They're launching a pledge to insulate all Britain's homes by 2030.
Drafty houses account for a significant part of our national carbon footprint as we leak warm air, heated by burning fossil fuels, into the atmosphere.
It's a sensible policy and one they'd need to deliver in order to meet their other manifesto pledge of a net-zero carbon economy by 2045.
Making homes more efficient could also help them deliver on another very ambitious green pledge - 80% renewable electricity by 2030.
If we use energy more efficiently, we'll have to generate less.
But with current projections suggesting only 50% renewables by 2030, they'll have their work cut out.
But many analysts predict nuclear power would be essential to fill gaps in the renewable power grid on the days that the wind isn't blowing and the sun isn't shining.
There aspiring policies for UK nature too - like planting 60million more trees restoring peat-lands.
The latter could be a win-win peat bogs lock up carbon and prevent flooding.
Everyone loves trees, but with many parties promising tree planting schemes some environmentalists are warning covering Britain's marginal land with plastic tubes filled with saplings is not the same as encouraging what's left of our natural wilderness to expand and thrive.
Hidden away in their manifesto is perhaps the most ambitious pledge of all - to take on the City over climate change.
The Lib Dems have said they will remove hidden subsidies for fossil fuels and regulate of financial services and UK registered and listed companies to set carbon cutting targets consistent with the Paris agreement.