbon emissions. So it's absolutely right that in the face of significant, repeated reluctance from our Coalition partners, we have stuck to our guns and taken a green leap forward.
From 2016, all homes will be "Zero Carbon Homes." This policy is a golden opportunity: every new homeowner can enjoy the security of cheaper energy bills, a warmer, drier home and better health as a result. The human tragedy of climate change is brought centre stage, not dismissed.
Regrettably, a quick google of the coverage surrounding this announcement shows up how easily wishing for the "perfect" solution can end up in throwing the baby out with the bathwater, as comments like "watering down" become the overriding narrative.
But let's look at the options for the home building industry - the ones who have to deliver this:
1. Code three: our current standard. Looks OK on the face of it (insulation, double glazing and an efficient boiler). Terrible for residents' energy bills and the climate.
2. Code four: a significant upgrade. Triple glazing, extra insulation, solar panels in some cases. Last year only 10% of homes built met this standard. Now all new homes will meet it.
3. Code five: a vast improvement. No net carbon emitted by any home, either in construction or throughout its lifetime, including from heat, lighting or fixed building services.
Which did we chose? Code five. Now this might sound unrealistic, which is where the somewhat niche term "allowable solutions" comes in. Where it would be all nigh impossible to build a carbon-tight home "on site", developers aren't let off the hook. Instead they contribute to a central pot of money which will go straight back into locking up any remaining carbon leakage in other "off-setting" schemes and carbon reducing initiatives. The net result is, as Lib Dem minister Stephen Williams describes, "No Carbon. None. Nil. Nought. Zip. Zilch."
We need to build homes - 300,000 a year to catch up with our chronic undersupply and keep pace with our changing patterns of living. We cannot post-pone strong climate action.
I've been an environmental activist for 30 years. I joined Greenpeace even before I joined the Liberals! I joined the liberals in fact partly because we were, and are now, the greenest party. In Government, we have had to pitch battle every day against climate change deniers to make to make the case for tackling climate change.
Zero Carbon homes is, overall, a massive win, and we need green campaigners to come out and say so. Where there are difficult decisions to be made, such as the exemption for small builders which the Government is currently consulting on, we need to work collaboratively to find the best practical solution. We need to make sure that measures to protect small builders are not abused by big developers and that off-setting mechanisms are robustly policed. But relentless criticism will not add up to more green progress - it will just let those who consider our agenda "green crap" get their way.