Most of the products that are used in commercial flooring are glued directly to the substrate whether that substrate is wood, concrete or on top of another flooring product.
My discussion will focus on installations over concrete substrates. Most people do not realize that concrete is porous like a sponge. In fact, water is required for the cement to hydrate and solidify. It takes time for the chemical process called cement hydration to produce crystals that interlock and bind together. It is not advisable to install flooring on top of fresh concrete because the water must be evaporated in the hydration process.
Moisture vapors can and do permeate though concrete. If the concrete is on grade and there is no or an incomplete moisture vapor barrier, then moisture vapors can wick up through the concrete. Today’s flooring and adhesive manufacturers have answered the call to provide more eco friendly adhesives. Many of more environmentally adhesives give off less harmful vapors into the atmosphere. The effect is that these adhesives are often less forgiving and less able to resist moisture than the old solvent based adhesives that used to be on the market.