Moisture adsorption on FR4 PCB
If there are any fixing holes, it will be impossible to eliminate completely; using Parylene can (depending on how it is applied) provide a complete barrier. although moisture can still penetrate (very slowly).

This is very expensive solution (not least because there are very few suppliers who can perform this operation).

The other downside is it cannot be effectively removed without causing major damage to the PCB. I have some products where the use of this sealant is mandated (they are in ship-board aircraft).

I only mention this to show there are solutions, if there is a requirement for complete sealing.

I would note that different PCB materials have different moisture absorbtion rates.

'Ordinary' conformal coat comes in a wide array of types, each with their own pros and cons:

Acrylic, Urethanes, Silicones, Synthetic Rubber, Water based(!), some are UV Curable.

There are numerous manufacturers of these products and which one to use depends on the specific application.

As Sphero notes, there are system level considerations with ancillary materials to consider as well.