Floor underlayment is an important thing to get right when laying new flooring, a lot will depend on the flooring that is going to be laid down. Underlay is the layer between your floorboards or concrete flooring and your new floor covering.
It is important to get the right floor underlayment to your new flooring to help prolong the life of the material and keep it strong and level. Floor underlayment also helps to keep your home warm by insulating the floor and helping prevent coldness rising from the subfloor. Underlay is needed to protect your subfloor from dust, dirt or moisture that may be present on any floor, and to protect the new flooring from underneath against rough surfaces on the subfloor. Underlay can also act to cushion the flooring for comfortable walking and to make the floor quieter to walk upon. As walking on flooring that has underlay will echo a lot less even in a larger room.
Hardwood floors and laminate flooring will need an underlay of foam or polyurethane as these are more cushioning, as without underlay the wood will be squeaky and could snap underfoot. Rubber underlay would be used under tiles as they need something more robust and firm otherwise, the tile and grouting could crumble away. For most other flooring types like vinyl tiles, a layer of plywood is an ideal underlay, especially in kitchens and bathrooms as it can withstand any moisture. If you have a large room to fit floor underlayment to, you may have to consider cheaper types of underlay but, this can be a false economy as cheaper types don't always have the protection needed against mildew and dampness.
Cork floor underlayment is a great environmentally friendly underlay choice, it is also very cushioning and has the best sound absorption.