Hi Shew,
There are many factors involved in the durability of a mattress or the components in it and there is far more misinformation on the web about this than there is accurate information.
First of all … there are 3 main broad categories of foam. These are latex, polyfoam (polyurethane), and memory foam ( a form of polyurethane which is formulated with added chemicals to favor the “gelling” chemical reaction instead of the “urethane” reaction to give it more viscoelastic qualities).
The “gold standard” for foam durability is latex foam although higher density versions of other types of foam can also be very durable choices.
There is more about the different types and blends of latex and their relative properties and durability in this article and in post #6 here. In essence I would tend towards either 100% natural Dunlop latex or either blended or natural Talalay (except in the lower ILD’s where 100% natural Talalay would tend to be less durable than the blended version).
With polyurethane and memory foam … the single biggest factor in durability is an “apples to apples” comparison of density (actually this would be “unfilled” or polymer density which doesn’t include the weight of any added fillers which can add to the apparent density of the material but lower the durability). In other words a 2.6 lb polyfoam will be more durable than a 1.8 lb polyfoam all other factors being equal. With memory foam all densities are higher because of the added chemicals that give it varying degrees of viscosity but the same rule holds true when comparing memory foam to memory foam and 5 lb memory foam would be more durable than 4 lb memory foam.
As a general rule I would use 1.5 lb density polyfoam as a minimum guideline for a two sided mattress, 1.8 lb density polyfoam as a minimum guideline for a one sided mattress, and 4 lb density as a minimum guideline for memory foam in a mattress. All latex would be a high quality material although natural latex is a more costly material than synthetic latex. There should be no more than “about an inch or so” of lower quality/density materials in the comfort and quilting layers combined (around an inch or less would not have a significant effect on the durability or longevity of the mattress). Once you are at about 2" or more of lower quality materials they are likely to be the weak link of the mattress.
Some of the other factors involved in durability are
Softness/Firmness: Softer foams are less durable than firmer foams because they are subject to more mechanical compression which stresses the foam more. So if everything else was equal … a 1.8 lb polyfoam that was 15 ILD would be less durable than a 1.8 lb polyfoam with a 28 ILD rating.
Position: Upper layers of a mattress are also subject to more compression than the lower layers of a mattress so the “position” of the foam will also affect the durability of a foam. For example a 1.8 lb polyfoam used in the top layers will not be as durable as a 1.8 lb foam used in the deeper support layers of a mattress because it is subject to more frequent and deeper compression. It is almost always the upper layers of a mattress that are most prone to softening and breakdown which is why it’s so important to make sure they include higher quality materials.
Layers above and below: The layers that are used above and below a particular foam will also affect the durability of a foam because they will modify the response of the foam. In other words a softer foam used above another layer will result in more compression of the layer below while a firmer foam used above another layer will reduce the compression of the foam below it. Firmer foams below a layer will lead to less “bending” into the lower layer while a softer foam below a layer will lead to more “bending” into the layer below it.
One or two sided: A two sided mattress can be flipped and the foam on one side can rest and recover and is only used half as much so will last much longer. It’s important though to make sure that the comfort layers of a two sided mattress aren’t too thick which can compromise the support of a mattress because of the thickness of the soft layers on the bottom. A good general guideline for a maximum thickness would be in the range of about 3" or less in the comfort layers. There is more about one sided vs two sided mattresses in post #3 here.
Replaceable layers: Some mattresses have individual layers and a zip cover where each layer can be removed and replaced. Some local manufacturers will also replace individual layers in a mattress. A mattress will usually soften and break down from the top down so both of these can be a benefit in terms of durability because a single layer that has softened (usually in the upper layers of a mattress) can be replaced without having to replace the entire mattress.
Formulation: While density is the single biggest factor in durability … other chemicals or ingredients added to a foam can also affect durability. For example … “filler materials” that are added to a foam (such as “sand” or other particles including “gel” particles) can lower durability of the base foam they are added to and chemicals such as plasticizers or the presence or absence of antioxidants will also affect the durability of a foam.
Thickness: Thicker layers of lower quality foams will have a bigger effect on the durability and longevity of a mattress if they soften than thinner layers that are mixed in with higher quality foam.
The person on the mattress: Heavier people or people with heavier “areas” or who are more “active” on a mattress will wear out materials faster than lighter, more evenly proportioned, or less active people so higher density foams than just the “minimum” guidelines can be more important.
Other factors: There are also other more “arcane” factors that can affect the durability of a foam such as the shape of the foam cells, the strength and elasticity of the crosslinks, the resilience of a foam, and the compression modulus (which affects the mechanical compression qualities of the foam) which will also affect durability.
In general terms though … paying attention to the quality/density of the upper layers (which are the most prone to softening and breakdown), and the layering of the mattress (how thick are the lower quality foams and where are they in the mattress), will greatly improve your odds that you will end up with a more durable mattress. In other words, the “weakest link” of the mattress will determine the durability of the mattress as a whole.
It’s also important to know that durability is relative to the person that sleeps on a mattress and no matter how durable the materials themselves may be … in practical terms a mattress will only last as long as it maintains the support, pressure relief, and personal preferences that allows someone to sleep well on a mattress. Foam softening or other changes in mattress materials or components may have different effects on different people and a mattress that has softened or changed to the degree that it no longer provides the support, pressure relief. or personal preferences for one person may still be fine for someone else. It’s usually not the final breakdown of materials that leads to the need to replace a mattress but the gradual loss of comfort and support that finally “crosses” a line and at some point is no longer suitable for that person to sleep on. Each person’s “line” can be very different.
One final comment which has to do with warranties. In general there are 3 stages of foam softening and breakdown that all foams will go through. They are an initial softening period over the first 90 days or so (a bigger factor with polyurethane and memory foam than with quality latex), followed by a more gradual softening over longer periods of time (determined by the type, density, and stresses on the foam), followed by the breakdown of the polymer itself which leads to impressions which don’t “come back” when there is no weight on the mattress.
Warranties only cover the depth of impressions and “exclude” any visible impression that is less than the warranty exclusion for that particular mattress. Foam softening on the other hand (which is the biggest reason that a mattress becomes unsuitable for sleeping on for a particular individual) is not covered in the warranty because when you take the weight off the mattress it will still have just enough resilience left to “come back” enough that any visible impressions are still within the warranty exclusion. Because of this … warranties are more of a marketing tool (to create the impression of higher quality) than they are an indication of how long a mattress will last. Knowing the materials in the layers of your mattress are a far more accurate indicator of durability and longevity than the length of the warranty which only covers “defects” and softening is not considered to be a defect which means it isn’t covered by a warranty.
Phoenix