Loft insulation
Many houses have old loft insulation between the joists, but this gets flattened and compressed over time by storing things in the loft, and can therefore loose its’ effectiveness.
Loft insulation (between and over the rafters) is very effective easy to install. The benefits of having decent loft insulation are quickly apparent, with c. 270+ mm a minimum desirable thickness. Even if there is some existing insulation in place, this can be easily topped-up to help reduce CO2 emissions and save energy. Installation usually only takes a few hours with minimal disruption to the occupants.
There are also DIY kits available at stores which enable raised platforms to be fitted above the insulation so that you can have a clean storage area without flattening the insulation (reducing the insulative properties).
For further information, look at the Energy Saving Trust’s website:
https://www.energysavingtrust.org.uk/home-insulation/roof-and-loft