Hi Sonic,
I absolutely agree with most of your points … except that ILD is measured on a certain standard thickness of a material (more about that in a bit) and then any thinner layers that are cut from that thicker core are still rated at the same ILD … even though they are now a thinner layer. The ILD “rating” in other words doesn’t change as layers are cut from a thicker core of a certain ILD. The point you are making about layer thickness being a major factor is absolutely on the money though and I can’t stress enough how important this … and a few other factors … are.
There are 5 main components or “specs” that determine how layers perform and interact in a mattress and several smaller components to the pressure relief / alignment “equation”. Before I get started … for those who are looking for more “simple” information … this post may be a little on the long or technical side 
One of these as you mentioned … and the one that is the most commonly talked about is ILD (or IFD which is very similar and is used more with polyfoam). ILD means Indentation load deflection while IFD means indentation force deflection. IFD is done with a small pre-weight to very slightly compress the surface before the testing weight is applied but they are functionally the same (assuming the same layer thickness and percentage compression is being tested). The PFA has some good information about IFD and several of the other factors in this post.
It’s “standard definition” is the amount of weight that it takes to compress a 4" thick layer, 20" x 20", 25% of its thickness using a 50 sq inch circular metal press or “foot”. Once a foam is rated … then thinner “cuts” of this foam from the rated slab are still rated at the same IFD. So if a “slab” of polyfoam or other material is rated at a certain IFD (using a 4" piece) then all thicknesses that are cut from this slab are rated at 24 IFD. If a 6" layer of latex for example is rated at a certain ILD … then six 1" toppers that are cut from it would all be rated at the same ILD.
This gets confused though because different materials and different areas of the world use different methods of measurement. Some latex (like talalay which is produced in 6" cores) for example measures IFD @ 25% with a 6" slab (which would then be rated higher than a 4" piece) while some areas of the world use IFD @ 40% compression instead of 25% which also results in a firmer rating. Other areas or materials (such as latex green) use kgf (kilograms of force) which “looks like” much softer latex than a standard IFD test would produce (notice how low the numbers are). Sites all over the internet have simply posted the kgf rating except they call it ILD which is quite misleading as to how soft/firm it really is so some people that are using Latex Green layers for example believe that they will be softer than they really are and may have great difficulty getting their layering right. Because of the variations … it is sometimes difficult to “translate” the softness or ILD of materials from one measurement to another unless the differences are taken into account or known through experience.
But the differences are only part of the story. Because standard IFD is measured at a 25% compression … it is only really accurate if the mattress layer is compressed exactly that amount. In real life a layer is compressed either less or more than 25% (and different parts of the body have different weights as well which compress a mattress to different depths) so it may “feel” either softer or firmer depending on body weight and depth of compression. How much a layer compresses is of course also affected by where in the mattress the layer is and the “relative” ILD’s of the layers because different materials or even the same type of material from different manufacturers may not be easily comparable based on ILD alone.
This brings us to the second critical part of what creates the “feel”, “softness/comfort”, and supportive qualities of different materials … and this is called compression modulus. It is also called sag factor or support factor. This measurement takes the IFD at a 25% compression and a second IFD measurement at a 65% compression and the ratio between them is the compression modulus. Some foams like polyfoam typically have a lower compression modulus which means they don’t get as firm with deeper compression and so will feel softer and bottom out more easily than those with a higher compression modulus. This measurement … even though it is rarely discussed anywhere (at least on north american sites with the exception of the PFA site again) … is just as critical a part of comfort and support as ILD and yet it is little realized or taken into account. Part of the reason that true HR (high resilience) polyfoam is a higher performance material compared to lower grade polyfoams is that by definition it must have a compression modulus of at least 2.4 (and a density of 2.5 as well) while lower grades of polyfoam have a compression modulus less than this (and often well below 2). Much of the polyfoam that is being sold as HR or high resilience foam isn’t at all. Talalay latex is in the range of 3 while Dunlop is in the range of 4. This is why latex in general and Dunlop in particular can be soft on top and get firmer with increased compression and why single slab Dunlop layers were often used as a complete mattress. It could be both soft on top and supportive underneath in a single layer. The equivalent to compression modulus in an innerspring would be spring rate.
Third … layer thickness plays a key role in how the comfort layers interact with the support layers and in tailoring a mattress to different body weights, body shapes, and sleeping positions. This site includes a fair bit of information about this and the 2 main layering schemes in this overview and the subsections about progressive construction and differential construction.
The fourth of the important specs that determine why some materials perform or feel differently than others is point elasticity which plays a key role in how well a material can “mirror” a body shape and relieve pressure (and why latex and memory foam are so good at this as they both have very good point elasticity). This is more applicable to comfort and transition layers than to support layers of a mattress. More information about this is in several parts of the site (you can search on point elasticity).
Fifth there is resilience (how strongly a material springs back) which is measured by how high a ball which is dropped on a material bounces (measured as a percentage of the height from which it is dropped) which can play a role in how well a material, especially in the comfort layers, supports the more recessed areas of the body, how easy it is to move on the mattress, and how much energy it absorbs vs returns. The “opposite” of resilience is hysteresis which is how much energy is absorbed by a material and not returned when the compression force is removed. Memory foam for example has high hysteresis and low resilience while latex has high resilience and low hysteresis.
Finally there is the effect of the sleeping environment because some types of material such as memory foam or gel memory foam are sensitive to temperature and humidity to different degrees and can change their softness based on different conditions in the bedroom.
While all the details about how these main “specs” of different materials and types of comfort and support layers can interact together are outside the scope of a single post (which would take many pages or even books) … without a basic knowledge of how these factors interact … it is very difficult to imagine or predict how a certain layering using certain materials may perform or “feel” for any specific individual. This is especially true if people only use ILD to make decisions without taking into account the major differences that the other factors can make in their choices. While this is rather technical to go into in each case … these “specs” are mentioned in the glossary section of the site and discussed in more detail in various subsections of the “mattresses” section of the site for those who like to get more technical.
One of the reasons I am a “fan” of latex is because it can be made in a wide range of IFD’s (ILD’s) … has the highest compression modulus of all foams … has (along with memory foam) the best point elasticity of all the foams … has the highest resilience of all the foams … is the most breathable (coolest) of all the foams … and has better durability than all other foams and most other mattress materials … including steel innersprings.
Finally to your comment about a single core of say 6" vs a “split core” of 2 x 3". When core layers are “split” it reduces resistive forces inside the layer itself. When a foam layer is compressed … it affects all the surrounding material that is attached to it. The material below is compressed and the material beside the compression point is stretched and pulls back against the compression (in addition to the resistance of the foam below the compression point). This creates various interacting forces inside the mattress including shear forces. When a layer is split … it reduces these “resistive” forces and the 2 layers can react a little more independently and act as if they are slightly softer and you will sink into them a little deeper. Having said this … if the ILD of the two 3" layers are the same as a 6" layer and there is a comfort layer on top of them then most people wouldn’t notice a difference. In the same way a comfort layer that is made of three 1" layers of say 19 ILD will function as a “softer/thinner layer than a single 3” piece of the same ILD but in the top layers and with thinner layers the difference would probably be more noticeable. Because of this … in “theory” a single core can be a little firmer than 2 half cores of the same ILD although the difference probably won’t be large enough to feel. Having several layers instead of a single layer also slightly alters what I call the “order of compression” which is the dynamics of the order in which layers in a mattress progressively compress compared to how a single layer will compress. This is also affected to smaller degrees by surface tensions of the layers.
There are certain circumstances where a single layer would be an advantage while in others … the ability to choose a firmer “bottom half” of a lower layer can also be an advantage. One is not better than another as long as it can be “predicted” how a particular construction will respond in a reasonably accurate way. In some cases the differences are so minor that they do not play a significant role.
Density too can play a role in some foams … especially memory foam … in terms of its performance and feel. Density in general is not directly related to ILD (in polyfoam for example, any density can be made in a wide range of ILDs) and is more closely related to compression modulus of foams. Comparative density between different materials can also be misleading but is more meaningful when comparing different versions or qualities of the same material.
Depending on the material … there are also other factors that play a role in the performance of certain materials such as creep forces and temperature sensitivity that are part of all viscoelastic materials, cellular structure and/or breakdown of a foam, wear resistance, weathering and heat aging resistance, and many others. None of this takes into account the effect that different base layers and tickings and quiltings can have on the performance of the mattress layers because every layer and component in a mattress will have some effect on every other layer above and below it … and some of these can be significant as well.
All in all it is partly science and partly an art to “imagine” and predict how the layers of a mattress will interact together and with an individual body shape, weight distribution, and sleeping position. The good news is that if people choose higher quality materials (that have better specs) and test and pay attention to ILD and layer thickness (which can vary with any material) … then it becomes much more likely that they will learn the general layering and ILD combinations that work best for them and the odds of success or “duplicating” a mattress when that is necessary are much higher.
Carefully testing a mattress for PPP (Posture and alignment, Pressure relief, and Personal preferences) will replace most of the more technical knowledge that is discussed in this post. Best of all would be to have the help of a knowledgeable person at a local manufacturer or sleep shop that has the experience and knowledge to help you find a layering scheme that “bypasses” the need for any technical knowledge at all where you can just use your own careful and objective testing and experience as your “guide”
Either way though … there is something about putting together or finding a perfect mattress that is really satisfying … and it can certainly make a difference in how well we rest and just as much in how we feel when we are awake.
Phoenix
