The pros and cons of memory foam ... beyond the hype.
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This is a "first draft" and of course is based on what I have learned and/or come to believe from my own research and personal experience. It is not meant to be any kind of "final statement" on memory foam and is subject to any "corrections" necessary along the way as new information, opinions, or materials come to light from either my own research and experience or that of others. Note added later ... there is a second draft in reply #8 which is a little less "technical" and perhaps a little easier to read. This one includes a little more technical information for those that are so inlined. Visco elastic foam or "memory foam" is a very unusual material and perhaps partly because it has such unusual qualities compared to the many other foams used in the mattress world, there is also a lot of misinformation "disguised as fact" about it. It is also subject to many different "formulas" in it's manufacturing by different companies which change it's qualities ... for better or for worse ... which in turn becomes the subject of lots of the hype you will hear in the world of marketing and selling mattresses. So what are the qualities that "define" visco-elastic foam or as it's more commonly known ... memory foam? There are several elements which give memory foam its unique characteristics and make it different from all other foams. These are mainly viscosity, elasticity, response time, resilience, and the variability of it's qualities with changing external factors, mainly temperature. While I am not representing what follows as being completely scientific (I am not a mattress scientist), and I have made some generalizations, I believe that the information will help with asking better questions and hopefully making better choices if you are considering memory foam as a possibility for your next mattress. Viscosity: In very general terms, this is a measure of a viscous (non elastic or "liquid") material's ability to "flow away" from pressure (off to the side) as opposed to a non viscous or elastic material which progressively compresses (underneath you). In practical mattress terms it means how long it takes to "sink into" a viscous material (a material that "flows" like memory foam) with a constant weight (like your body) laying on top of it and also affects how far you will sink into that material. Viscosity is usually considered to be a property of liquids which are "displaced" under pressure (flow away in many directions) instead of being progressively compressed under pressure (in a single direction) like non viscous elastic materials or "non memory" foams. If you drop a marble into honey for example it will take longer to get to the bottom of the jar than dropping that same marble into a jar of water. Both honey and water are viscous materials but honey has a higher viscosity (resistance to flow) than water and will flow away from pressure more slowly. Memory foam acts somewhat like a high viscosity liquid in that it partly "flows away from pressure" (with the application of heat) in addition to being compressed under pressure. In both cases the marble will eventually reach the bottom of the jar since both honey and water have no elasticity. If you were to jump on to a memory foam mattress and land on your back it would feel quite hard. Water which is an even more viscous material would feel even harder as it also takes time to "flow" away from pressure and does not compress (which is why belly flops can be painful) even though it feels softer to more gradual pressure. Back flopping or belly flopping onto a similar "softness" of regular polyfoam or latex would feel much softer since they compress under pressure and do so with progressive resistance in a much shorter period of time. The combination of "flow" and "compression" gives memory foam it's ability to conform to your body and distribute pressure. Elasticity: In general terms, this means how much a material "rebounds" back to its original shape or length after it has been compressed or stretched. The opposite of elasticity is plasticity (a material that stays deformed). A fully elastic material will rebound all the way back into it's original shape or length. Both memory foam and regular foam have high elasticity since they rebound all the way (at least until they break down) back to their original shape. The difference is that most foams will rebound quickly while memory foam will rebound more slowly. This is because it's internal composition requires it to partly "flow back" in order to regain it's shape (in the same way that liquid honey in an enclosed container has to flow back to regain it's "shape") and partly because it needs to "change back" into a more elastic material when pressure (and heat) is removed. In other words, memory foam needs more "time" to both fully compress and to fully rebound. Regular foam needs much less time to do both. Response time: Memory foam is often called "slow response foam" and this slower response happens in both "directions" (sinking in and coming back). The reasons for this are the result of it's composition and are related to its changeable viscosity, elasticity (it comes back as well as other foams but takes longer to do it, thus its "memory") , and resilience (which is a measure of elasticity over time or what is often called "pushback" or a feeling of "bounciness"). Resilience: This is often confused with elasticity but there is a difference. Resilience is measured by a material's ability to make an object dropped on a mattress material "bounce". It is the rebound "force" in other words and is connected to time where elasticity is not. To make something "bounce" a material has to have both a high elasticity (ability to rebound back to it's original shape) and it has to spring back quickly (short response time) or the dropped object won't bounce. Most "non memory" foams have a much higher resiliency than memory foam since they rebound all the way (just like memory foam) AND do so quickly (unlike memory foam). This is primarily because compression "stores" energy while flow "distributes and absorbs" energy. Resilience is a major factor in a material's ability to provide proper support to a sleeper. Support in a mattress needs to work in "real time" as the body changes position and to provide the level of pushback required to support the more recessed areas of the body and keep the spine aligned through the range of changing positions and pressures. The "force" behind this pushback (from more resilient foams or springs) is a combination of elasticity and time. This is why the non memory foam layers in a mattress are so important to the support needs of a sleeper since they add the needed resilience to a mattress' construction to keep your spine aligned. Without resilience you are "stuck" with the spinal alignment/misalgnment you end up with at the full compression of the foam and your mattress may also not have the same range of ability to keep you aligned with different weight distributions and different sleeping positions. Variables: Memory foam also has another quality that makes it different from other foams and that is it's ability to "change" from elasticity to viscosity through temperature sensitivity. As the temperature rises (either from body heat or from ambient room temperature), it becomes more viscous and less elastic (more "liquid" in it's properties) and will partly flow away from pressure more like a thick liquid. This transformation into "liquidity" allows it to become "softer" in response to gradual pressure. This viscous quality will also increase or decrease within a range of temperatures just like most liquids which get "thinner" or "less dense" with increasing heat. As it cools down, it becomes more elastic once again and returns to it's original form. Regular foams on the other hand are not nearly as sensitive to the heat which is encountered in typical use and have a more consistent compression response (remain elastic) regardless of body heat and ambient temperatures. It is important to remember here that a change in temperature will change HOW LONG it takes for memory foam to compress to it's maximum compression (it's viscosity will change) and also to a lesser degree HOW FAR you sink in to the material. Memory foam in other words is not completely like a liquid where it is only a matter of time till you get to the bottom. It has viscous qualities (ability to flow away from pressure) but the sinking in will always stop at some point as it is also a solid material which compresses. If this was not true then no matter what type or thickness of memory foam you were laying on you would eventually go right through it. How far you sink in to memory foam and how soft or firm it feels is related to it's density (measured in lbs/sq ft) and it's "softness or firmness" at normal temperatures (expressed in ILD) as well as it's ability to change within the temperature range it encounters. There are other variables which can change the viscosity or other characteristics of certain formulations of memory foam (including mechanical stresses) in the same way that shaking catsup will make it flow more easily (change it's viscosity) but at this time the primary variable in how a memory foam changes it's viscosity is related to temperature sensitivity. Different manufacturers produce different "families" of memory foams with different qualities (such as breathability, density, and softness) and different sensitivities to temperature and temperature ranges as well which means there is a wide variety of different types and "feels" of memory foams available to a consumer. So what does all this mean in mattress terms? This unique combination of qualities lead to the advantages of memory foam for some ... Memory foam's greatest advantage, through its unique ability to both "flow" and "compress", is generally considered to be it's ability to distribute and relieve pressure. This does not mean however that it is dramatically better than the best of other foams as there are other types of foam that through compression alone are very close to memory foam in their ability to distribute pressure to levels below detection. In general though memory foam has a slight edge over the best of these. I should also mention here that there are other non foam materials which are at least the equal of memory foam in their ability to distribute and relieve pressure (buckling column gels are an example) but even these have only a slight edge in providing pressure relief to levels below detection over the best of other foams, at least in the majority of applications. In hospital applications (such as the relief of pressure sores) or with highly sensitive people, this slight difference can be important. In most applications however this difference in pressure relief is not as critical or even noticeable. In most cases the difference is more in "feel" rather than detectable pressure relief. Another memory foam advantage is that unlike a liquid, memory foam requires body heat (in differing degrees with different formulations) to lower it's viscosity enough to "flow" so the areas of foam that are further away from your body will be more elastic and/or have a higher viscosity 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 or resistance 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 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 is also connected to memory foam's ability to isolate movement between people sleeping on a mattress. This movement isolation is better than most other foams or mattress constructions although there are others (such as latex) which have a similar ability to isolate movement. This quality of memory foam is another real atttraction to many people. Memory foam, at least if it is a good quality and higher density memory foam, is also more durable and will keep it's qualities and last longer than many other polyfoams that are used in a mattress, especially in the upper layers. It is not unreasonable to expect the best quality memory foams to last 8 - 12 years (depending on it's 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 stuffings used in "upper end" mattresses, all of which can last much longer without breaking down. The same qualities of memory foam which are attractive to some can also be responsible for it's weaknesses for others .... Because of it's low resilience, memory foam is a poor support material. This is why it is so important to pay attention to the qualities of the layers underneath the memory foam in a mattress as this is where the support (ability to keep your spine aligned) comes from. Treating memory foam as "supportive" as opposed to "pressure relieving" will usually lead to a potential purchaser paying less attention to underlying parts of the mattress and their resilience and possibly choosing a mattress with a poor ability to provide correct support for a particular body weight distribution and/or different sleeping positions. Some side effects of memory foam's greater sensitivity to heat can also lead to sleeping issues for some people. The deeper in a mattress someone sleeps, the more likely someone is to have issues with "sleeping hot". This of course is a quality of all foams where you "sink in" to some degree but it is compounded by the makeup of memory foams in general which allow a greater degree of sinking in and are typically less breathable (allow for less evaporation) than other foams. Even the newer generation memory foams which are more breathable tend to be less so than other foams that are readily available. The greater breathability of other foams which have a more "open" cell structure (like latex for example) tend to lessen the heat issues even for those who like to sleep more "in" a mattress using softer "non memory" foams. This same sensitivity to heat can also lead in some cases to a mattress becoming "too soft" or "too hard" depending on the external temperature in your bedroom, and changing it's feel from season to season or from what you experienced in the store. Different types of memory foam can be more or less sensitive to this but it can be more important to control the temperature of your bedroom with some memory foams than it is with others or with other "non memory" foams. Another potential weakness of memory foams is that they take "time" to adjust to different positions. This can be an issue for those who change positions often or are sensitive to the "time" it takes for the memory foam to conform to their new position (creating short term "pressure"). Again different types of memory foams will take shorter or longer to conform to a new position. This "time to compress or rebound" that changes with temperature is both part of memory foam's strength for some (creates a "stable cradle") and its weakness for others (doesn't conform to new positions quickly enough). Some people may also be sensitive to a lack of resiliency or "pushback" which allows them to change positions with a little "help" from the mattress. Finally there are the "offgassing issues" of some of the poorer quality foams that are so common in the market. While all memory foams (including the "green" ones) use some "nasty" materials in their manufacture, some of them have more of this material left in them by the time you sleep on them than others. For those that are sensitive to this offgassing (either to it's smell or to the vapor itself regardless of smell), this can lead to issues ranging from a reaction to the unpleasant smell itself all the way to respiratory issues caused by the vapors. Your best protection against this is to make sure you know who manufactures the memory foam used in the mattress and not just accept the "re-branded name" that has been given to it (and to you). If the foam in your mattress has been certified by Certi-pur or Oekotex (or a similar organization), you can be reasonably sure that any smell or offgassing has been tested for any harm it may cause you. In the case of Certi-pur certification, the foam will also have been tested for it's durability and so is less likely to lose it's beneficial qualities in a few months or even a very few years after purchase. Without knowing who manufactures the memory foam inside a mattress you are considering and knowing what specifications and certifications it has, I would not buy it. There are just too many retailers and manufacturers who are more than willing to provide you with this information to waste any time with those who either can't or won't. 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 need, the more likely you will be to buy a mattress that is perfect for YOU. Phoenix This message was modified Nov 17, 2010 by Phoenix
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Re: The pros and cons of memory foam ... beyond the hype. Relaxo
(Nov 9, 2010 11:32 PM) -
Re: The pros and cons of memory foam ... beyond the hype. roy1
(Nov 15, 2010 2:08 PM) -
Re: The pros and cons of memory foam ... beyond the hype. Phoenix
(Nov 15, 2010 3:13 PM) -
Re: The pros and cons of memory foam ... beyond the hype. sandman
(Nov 15, 2010 3:43 PM) -
Re: The pros and cons of memory foam ... beyond the hype. Phoenix
(Nov 15, 2010 3:53 PM) -
Re: The pros and cons of memory foam ... beyond the hype. markbnh1
(Nov 15, 2010 6:42 PM) -
Re: The pros and cons of memory foam ... beyond the hype. Phoenix
(Nov 16, 2010 1:49 AM) -
Re: The pros and cons of memory foam ... beyond the hype. funride
(Nov 30, 2010 2:08 AM) -
Re: The pros and cons of memory foam ... beyond the hype. sandman
(Nov 15, 2010 8:29 PM)