Underlayment…the hidden piece of the flooring puzzle. Hidden but vital. And with each type of flooring you might be installing, you need an appropriate underlayment. One thing to note in case you didn't know, underlayment is not subflooring.
Subfloors are typically plywood or cement (as in a basement) that is the base structure of your floor. Underlayment, then, is the material you put down between your flooring and your subfloor. Underlayment adds a layer of protection for your subfloor against extra moisture as well as adds sound absorption and cushion. Always check with the manufacturer of your flooring to find out the best underlayment for your product.
The easiest one is carpet. Its underlayment is, of course, padding. It comes in varying thicknesses and can be made of different materials, but the idea is about the same for all carpeting. Hardwood floors and laminates typically require a moisture barrier underlayment, such as WhisperStep, which is basically a thin sheet of plastic, along with a foam layer. This helps the stability of the flooring, and it helps keep moisture from rising up through your hardwood if it is being installed over cement subflooring. If your hardwood is going to be glued down, make sure you glue the underlayment to the subfloor first.
Laminate and engineered wood "floating" floors need to have a some padding as well, which is basically a thin layered plastic. If you don't install underlayment under laminate flooring, anything — especially your footsteps can sound noisy — and you won't have the same cushion in your step like you would feel in solid hardwood flooring. Cork is becoming a popular underlayment for all types of floating flooring because of its cushion, moisture resistance, and strength. It can also be used under ceramic tile flooring.
Ceramic tile should have what they call an isolation membrane underlayment. This type of underlayment is used often on cement floors that have cracks that could subvert the continuity and levelness of the floor, which can cause your tiles to crack as well. Isolation membranes help fill the gaps and accept the differing pressure points on a very slightly uneven floor.
If there is any significant unevenness, then the floor needs to be leveled out before installing any type of flooring, and especially ceramic tile. Vinyl and vinyl tiles can be installed directly over plywood subflooring, but be sure to check the quality of the plywood. It must be smooth and even and quality-grade, or it can lead to a voided warranty and possibly failure of the floor.