Leafhead, or should I say Rambo! As far as I know whiteflies are confined to plants. Applying the neem oil emulsion to the greenhouse structure might have some benefit but I seriously doubt that you will find a label that allows this app.
"Neem oil can control sucking insects like aphids, whiteflies and stemborers. 30-40 ml of neem oil is added to 1 litre of water, stirred well and then an emulsifier added, e.g. liquid soap at 1 ml per litre. It is essential to add the emulsifier and mix properly. It should be used immediately otherwise oil droplets start floating. A knapsack sprayer is better for neem oil spraying than a hand sprayer." source.

The directions for mixing should be on the package and any commercial product meant to be sprayed should have an emulsifier already in it. Some products are sold ready to use and diluting them reduces the effectiveness of the product. Since we have no idea what you purchased we cannot offer the advice you want.