The Qualities, Advantages, and Disadvantages of Memory Foam
Viscoelastic foam or "memory foam" is a very different material and perhaps partly because it has such unusual qualities compared to the many other materials used in mattresses, there is also a lot of misinformation "disguised as fact" about it. It is also subject to many different "formulas" in its manufacturing by different companies which change its qualities ... for better or for worse ... and this too becomes the subject of lots of the hype and misinformation you will hear in the world of marketing and selling mattresses.
What are the basic differences in materials used in a mattress?
There are two basic ways that a material in a mattress reacts to weight and pressure.
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Viscous materials tend to flow away from pressure like a liquid or honey and tend to distribute and absorb energy
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Elastic materials tend to store energy under pressure and to different degrees push back against compression
Water and Air
Materials like Water and Air, are viscous. They do have some pushback in a mattress enclosure but this comes from the elasticity and resilience of the enclosure or from other materials in the enclosure that holds them not the air or water itself. Viscous materials can be very hard initially with sudden pressure but feel much softer under more gradual pressure (try leaping onto an airbed or waterbed core). This is because viscous materials take time to "give way" to pressure. When they do give way, they give way in any available direction (out to the sides for example) and spread the pressure out through the layer.Materials in a mattress that absorb compression forces and redistribute pressure away from pressure points are great for pressure relief but not usually as good for support. Layers that store energy and push back can also be very good at pressure relief in softer versions but are also better for support. This also depends to some degree on the point elasticity of the material and its ability to form a conforming cradle that mirrors the shape of the body. All viscous materials are good at this. Some elastic and more resilient materials are better at this than others. The different layers in a mattress are usually designed in such a way that the complete mattress will have both supportive qualities and pressure relieving qualities. The core of the mattress which is the middle and bottom parts (usually innerspring, latex, or higher quality polyfoam) is the part that is primarily responsible for supporting the heavier parts of your body and keeping them from sinking in too far. The comfort layers which is a few inches (usually polyfoam, latex, memory foam, natural wool, horsehair, or synthetic fibers) are responsible for redistributing pressure so you don't get "pressure points" when you sleep. They are also responsible for supporting the inner or more recessed parts of your body (like the small of your back, waist, upper thighs, etc) so that gravity doesn't pull them down against the natural position or curvature of your spine. These parts of your body don't usually sink in enough for the deeper support layers to truly support them.
So what does all this mean in mattress terms?
Its unique combination of qualities leads to the advantages of memory foam for some...but the same combination of qualities that are attractive to some can also be responsible for memory foam's weaknesses for others ...
Advantages
Memory foam's greatest advantage, because of its combination of properties, is generally considered to be its ability to distribute and relieve pressure. This does not mean however that it is dramatically better than the best of other materials as there are other types of foam like latex or high-quality polyfoam that in their softer versions through compression alone are very close to memory foam in their ability to distribute pressure to levels below personal detection for most people. Even natural fibers that have broken in and formed a "cradle" to your body can distribute pressure very well and are often used in high-quality mattresses. I should also mention here that there is one other material called "buckling gel" that in some cases may even be superior to memory foam or latex in terms of pressure relief but it is newer, not as common, and rather expensive. If a mattress relieves pressure below levels that you personally can detect, then which material is used in the comfort layers to do this is not so relevant except for other reasons such as its ability to support, its breathability, and its durability. In hospital applications (such as the relief of pressure sores) or with highly sensitive people, this slight difference in pressure relief can be more important however in most applications the difference is not as critical or even noticeable and the difference in materials used for pressure relief is more about how they feel and personal preference.
Another advantage of memory foam over a liquid or air is that memory foam requires body heat (in differing degrees with different formulations) to become viscous enough to "flow" so the "unmelted" areas of foam that are further away from your body will be firmer and resist pressure without "flowing away" from that pressure. This means that you are enclosed in a "stable cradle" of material that is "softer" close to you while it remains "firmer" further away from you. This combination of softness and firmness feels very good to people who like to sleep "in" a mattress that also feels "stable". Without this ability and the difference in viscosity and elasticity in different parts of the mattress, you would be sleeping "in" a viscous material which felt more like water or jelly without the feeling of "stable softness" that comes from a good memory foam layer. Other more elastic foams and natural materials to greater or lesser degrees are able to combine a feeling of softness with stability as well but memory foam is the leader here.
The qualities that provide this "stable cradle" effect are also connected to another of memory foam's attractions to some, which is its ability to isolate movement between people sleeping on a mattress. Its ability to absorb energy and isolate movement is better than most other materials because of its ability to both absorb energy and respond locally to movement. Latex in a comfort layer is also excellent here because of its ability to "localize" compression better than other materials but it doesn't absorb energy nearly as much. Latex, certain high-quality polyfoams, and certain innersprings (like pocket coils) in the support layers also contribute to a mattress' ability to isolate movement and what is under your memory foam is important if this is a desirable part of a mattress' qualities for you.
Memory foam, at least if it is good quality and higher density memory foam (5 lbs density or above), is also more durable, will keep its qualities, and last longer than many other polyfoams that are typically used in a mattress, especially in the upper comfort layers. It is not unreasonable to expect the best quality memory foams to last 8 - 12 years (depending on its use and the "stresses" it is exposed to). This is not as long however as some very high-quality polyfoams, latex foams, better quality innersprings, or natural fibers used in "upper end" mattresses, all of which can last longer without breaking down.
So there you have it. Hopefully I have covered the main points of the generic differences, strengths, and weaknesses of memory foam as a whole. In my experience and research, memory foam has been the subject of more misinformation and hype than almost any other material in a mattress and this confusion and misinformation has in my opinion led to too many poor or at least inappropriate mattress buying decisions. It is sometimes a frustrating process to "get to the bottom of things". Memory foam is certainly a valid choice in a mattress material for some people and there are many who love it however I believe that knowing more about what it does in comparison to other materials and why and how it does it, is an important part of buying a memory foam mattress. There are many choices of mattress construction available and the more those choices are based on fact, personal experience, and individual needs and personal preferences, the more likely you will be to buy a mattress that is perfect for YOU.
