A Guide To Underlay — What Is It, Which Floors Need It, and How To Choose

If you are planning to install new floors, you are most likely focused on the style, colour, and material that will look best and match your needs. However, there is another consideration to keep in mind: underlay.

Because of its necessity, there is an extensive selection of underlay options to choose from. Here, we’ll outline what underlay is, why underlay is needed, and the best underlay for each flooring material to help give you an idea of what to look for.

 

What Is Underlay?

Underlay is a treatment that exists between the structural floor and the floor covering.

Many different underlay types are available and serve a variety of purposes; from underfloor levelling, to acoustics, to comfort. Though the purpose varies depending on the material used, most floor coverings require some type of underlay.

Underlay varies in thickness from 1 mm for a simple hybrid floor underlay, through to 20 mm or greater in the case of high density rubber (for high end acoustic attenuation). The purpose of underlay varies, and it is even possible in some cases to have two different types of underlay that perform different purposes.

 

What Is The Purpose of Underlay?

Underlay, like the floor covering, is not a part of the structural subfloor of a house. Instead, it exists to enhance the floor’s visual appearance, physical comfort, or acoustic properties.

Depending on the type of floor covering you choose, it may require a different kind of underlay. For example, the thin 2 mm foam underlay used under timber or laminate floors is there primarily to disconnect the floor covering physically from the sub-floor. In the case of carpet, though, a 12 mm rubber underlay is there to feel great!

Cement fibre sheet underlay that is used under ceramic tile floors in certain circumstances acts as a compression plate over the top of timber or yellow tongue floors to prevent the tiles from cracking or the grout coming away. Additionally, underlay can provide cushioning for comfort, thermal insulation and a reduction in both structure borne and airborne noise.

Underlay is important because:

• Hard underlay types provide a smoother, more predictable surface by covering subfloor imperfections, and aid in load transfer where required
• It can provide a better looking end result
• It improves structural stability when a hard underlay is used over yellow tongue or timber sub-floors
• It minimises the hollow sound and airborne noise from foot traffic
• It helps to resist damage from rising underfloor moisture
• It reduces structure borne noise transmission in multi-level dwellings
• It adds softness and physical comfort
• It increases the longevity of flooring by providing support and protection

 

 

Do All Flooring Types Require Underlay?

In most cases, an underlay of some kind will be required. This may be very simple, such as underlay under a floating floor or carpet, through to major treatment that may be used under timber flooring in multi-unit dwellings.

There are some cases, such as with LVT or sheet vinyl flooring being directly laid over concrete, where underlay is not required. However, even in the case of new homes, some sort of floor preparation treatment will be required.

If there is damage to the structural subfloor, it is best to check with your flooring installer whether some form of floor preparation is required prior to underlay being installed. In some cases, certain floor coverings can be laid over the top of old ones. Before attempting this, always check with the product manufacturer for their installation requirements.

 

What Are the Different Types?

As already mentioned, there are a number of different underlay types, and each has a different and specific purpose that it performs. There are, however, two primary categories of underlay, and these are rigid underlays and soft underlays.

 

Rigid Underlay

Typically, the role of rigid underlay serves one of two roles:

• Provide a flat surface for the floor covering to lie on top of
• Act as a compression plate that spreads the weight from human footfall over a larger area to reduce the possibility of damage to the floor covering

In either case, rigid underlay systems are always fixed to a subfloor with adhesive, screws, staples, nails or a combination of these methods. The rigid underlay system must be firmly secured to the subfloor to prevent any movement in the floor covering as this would compromise the durability and integrity of the entire system.

There are several types of rigid underlay in common use:

• Plywood underlay is typically used under timber floors that are being direct stuck or secret nailed. This is because it is very difficult to secret nail a timber floor to concrete. On upper levels that have yellow tongue floors, adhesive will generally not bond well. Therefore, ply sheeting is screwed to the subfloor, allowing the timber to be either secret nailed or glued to the ply.
• Masonite and MDF (medium density fibreboard) underlays are typically used with vinyl products such as LVT and sheet vinyl on upper levels where there is a timber or yellow tongue Subfloor. Masonite or MDF sheets are typically nailed with a nail gun to the subfloor and the vinyl either loose laid or — in most cases of best practice — glued to this underlay system. Masonite is the more common option as it has better water resistance properties than MDF.
• Fibre-cement (FC) sheeting is used on timber or yellow tongue sub-floors when they are to be covered with ceramic tiles. As these sub-floor types have more ‘flex’ in them than concrete floors, the FC sheet provides stiffness under the tiles, and helps spread the weight placed onto individual tiles over a larger surface area. In addition to a rubberised adhesive and grout, this helps to prevent damage to tile floors on upper levels due to movement underneath the tiles themselves.

 

Soft Underlay

Soft underlay is typically used for 2 reasons: underfoot comfort and acoustic attenuation. Sometimes, an underlay even performs both functions.

These types of underlay are usually purchased in rolls and simply rolled out loose onto the subfloor. Once installed, the weight of the floor covering will hold the soft underlay in place.

Soft underlays include:

• Carpet underlay is either made from foam, rubber and felt, with many different types and subtypes. Felt has fallen into disuse due to the comfort and price benefits of both foam and rubber. Many carpet retailers will have a personal preference toward either foam or rubber variants. Sometimes this is driven by which supplier will give them a better price on a product. However, many manufacturers of carpet underlay produce both foam and rubber, so the argument is largely redundant. Both foam and rubber carpet underlay are there for comfort, and little else. Sometimes underlay manufacturers will talk in terms of the ‘thickness’ of their underlay, but this is not the only factor to consider. The density of the underlay is often the more important factor. If there is less density in a product, but greater thickness, it means that the product has more ‘air’ in it and will flatten in a shorter time. These underlays do not have a moisture barrier built into them, as both the underlay and the carpet on top of it are able to ‘breathe’, and hence to not trap moisture within them.
• Hard flooring underlay is usually either ‘open cell’ or ‘closed cell’ foam. Felt variants exist but are not in common use for floated timber and laminate floors. Open cell foam is the less expensive option and bubbles of air can be visibly seen in the product, and it is often translucent. Closed cell foam is solid in appearance and is of a higher density, and will last many times longer than open cell foam. Both usually have a moisture barrier on the back of them to help resist against rising moisture. However, in many cases, this membrane is not waterproof, being as low as 70 microns in thickness. To be completely waterproof, this barrier should be at least 200 microns thick. This is not always necessary, such as on a second storey yellow tongue floor, but is essential over concrete floors. This type of underlay is floated rather than fixed and uses the weight of the floor covering to hold it in place.
• Hybrid underlay is a foam-rubber underlay that comes already fixed to the back of hybrid flooring. The great thing about this is that it is already there and no considerations need to be made – you are simply ready to go. Very early versions of hybrid flooring did not have this underlay and required a higher density underlay such as the type used for timber and laminate floors. Fortunately, there are few, if any, of these remaining in the market. It is important to note that hybrid flooring is not designed to be used with other types of soft underlay. Many manufacturers will void their warranties if another underlay is used, as the additional compression under the floor covering can compromise the integrity of the joins of the hybrid flooring. Always check with the manufacturer before attempting this.
• High-density rubber underlay is typically used under timber floors or tiles in commercial applications. It is made from shredded card tyres, and generally comes in sheets. Rolls are available, but due to the weight they are often impractical. The rule with this underlay is if you float the floor, then you may float the underlay. If you are gluing the floor, then you must glue the underlay. This underlay is used in place of hard flooring underlay when used with timber or laminate floors, and usually provides no acoustic benefit. However, hard flooring underlays cannot be used when a floor is being glued, so in this case high-density rubber becomes the only option. For tiles, it is the only underlay option available that provides acoustic attenuation. Additionally, the tiles must use a rubberised adhesive and grout to be successful with this kind of underlay. Different densities are available, with heavier densities being used for tiles and lighter densities for timber. Some types mix cork into their composition but many are pure, recycled rubber.

 

Special Considerations

With underlay, thick is not always best. With carpet underlays, thickness with good density is the key. If the underlay is too thick and too soft, it will feel strange and flatten quickly. If it is short and overly dense it will feel too hard.

With hard flooring underlay, the reverse is more true, due to the floor spreading your weight over a larger area. Lower density, thinner underlay is the key. Low density, thick underlay will compress too much when walked upon and eventually damage the product.

Always use an underlay as intended and do not try to get creative or cut corners. For example, using a carpet underlay underneath a floating floor is a quick path to disaster.

Alternately, putting a hard floor underlay under a carpet will feel like you are walking on concrete.

 

 

What Is the Best Type for Floated Timber Flooring?

For timber flooring that is being floated, it is important to find the balance of density combined with a good moisture barrier.

Having a high density underlay is better to deaden the sound of airborne noise, but a softer density is better for structure borne noise (the noise that re-radiates through the structure of your house, and what you hear from the floor above).

It is always recommended that you use closed cell foam, as it will last the life of the floor. Open cell foam will flatten over time, rendering it useless. It will happen gradually, so you will not necessarily notice the change until suddenly the floor sounds louder than it used to.

Also, always make sure that you have an appropriate moisture membrane on the product. 200 micron is not always required, but if you have it, you will never go wrong!

 

What Is the Best Type for Laminate Floors?

All of the same rules that apply above for floated timber floors also ring true for laminate floors. The underlay used under these two types of floor covering is exactly the same if the floor is floated.

 

What Is the Best Type for Vinyl Flooring?

Underlay is typically not required for vinyl floors if they are laid on concrete. If the floor is uneven, this is better dealt with by a floor preparation specialist. The preparation professional will perform a process known as ‘Flooding’ and ‘Grinding’. This process involves grinding high points off the floor and then smoothing it with a self-levelling cement compound to fill the low points.

On upper floors, masonite is probably the underlay of choice. Both FC sheet and ply would work, but masonite is lighter, easier to work with, and less expensive — all while doing the job equally as well for vinyl floors.

 

What Is the Best Type for Hybrid Flooring?

Hybrid flooring has the underlay built into it already, so you simply take it from the box and put it onto your floor. No additional underlay is recommended for hybrid. In fact, we advise against it.
 

 

What Is the Best Type for Carpet Tiles?

Most often carpet tiles are glued down directly to the subfloor with no underlay. However, in those cases where it is required for acoustic reasons, high density rubber or foam with a short height is the best underlay to use.

 

Final Thoughts

Choosing the proper underlay is just as important as choosing the best flooring for your space.

If you have more questions on how to tell which type of underlay you need, our team of experts would be glad to help. Our experts are here to guide you through the entire process of choosing floors for your home or business, including finding the appropriate underlay.

We can also connect you with over 200 skilled tradesmen throughout Australia if you would rather let someone else handle the hard work and preparation involved in installation!