Fix the first deck of [ply] board, then just lay the insualtion board on top, and then lay the top deck of [ply] board and screw this through to the joists. No glue required
You can buy insulation with the top layer of ply already glued to it, but IMO you are better off with a separate top deck which you can then cross bond with the insulation to bet a better surface
A VCL is an essential part of a warm roof construction. It goes above the decking and below the insulation. Single Ply can be mechanically fixed on top of the insulation without the need for another layer of ply.
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local