Comfort layers consist of the upper few inches of a mattress and are a very important part of all mattress construction. They are primarily responsible for pressure relief which is one of the two main functions of all mattresses. The comfort layers also include any quilting layers used in the very top part of a mattress since they act together to give a mattress its pressure relieving qualities. There is a wide variety of materials used in comfort layers. Unfortunately, some that are quite commonly used are completely unsuitable for use in a mattress, and especially in the comfort layers. Other materials have very high quality and effectiveness. Comfort layers can be made of a single layer or several layers of different ILD or materials.
Like the support layers, comfort layers have a primary function and a secondary function. The primary function is to provide pressure relief while you sleep by forming a cradle that is shaped to your body profile. If you change positions as most of us do when we sleep, then the ability to quickly change the shape of the cradle with different sleeping profiles is also important. The secondary function of a comfort layer is to support or help support the more recessed areas of the body (such as the lumbar) and prevent them from sagging.
There are also three main methods of mattress construction. Progressive constructions use a thinner comfort layer which "borrows" from the mattress core to aid in pressure relief and in supporting the recessed areas of the body. Differential constructions use thicker comfort layers which provide most of the pressure relief by themselves and also contribute most of the support for the more recessed areas of your body profile such as the lumbar. Zoned constructions may use either thinner or thicker comfort layers.
For comfort layers to be able to both relieve pressure and fill in and support the gaps in your profile, they need softness (to allow you to sink in deeply enough to form a cradle), progressive resistance (to increasingly stop the sinking so you don't go all the way through the layer), point elasticity (the ability to shape itself exactly to your profile), and resilience (the ability to push back and hold the more recessed parts of you up).
Different materials have different combinations of each quality and are suitable for different types of construction.
Latex is made either from a milky liquid that comes from a rubber tree and is processed into a foam using one of two methods called Talalay and Dunlop. It is basically "rubber foam". It can also be made artificially with a synthetic alternative called SBR which is a made from petrochemicals and is less expensive. When it is made from natural rubber it is called NR latex. Blends of NR and SBR are widely available and very similar to NR in quality while pure NR latex is less common, particularly in Talalay. While 100% SBR is chemically very similar to Natural Latex, by itself it has less of the desirable elastic and "supportive" properties of NR latex and is often not as durable.
Talalay is a more complex and expensive production method that produces a lighter and more consistent foam which is especially attractive to many as a comfort layer because it can be made softer than Dunlop in ILD's as low as 14. While Talalay made from NR has more elastic qualities than the blended version and is slightly "springier", and more "supportive" ... in the softer ILD's below 18 Talalay may be more durable as a blend than as NR latex.
Dunlop is a more common and widely used method which produces a denser and slightly less consistent foam with qualities that make it attractive to many for support layers or to those who like firmer, less "springy" comfort layers. It is not made in as soft ILD's as Talalay with the softest generally being in the high teens to low 20's ILD range. Although the properties of 100% natural Dunlop are generally preferable to synthetic blends ... there are now newer forms of continuous pour Dunlop latex that are mostly synthetic that are comparable in softness to the softest Talalay and are comparable in terms of durability to natural Dunlop latex.
Latex is among the highest quality materials that can be used in a mattress and is unusual for its abilities to be both soft and supportive at the same time. Because of its ability to shape itself to the profile of a body, it has similar pressure relieving qualities to memory foam, especially in the lower ILD's. It is also the most breathable and durable of all the different types of foam. Its durability makes it especially suitable for use in a comfort layer as this is the part of a mattress that is the most stressed and subject to breakdown and it can outlast innersprings or any other type of foam. Its only disadvantage is that it can be more expensive than other foams. In general NR Dunlop and blended Talalay are similar in cost with NR Talalay being more expensive.
Memory foam is similar to polyfoam with some added chemicals to give it its unique properties. It is a very soft material, especially when heat and pressure is applied and it "melts" into a semi solid form. It does an excellent job of forming a cradle and relieving pressure and has become more popular as a comfort layer for this reason. There are some negatives associated with this however because it has very low resilience which can reduce its ability in some cases to hold up the more recessed parts of you, in particular your lumbar area. It also has less progressive resistance as it is compressed because of its ability to keep softening under heat and pressure so it may allow heavier parts of you to sink more deeply into a layer that uses it. For these reasons (lack of resilience and progressive resistance), it is best to use the thinnest layer of memory foam that will give you the pressure relief you need either by itself or in combination with the support layer below it or other materials. With too much thickness, particularly with lower densities under 5.0 lbs per cubic foot, it can allow the heavier parts of you to sink down too far either initially or over the course of the night and you may end up with a sore back from a misaligned spine.
Memory foam typically comes in an ILD range of under 10 to 20 and even memory foam that feels very firm when it is cooler will slowly become much softer with the heat and pressure of your body over the course of time. It is dependent in any thickness on the support layer below it for primary support for the mattress because memory foam by itself isn't supportive enough to be used in the deeper support layers. Thin layers of memory foam may need a slightly softer support layer with more progressive resistance underneath it (a material that is softer on top with initial compression and becomes firmer more quickly with progressively deeper compression) while thicker layers of memory foam (should you choose to go in this direction) may need a much firmer layer underneath it to stop any further sinking down of heavier areas of your body. Memory foam can also be very successfully used in layered combinations with other materials that can "make up" for some of its lack of supportive qualities.
There are different qualities or grades of memory foam and 5.0 lbs per cubic foot and higher is considered to be the highest quality, 4.0 - 4.9 is mid range quality, and 3.0 - 3.9 is lower quality (and only suitable for toppers or lower budget mattresses). Memory foam less dense than this should be avoided. Higher grades are more durable and will keep their qualities for longer than lower grades but will not generally last as long as innersprings, latex foam, or natural fibers. Strangely enough, higher density memory foam can feel softer as it "melts" or softens with body heat than lower density memory foams which are often either too soft ("melt" too easily) or too firm (like a light styrofoam) depending on how they are made. Higher densities can also have more progressive resistance as you sink into it and is sometimes available in densities as high as 8 lbs per cubic foot. Because memory foam is more closed celled than other foams, which is part of the reason for its "slow response", it is also less breathable than other foams and while the newer generations are more breathable than older formulations, they still lag behind other foams in this area.
The lower grades of polyurethane foam are one of the biggest reasons for foam softening and body impressions in most mainstream mattresses sold today. There are many different versions and grades and densities of polyfoam that range from very low quality/durability to some very high quality materials but I would be very cautious and make sure that any mattress you are considering doesn't use more than "about an inch or so" of lower density soft polyfoam in the comfort layers (less than 1.8 lb density) because this can soften or break down relatively quickly and become a weak link in the mattress in terms of durability and the useful life of a mattress. Polyfoam comfort layers that use a middle grade of polyfoam called HD (High Density 1.8 lbs or higher in a one sided mattress or 1.5 lbs in a two sided mattress or in lower budget mattresses) or that use a slightly firmer version of foam, are stretched tight over the border wire, hog ringed to the innerspring, and preferably used in a 2 sided mattress that will give the layers time to "rest" and recover and last longer and can be much more durable than lower density versions of polyfoam that are commonly used in mainstream mattresses and can be used to make a good quality and durable mattress. Even here I would only purchase a mattress that included this grade of foam from a knowledgeable local manufacturer or retailer who can confirm the specs of the materials in their mattress and is familiar with the foam densities and construction methods that can result in a much longer lasting mattress and are "budget appropriate".
The highest grades of polyfoam such as HR polyfoam (which is 2.5 lb density or higher and has a compression modulus of 2.4 or higher and has a ball rebound of 60% or higher) and some of the other higher density/high performance grades of polyfoam (typically from 2.5 lb density to 5 lb density) can approximate latex or higher density memory foam in some of their more desirable properties and durability. Unfortunately, most of the major manufacturers usually include several inches of lower grade soft polyfoam (less than 1.5 lb density) in the comfort layers of almost all their mattresses (even the ones that are often "called" latex or "memory foam" mattresses) and often even more. The results of this are seen in how quickly these mattresses form body impressions, especially in one sided polyfoam pillowtops and eurotops and even before an impression is evident, will lose the qualities that made it attractive in the store.
A good rule to follow would be to avoid purchasing any mattress that had more than "about an inch or so" of lower grade soft polyfoam in the comfort layers or quilting unless you know for sure that it is 1.8 lbs density or higher (1.5 lbs in a lower budget or two sided mattress or 2.0 lbs if you are in a higher weight range) and you have been given accurate specifications. This is generally only possible if you purchase a mattress from a high quality local or regional manufacturer or specialty sleep shop that will tell you exactly what grade of polyfoam they use and is more interested in providing you with a suitable mattress than they are in adding or selling inappropriate materials for the sake of profit. The highest quality grades of polyfoam can approach latex in their performance although they generally won't match it and still won't generally last as long. For reference ... HR polyfoam BY LAW must have a density of 2.5 lbs/cubic foot or higher AND have a compression modulus (support factor) of 2.4 or higher to be listed as HR polyfoam on a law tag. This last specification is almost never revealed by larger mattress manufacturers or retail outlets and is why many people will tell you that a certain polyfoam is HR when it only has a higher density when it is not (although HD polyfoam that is in the same density range will be just as durable even if it doesn't have all of the specs of HR polyfoam).
Natural and artificial fibers:
Natural fibers can be very comfortable in certain circumstances. They are far less elastic than foams however and because of this, they are usually more suitable for those who sleep in a single position and not in multiple positions. This is because they tend to form a semi permanent pressure relieving cradle, most suitable for a single sleeping profile, and do not have the ability to easily form multiple cradles as you change positions. It is normal and even necessary for these fibers to form a body impression as this is part of how they relieve pressure with the help of the support layers below it (unlike foam which can relieve pressure for multiple positions and where body impressions are an indicator of its breakdown). This forming of body impressions is part of the necessary breaking in process of natural fiber mattresses.
They are very long lasting and generally require high quality support layers that have the ability to conform to a body profile. They also require special manufacturing techniques and skills to keep the fibers from compressing more than is necessary and becoming too firm and because of this they can often cost more than other mattresses that may have better pressure relief and support. They are very breathable and good for temperature regulation and along with latex and quality innersprings they are among the most durable materials that are used in mattresses. Best in most cases for single position sleepers that have a "flatter" profile.
The comments regarding polyfoam would apply to most artificial fibers used in mattresses and natural fibers are a far better choice with few exceptions. They should be limited to thinner layers of not more than an inch (like lower grades of polyfoam) ... usually used in the quilting ... if they are used at all.
These are "mini" innersprings designed for use in the comfort layers of a mattress and can be both pressure relieving and supportive. Microcoils are a good option and certainly better than lower grades of polyfoam commonly used in the comfort layers of a mattress. They require a thinner layer of foam or fiber above them so the feeling of the coils is isolated from the sleeper and should be used with firmer support layers. They can be an attractive and comfortable choice in a mid range mattress.
Buckling column gel:
This material has only recently become more popular and there are many who believe it provides the highest level of pressure relief of any material on the market ... with memory foam and latex close behind. It is very expensive and has an unusual "crackling" feel and like memory foam requires a firm and high quality support layer underneath it as it does not support heavier areas of the body and cannot be used in the support layers of a mattress. Used in a comfort layer though, it also does a good job in supporting the recessed areas of the body such as the lumbar because of the stiffer columns that do not buckle. This may represent an expensive but attractive choice to some with more severe pressure issues while they sleep.
There are other materials that may also be suitable for use in a comfort layer and many combinations of these as well however most of these are less common and tend to be more expensive, although some of them can also be very high quality materials. Feel free to ask about these in the forum if you are considering a mattress that includes them.