Hi Tomcat,
It’s not quite so simple as recommending or choosing a mattress based only on the density of the materials and I don’t give specific recommendations because there is much more involved in a mattress choice and even the durability of a mattress than just density alone and this could lead to some very poor choices for those who took this information too literally. There is more about the factors that can affect durability and the useful life of a mattress relative to each person in post #4 here but there is no way to quantify this relative to a specific mattress design or for each person outside of using “guidelines” and comparisons between mattresses.
Generally yes, the density of a material is the single biggest factor in its durability but only up to a point and it also depends on the position of the material and on the other factors involved in the previous link.
In the case of polyfoam and memory foam … the polymer density of the material is important (the density before any fillers are added) and in the case of HD polyfoam higher densities up to about 2.2 lbs or so will be more durable after which higher densities are usually reached with the use of fillers. With HR polyfoam (which uses a different combination of chemicals) the polymer densities can be higher.
In the case of memory foam … higher polymer (unfilled) density will improve durability up to about 6 lbs or so after which the benefits of increased density are diminished. Memory foam that is in a 5 lb or higher range would be a very durable material.
If you are using lower density foams then the layers above it and it’s thickness will also make a difference in the durability of the material and how much of an effect foam softening will have. An inch of medium density memory foam in the top layer for example will have little effect on the overall effect of foam softening or the durability of a mattress while a thicker layer of medium density material in the top layer will have a bigger effect (there would be more medium density material exposed to the compression forces). The same thicker layer of 4 lb memory foam that could be an issue in the top layer may not be an issue underneath a higher density material that absorbs most of the compression forces and will be more durable.
The base layer of a mattress is not usually the weak link of a mattress unless there are thinner layers of foam over it or with higher weights in which case the base layer will be compressed more and could also become a bigger factor in terms of durability. Dreamfoam’s sister company for example uses 2.17 lb base foam in most of their mattresses but they are also in a higher budget range than most of the Dreamfoam mattresses. The top 5" or 6" of a mattress or so are the most important in terms of the durability of a mattress and the “best” place to lower foam density for the sake of cost and budget with the least risk to durability would be underneath this. Density also has nothing to do with firmness so a lower density material can be just as firm and “supportive” as a higher density material … it just won’t maintain its properties for as long.
4 lb memory foam is in a medium density/durability range and would be fine for most people. 5 lb memory foam is in a high density/durability range.
In general it’s best to look for a “weak link” in a mattress or to identify any “cautions” that are on a borderline using “best judgement” rather than try and guess how long any specific mattress will last for any specific person which may not be possible to quantify with all the many variables involved (including the suitability of a mattress choice and how much “room” you have for foam softening before a mattress loses it’s comfort and support to the degree that it would become unsuitable for you to sleep on).
I would also keep in mind that using durability alone as the most important factor can result in choosing a mattress that is very durable and would last for an exceptionally long time but may not be a good match for you in terms of PPP in which case the durability of the mattress would have little value to you. The opposite is also true and a mattress that is “perfect” in terms of PPP but is on the edge or being too soft for you or uses lower quality materials that can soften or break down more quickly would also have little value if it loses comfort and/or support too quickly and you are looking for another mattress in only a year or two. All of these and the other parts of a mattress purchase (including cost) that are important to you all need to be factored in when you are deciding which mattress is “best” for you.
All you have to do is let me know your city or zip code and I’ll be happy to link you to the nearest list on the forum that would include the better options or possibilities I’m aware of in your area.
It has taken me many hundreds (or closer to thousands) of hours of google searching, following links, other sources, and phone conversations to put the lists together that are on the forum although there will always be some that I miss and retailers also change what they carry on a regular basis so it’s always an ongoing process that is constantly being updated as new information (including from the discoveries of the members here) come to light.
Phoenix