Sealing layer in conservatory. Construction plastic?

[ab22cw]

I need help with the floor in our new sunroom. We will be laying floor tiles. What we currently have from the ground up is:
Ground
50cm air
20cm Styrofoam and joists (cc30)
Particle board

Based on advice from the carpenter, we planned to self-level on top of the particle board and then lay brick tiles.
The carpenter also said we need to apply a waterproofing membrane on the particle board. The waterproofing we found, intended for outdoor use (pool), costs 9000 for 15m2, which is a bit too expensive, in my opinion.

I assume the waterproofing needs to withstand temperatures between minus 15 to plus 45 degrees.

My question;
Can I skip the expensive waterproofing and place a plastic film (construction plastic) between the wooden board and the leveling compound?
Or
Can I replace the wooden board with something else, preferably something that doesn't require waterproofing and leveling that I can lay the floor tiles on?

Why is waterproofing needed upwards? Is it for protection against spills or humidity?

 

[Thomas_Blekinge]

Why do you need a sealing layer? I don't get it.
Building plastic doesn't have good adhesion to the filler, so I wouldn't have done that. But I don't understand the point of a sealing layer.

 

[Jonatan79]

No moisture barrier is needed. You apply primer to the particle board and then self-leveling compound to lay the floor tiles on. It's also fine to use floor gypsum directly on the floor particle board.

 

[torparavgrund]

I also think it sounds very strange. Could the carpenter have somewhat carelessly said "waterproofing" when he actually meant rolled primer?