Sun light block on glass in greenhouse

Go back to an old way, mix carbonate of lime with water to the thickness of paint. With a large brush paint on the inside of your greenhouse. Cover your plants with a sheet of newspaper to collect the splash.

Its a product called Coolglass. You mix the powder with water and spray it on the outside of the greenhouse. It is very effective and wipes off when the sun has gone!!

Coolglas comes as a spray can or sachets in most garden centres