The ILD (aka IFD) numbers we toss around are at 25% compression. The Polyurethane Foam Association refers to this as "surface feel" or "firmness".
Foams are also measured at 65%, but this number is almost never mentioned. The importance of this number is that two foams can have the same ILD at 25%, but quite different ILD's at 65%. The one with the higher 65% number will be more supportive. This is sometimes referred to as "deep down support."
The ratio of the 65% ILD to 25% ILD is "support factor," or "compression modulus."
Support is a key function of flexible polyurethane foam. In many
ways, it is the most important function foam can provide.
In many ways, foam support and firmness are interrelated. In fact,
compression modulus is measured by taking the ratio of two foams
firmness measurements. And foams with high levels of support can
actually feel firmer than foams with the same density and surface
firmness measurement but which have lower support levels.
Generally speaking, the higher the density, the more "supportive" a foam will be. Typical poly foams will have support factors under 2.0, "High performance" foams will have support factors above 2.0.
The higher the density, typically the better the ability of the foam to provide support.This thread in the old forum contains some good discussion on the difference between "firmness" and "support".
Support is also important in that foam with higher support can be
specified with softer surface IFD, without sacrificing the ability of
the foam to bear a load.
Alliance for Flexible Polyurethane Foam glossery
FPF - The new acronym used to describe flexible polyurethane foam.
High Resilience (HR) FPF - High Resilience FPF have a high support factor and greater surface resilience than conventional FPFs and are defined in ASTM D3770. High resilience FPF has a less uniform (more random) cell structure, different from conventional products. The different cell structure helps add support, comfort, and resilience or bounce.
Resilience - An indicator of the surface elasticity or "springiness" of FPF. It is measured by dropping a standard steel ball onto the FPF cushion from a given height and measuring what percentage the ball rebounds.Here is a paragraph from the thread link above, from a post by yogiyoda. It's a lucid and concise summary.
Surface Firmness - The number of pounds of force necessary to indent an FPF sample by 25% of its original height.
Based on my understanding, density and support factor are important measurments in foam. Density is related to longevity. Density is related to Support Factor. Support Factor has just as much of an effect on the feel and perfomance of foam as the foams ILD. A high Support Factor is important in ensuring comfortable sleep, especially for stomach sleepers.
I have been trying to find support factors for various foams, specifically LI Talatech talalay latex and HR poly foam. I'm coming up empty. I read that support factors typically range from 1.8 to 2.6 for FPF. The following link is to a .pdf document from BASF claiming their Pluralux HR foam outperforms latex. It shows latex with a support factor of 2.7 and Pluralex at 2.9, but they don't specify process (Dunlop or Talalay), ILD, or density for the latex.
This PFA document includes a graph showing "4.2/28 Latex Rubber" (4.2 PCF, 28 ILD) having a support factor of 2.5, with "2.5/28 HR" having a support factor of 2.4. In resilience (ball drop test), they show latex at 68, HR at 58.
|Thank you haysdb for all this great information that I am saving.|
The problem with talking about support factor is that it's of limited practical use since we almost never know either the support factor or the 65% ILD so that we can calculate it ourselves.
What I personally come away with is that a person seeking firmness needs to look for not only high ILD's but high density as well. Note that Latex can be (mostly) excluded from this discussion because latex density and ILD are directly related - as latex ILD increases, so does density. This is not true for other types of foam. With memory foam, density has always been a part of the discussion, so with memory foams it's well understood that density and "support" are directly related.
Support factor becomes most meaningful when talking about polyurethane foam. There isn't much discussion of PU foam here, but this is definitely where PU foam gets a bad, albeit richly deserved, reputation. The problem (I think) isn't that PU foam is bad, per se, but that low density PU foam is bad. High density (HD and HR) poly foams can be of a quality approaching that of latex . The problem is, such high quality PU foam is rarely used. Support factors of PU foams can range from 1.6 or even less, to as much as 2.9. 
This is more theory than something I can prove, but I believe a conventional mattress (either innerspring or all foam) can be of high quality if made with high quality PU foam. The density of PU foam matters because it's directly related to support factor and longevity, and more attention needs to be focused on this.
 High Resilience Polyurethane Foam with the Performance of Latex Foam
Latex-like flexible polyurethane foam and process for making same (a link to BASF's patent)
 I don't actually know what the low-end is, but I do know that poly foam exists with a support factor as high as 2.9. I have already linked to that source earlier in this thread so I will not do so again here.
Post deleted by author. I incorporated this post into the previous one.
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