In a multi floor building if you hear noise from above, their is a good chance bad advice was given on noise reduction co-efficiencies and sound proofing of underlays !

Some of today's carpets require special padding if you want your carpet to last ten years. If you do not follow the manufacturer's padding and installation guidelines, you could void your carpet warranty. It is up to the professional carpet trader to inform you of the correct padding and installation method for each particular need. Remember as a general rule of thumb, a quality UNDERLAY will cost you more and will withstand the ware and tear of heavier traffic.

WARNING: Beware for a commonly used marketing ploy 'FREE UNDERLAY', this may be the most expensive underlay you will ever get and is offered by many retail stores, the reason it will cost you dearly, is these are common cheap lightweight padding like Black/Gold waffle rubber or frothed or re-bonded type of underlays, what you don't see is that after only a few months under your carpet the underlay (especially on stairs & in hot weather) will be flattened and will have lost it's new qualities. You will not really get this underlay for free as it is in the overall cost of the job anyway, it a common "used car salesman" type marketing ploy.

Bridgestone AirStep Waffle Rubber: (AirStep underlays)

This type of padding is commonly used today but often improperly. The waffle part of the padding gives it a thickness that is mostly air, and as a result, many of this type of lightweight underlays (less than 255g) is still too soft for today's modern petrochemical and light weight backed carpets. Despite claims to the contrary, today's "Black & Gold" rubber underlays are of domestic weight and are inferior as they are made in lighter weight and have now become a marketing ploy (#), the rubber used to make these padding's is held together with clay type binders that break down rapidly with use, if a section of carpet is in a doorway or struck by direct sunlight for a sustained period, on average only after only a few months the "Black or Gold" rubber underlay will have virtually lost most it's resilience as they are less in weight/quality, than that of a Semi-commercial 9mm Blue rubber underlay, so be aware !

Bridgestone AirStep CushionPad Textile and Fibre underlay:

CushionPad Underlays have replaced the old Traditional Jute underlays,  commonly used when one wants to limit the movement of carpet and they range in thickness between 6mm and 14mm, these underlay are now made form a blend of wool and cotton fibres & recycled textiles.  Most often this padding is used under our Natural Floorcoverings, area Rugs, Axminster and commercial  carpets.

If you can still locate the old Hair & Felt Underlay, key is density and bedding qualities. Stay with at least 1590g per sq mt (3.5lb) density on the synthetic fibre BACKED carpet. For quality carpets we recommend the lightest weight of 2270g (5lb) density with a 60% Hair and 40% Jute pads, the heaviest weight goes up to 3640g per sq mt density (8lb).

5mm Bridgestone AirStep AcoustaLay Underlayment (Soundproofing underlay):

This type of Layment is 5mm thick, 100% recycled Rubber and designed to be installed beneath the Underlay and smoothedge, or it can also be glued to floor or installed above a Ceramic Floor to create a quieter environment above and below the area!

Dunlop Re-bonded: (Dunlop underlays)

This type of padding is used most often by the floorcovering industry. It is made from of scraps of the high density foams used in furniture making that are bonded together. Re-bonded padding comes in various thickness', colours and densities . The density is rated at so many kilograms per sq metre. For example, a 2.5 kg re-bonded underlay would weigh 2.5 kg (5.5 lb)  per cubic metre (unless stated otherwise). The carpet and rug institute recommends a pad of at least 1.15 kg and 8 mm thickness for light traffic (your living room), and a pad of 3 kg and 8 mm for heavy traffic (hallways). These are minimum guidelines, and I suggest that a 6 to 8 lb+ pad for longer wear. Also, if you want a little more luxury, you can use a thickness of 12 mm (1/2 inch).

3mm Dunlop Acoustic Underlayment (Soundproofing underlay):

This type of Layment is re-bonded foam, designed to be installed as an Underlay, it can also be glued to floor or installed above a Ceramic Floor  to create as Quieter environment !

Slab Rubber (Double Bonded Installation)
(NB: Two areas of caution recommended when using this type of underlay with our natural floorcoverings. 1. The outer cut edges should to be tucked away by leaving a 6~8mm perimeter gap (as with smoothedge) to tuck the cut carpet into or the ends of the cut fibres will fray a little during installation and especially during normal vacuuming or cleaning over time. 2. When installing double bond underlay with the existing smoothedge that is 8mm high, if you fail to use a 6mm or thicker underlay +glue, will create a visible step-up from the underlay to the smoothedge.) However, these commercial underlays are the good if you use a minimum 6~8mm thick 1'800g m2 density slab rubber underlay. Unlike the waffle rubber padding, slab rubber does not contain big ripples of air. This pad feels much like a Very Heavy Re-bonded foam, but will resist furniture indentation and crushing for a much longer period of time. I recommend this pad for the high traffic areas or Rugs. This type of underlay is now one of the most used in a quality commercial installation as this underlay was designed to be stuck directly to the sub floor with the Floorcoverings being stuck to it, making this type of underlay one of the best especially, if you have radiant under floor heating. If you can afford this type of underlay you will not regret the extra cost incurred as the quality will long remain after the price is forgotten. The only problem is if you need to take up your carpet form water damage etc. the underlay gets wrecked & it is VERY difficult to remove the remaining underlay from the back of your carpet.

Foam Sheet Underlay: This type of padding is made from urethane foam and is available in different densities and thickness'. Generally this type of pad, regardless of the thickness, is not recommended for heavy traffic of any kind. All that air under the carpet just causes the carpet to move up and down to quickly & so much that the backing (_*) soon breaks down.

Frothed Foam: Some of this padding is a super dense urethane is approx. 6mm (1/4") thick and extremely durable. Recently a heavy version of this type of pad was recommended by the makers of carpets. This pad has virtually no VOC's (off gassing), can be used under all carpets. It is claimed that it will, reduce furniture indentations, prolong the life of your carpet better than any Re-bonded, fibre, waffle rubber or prime foam. It cost about the same as a good slab rubber padding and will last a long time.

Special Circumstances: There are certain carpets that require unique pad usage. One of these is a woven carpet. These carpets should be installed over a extra heavy super dense padding. In regards to radiant heated floors, if one looks at the R values of the various types of cushioning, it becomes readily apparent that SFRP (Slab Foam Rubber Pad) may be one of the best choices and even then, the thinner the better. SFRP in 6mm (1/4") thickness has an R value of only 0.31 - the hands-down winner by a massive margin. It has even 1/2 the R value of it's nearest competitor, Waffle sponge rubber, at 0.60, Frothed foam also works well with radiant heated floors but at 0.66, is better than some choices, but doesn't come close.

(_*) Modern carpets have a secondary backing that gives this type of carpet its dimensional stability & strength, as in a woven carpet is the primary backing and usually stronger !