Visco elastic foam or “memory foam” is different than the many other materials used in mattresses. There is a lot of misinformation on the material “disguised as fact.” Not all memory foams are created equal. They have many different formulas and compositions depending on who manufactures it. There is also a lot of marketing hype out there that borders on misinformation.
There are two basic ways that a material in a mattress reacts to weight and pressure.
1. Viscous materials flow away from pressure like a liquid or honey. They distribute and absorb energy.
2. Elastic materials tend to store energy under pressure and to different degrees push back against compression
Materials like Water and Air, are viscous
While these materials do feel like they “push back” it is actually the combination with the elasticity and resilience of the enclosure or from other materials in the enclosure that allows these materials to push back. 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, dictated by their enclosures, and spread the pressure out through the layer.
Springs and non memory foams (like “normal” polyurethane and latex) are elastic
Elastic materials recover quickly and with different amounts of “force” behind them (based on how quickly they spring back) to their original shape or length. Both are breathable enough that air just goes in and out with little to no resistance in all directions (compression and recovery) so the response from both of these are mainly because of the material itself not from what encloses them (although this will also have some effect). Of course foam has some resistance to airflow in both in and out directions so airflow plays a small part in its qualities while innersprings are not at all affected by air. Compression of an elastic material works in real time so it would initially feel much softer than a viscous material if you leaped on it but it only gives way in one direction (underneath you) so it will store energy underneath you (and push back) and may feel softer initially but firmer than a viscous material after a matter of some seconds. They have “bounce” and this bounce is known as resiliency.
Natural fibers are elastic and resilient to much lesser degrees but not viscous
Natural fibers are very breathable so air easily flows through them as they compress under pressure but the fibers are not nearly as elastic or resilient as non-memory foam or springs and they don’t “recover” as easily. Natural fibers are also not created equally. horsehair is the most resilient of the commonly used fibers, it recovers more strongly than wool which recovers more strongly than cotton. These fibers are so breathable that there is very little resistance to air flow under compression but since most fibers don’t have the resiliency to recover with as much force as springs or most foams and are less elastic, they will become more “permanently” compressed to differing degrees over time. They tend to need a resilient support layer underneath them.
Memory foam has a combination of viscosity and elasticity
This is why it is often called “visco-elastic.” This characteristic is what sets memory foam apart from other materials. It is made to recover over a matter of seconds and is strong enough to “refill” the air but it is not so resilient as to absorb the energy of compression to return with a “springy” feeling. It is less breathable than other materials and under compression the air both leaves and returns against more resistance than most other foams. This plays a part in why it is slower than other materials to compress and return. Memory softens in response to heat and humidity from your body or room temperature as it changes from a more elastic material into a more viscous material. The length of time it is continuously compressed can also affect how much the memory foam softens as well. This change or “melting” also takes time when you lie on it and it also takes time to change back to elasticity and return to its shape when you get up.
Memory foam is great for pressure point relief, but it must have a quality core, or support layer beneath it.
Layers that store energy and push back can be good at pressure relief in softer versions, but are usually considered 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 innersprings, 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 the few inches (usually polyfoam, latex, memory foam, natural wool, horsehair, or synthetic fibres) are responsible for redistributing pressure so you don’t get “pressure points” when you sleep. They are 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. The lighter parts of your body don’t usually sink in enough for the deeper support layers to play much of a part in supporting them.
What does all this mean in mattress terms?
Memory foam’s greatest advantage is 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. If a mattress relieves pressure below levels that you personally can detect, then knowing exactly 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 very 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.
Memory foam requires body heat (in differing degrees with different formulations) to become viscous enough to “flow”
Depending on the individual memory foam’s characteristic sensitivity to heat, the “un-melted” 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 can combine a feeling of softness with stability as well but memory foam is the leader in this respect.
Motion Isolation in Memory Foam
This is especially noticeable when more than one person is sleeping on a mattress. ESPECIALLY if one person likes to toss and turn at night while the other is a light sleeper! Memory foam’s 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 at motion isolation because of its ability to “localize” compression better than other materials but it doesn’t absorb energy nearly as much as memory foam. 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 very important if motion isolation is important to you.
Quality memory foams are more durable, will keep its qualities, and last longer than many other polyfoams
This is especially true in the upper comfort layers. It is not unreasonable to expect the best quality memory foams to last 8 – 12 years depending on use. That said, it is true that some very high quality polyfoams, latex foams, better quality innersprings, or natural fibers used in “upper end” mattresses can last longer without breaking down.