So I will try to explain this without screwing it up in an easy way to understand.
Image you have a group of glass cubes 12" on all sides. Kind of like those glass cubes they make fancy side walls with in homes, restaurants, and fancy hotels.
Lets say you fill each one of these cubes with different substances, up to the point where one more grain, drop or additional piece of substance would cause an overflow after filling the cube.
If you fill the cube with water, sand, rice, lead, latex, feathers, down, polyurethane and viscoelastic polyurethane, and HD foam.
Clearly, the one you fill with lead will be considerably heavier than all the others, but within each of the other substances you filled the cube with, they will all have different weights while occupying the same exact space.
Now you take your rice and place it in a cube and add water to it until one more grain of rice or drop of water causes it to overflow. The with weight of that cube will be heavier than the rice in the cube alone, still occupying the same space but the rice has now absorbed the water to increase the weight of that same cube.
So now you should have a visual and understanding that a cubic foot of different substances will have different weights. You will also notice that the rice that absorbed the water is softer (and heavier) than the dry rice alone.
When the ILD of a foam is measured, basically it is a measurement taken to determine how much force is necessary to depress the foam, usually at 25% of it’s depth. ILD Measurement Example. The first 25% of depression is usually the unit of measurement that is used to identify the ILD of a foam. It is not the only measurement taken, usually a second measurement will be taken at 65% depression. This will yield a second ILD. There is another measurement (not so important for this discussion) that takes the ratio of the two measurements to produce another number called “support factor.” This will offer additional information about how supportive the foam is.
Now all tempurpedic style foam (viscoelastic heat sensitive foam) is made from polyurethane, but with other additives added to it and infused with blowing agents to create various combinations. Much like adding water to the rice. Occupying the same space, but with a different texture, feel and weight.
Most would proffer that an ILD of 10-16 is softer, 26-32 is medium and 38-44 is firmer and then you have everything above, below or in-between.
But when laying on substance that has an ILD of 22, for all practical purposes it will depress the same, granted the different material make-up will feel different. Linen, cotton and wool all have different feels as does dunlop vs talalay, HD foam vs viscoelastic, memory foam vs latex and so on.
Now here is where I go off on a tangent. In the case of viscoelastic memory foam, since that is what we have been talking about. Tempurpedic Serenity and Temperpedic. It becomes a bit blurry when talking about ILDs. At least from my perspective (and I am sure someone will correct ME if I articulate this wrong), using an ILD to identify viscoelastic foam is kind of flawed.
What temperature did they do the test on VE MF? As we all know, VE MF softens in warmer environments and hardens in colder environments. Case in point. I have a TP automobile back cushion, which I leave in my car. As the weather has gotten cold lately, when I get in the car in the morning, the cushion is as hard as a rock. Nothing added, same piece of Tempur foam, just cold. After a while, the heated seat and car heat softens the cushion up. If you leave your home at 62degrees. When you get home, the topper will be more firm than after you get your body on it. So the ILD in this case is not as reliable as the use of the density figure (for durablity) and what additives were used to make it not only dense, but varying levels of firmness at a specific density.
Because Polyurethane is a synthetic, it can be made into different modalities. Fast response, medium response, slow response. It just depends on the additives infused when creating the final PE derivative product.
Latex is a bit different, as it is not heat sensitive like VE MF. It too can be manipulated for density and firmness, but the ILD is a bit more reliable in my view for latex than VE MF.
Hopefully, this makes sense. I may have missed a thing or two along the way, but I am sure someone will jump in and correct me.