Underlay: Definition, Function, Types And Beat Underlay-Floor Combinations

The durability and longevity of any flooring option depends on the type of underlay that has been used. The underlay lies just beneath the topmost visible floor covering. It consists of a layer of materials like plywood, concrete, hardboard or even a padding of thin foam. It is usually about one-fourth to half inch thick and its importance lies in the fact that it provides a flat and smooth surface for binding the topmost floor covering. Thus the purpose of the underlay can be said to be to:

• Smoothen the surface so that the flooring tiles or planks can be evenly laid,
• Improve adhesion between the floor and the subfloor,
• Provide better structural stability thereby adding to the building foundation,
• Adds a cushioning effect to the floor,
• Act as a shock absorber thereby offering protection to the floor,
• Help in soundproofing,
• Enable thermal insulation especially in places where the winters are very cold and biting.

Types of underlay

All underlays can be divided into two categories:

• Rigid underlay: Some of the common forms of rigid underlay are:
  • Plywood: These make for an excellent underlay especially for hardwood, engineered wood laminated flooring etc. They are available in the form of sheets which are smooth on one end and slightly rough on the other.
  • Underlayment panels: These are interlocking 2 by 2 ft. tiles which are relatively expensive but provide better moisture resistivity  and enable faster installation. They are ideal for carpet flooring as well as for laminate flooring.
  • Cement board: This type of underlay is only suited for mortared flooring like stone, porcelain, ceramic, terracotta flooring etc. Cement boards are easy to cut, smooth and mould resistant which is its biggest advantage.
  • Orient strand board: This type of underlay is generally used as a replacement for plywood underlay since it is cheaper, extremely durable and readily available. It is made by compressing wooden chips and gluing them together to form sheets.
• Soft underlay: Foam or cork is generally used as soft underlay. They are also said to be floating since they remain detached from the subfloor and are generally used to smoothen out imperfections of the underlay. Strictly speaking, they do not perform the function of an undelay and hence are generally used as a buffer either between:
  • The subfloor and the top floor or
  • The present underlay and the new flooring option being installed when floors are renovated.

Choosing an underlay

The choice of an underlay rather depends on the type of flooring option used. Since the underlay is also responsible for both thermal and acoustic insulation, it is always better to install an underlay before installation of the flooring option. While different types of underlay are used for different flooring options, some of the common combinations are:

• Underlay for tile flooring:  This underlay needs to be tough to prevent the overlying tiles from cracking, water resistant so that tiles can be used in bathrooms, kitchen basement etc. and be flexible enough to absorb movements related to the expansion and contraction of tiles. Thus cement board underlays and the DITRA uncoupling membrane made from polyurethane make for excellent underlayment choices for tiles.
• Underlay for laminated flooring: Foam underlay and acoustical laminated floor underlay are generally used in combination with laminated flooring. The foam underlay can either be used in combination with a moisture or vapour barrier or only by itself.
• Underlay for hardwood flooring: This underlay has to be hard and tough so as to be able to hold the hardwood planks and tiles in place. thus the commonly used underlay for hardwood is plywood, felt, cork, rubber etc.
• Underlay for carpets: Foam or rubber underlays are generally used in combination with carpet flooring.
• Vinyl underlay: Plywood or OSB make for an ideal vinyl underlay. But the choice of the same actually depends on the home-owner since OSB is a relatively inexpensive option as compared with plywood and is even used as its replacement.

The quality of the underlay is as important as the type of underlay installed. A good quality underlay will always help the floor look better and also perform the work that it is supposed to do. Hence it is very important not to cut corners with regards to the underlay when either installing a new floor or renovating an existing one.