This question was answered on 8th June 2018
The UK currently applies the reduced rate of 5% VAT to a wide range of energy-saving materials (ESMs) when they are installed in residential accommodation including: insulation; central heating system controls; hot water system controls; solar panels; wind turbines; water turbines; ground source heat pumps; air source heat pumps; micro combined heat and power units; and boilers designed to be fuelled by wood, straw or similar vegetal matter. Further information can be found in public notice 708/6 VAT on energy-saving materials and heating equipment, which is available on www.gov.uk.