If I'm looking at foam that has a density of 1.3, and an ILD of 36, and another piece of foam that has a density of 2.8 and an ILD of 36, what exactly is the difference? I know that ILD tells the firmness, but i thought firmness was dependent on the density...? Are these two pieces of foam similar in firmness but one will break down more quickly over time?

Thanks!

(Specifically, trying to figure out eggcrate topper for mattress...The super-soft (1.2, ILD 12) sounds nice, but i'm afraid it's too soft...But the others all have much higher ILD's (though not necessarily higher densities..i.e. 1.3, ILD 36))

Density is weight per cubic foot of foam.  Higher density foams last longer than lower density foams.  You want your foam to weigh a lot.

ILD is indention load deflection (or something like that).  Basically how much force is required to push a foam down.

They are not related.  ILD is what determines softness or firmness.  The higher the number the firmer the foam.  The lower the number the softer the foam.  To give you an idea of ranges, Latex International makes their blended talalay in an ILD range of 14 to 44.  44 feels like you are sleeping on concrete.  14 feels like you are sleeping on a cloud.

However, beyond ILD there is feel.  A 14 ILD memory foam feels much different than a 14 ILD latex foam.  It's important to try various surfaces to determine what feels best to you.

Good luck.

So, ILD and density are not related...? I thought they were, but i didn't really know! I am familiar with the importance of high density, and that different foams have different densities, etc.

What, then, makes a foam 'firmer'? I always thought the density determined that (i.e. more air between molecules--lower density--would cause it to be softer).

Some high quality memory foams are very dense, but very soft.  Sensus & Venus are good examples.  Even the very soft 14 ILD LI blended latex is 4 lb. + density, if memory serves me properly.

I see....

what makes something softer or firmer, then?

That is a good question.  Hopefully someone that is involved with manufacturing foams will answer it.