Polyfoam density

I want to buy a topper for a firm innerspring mattress (with not much of a comfort layer) and have more questions about density. I have samples of 2.5 lb. density HR poyfoam in a 31 ild and a 21 ild, both 2 1/2 inches. After trying both, I think I would prefer a firmness in between the two, maybe closer to the 21 iild. I cannot find a 23-25 ild in a 2.3 or 2.5 lb. density polyfoam. However, I see one online that there is a 25ild but with a density of 1.8. I wanted to find a higher density foam for longevity so I’m not sure how a 1.8 will last. Also, does a 1.8 density polyfoam have a different feel from a 2.5 density of the same ild? Because of the higher resiliency, will the 2.5 have a stronger " push back" feel? I do find the HR to be not as relaxing as I’d like… kinda bouncy.

The other question is this… With the 21 ild 2.5 lb. density sample in a 2 1/2 inch, my heavier spots, hips and ribcage sink too much for my preference. In theory, will a 3 Inch feel firmer and offer more support than a 2 1/2 inch?.. same ild.

Thank you

Hi lilac2,

There are several factors or “specs” besides just ILD that will affect the softness of a topper so going by ILD alone to assess the softness or firmness of a topper or how it will feel to you can be somewhat misleading. There is more about this in post #4 here.

In very general terms … a lower ILD, a lower compression modulus, more thickness, and greater “point elasticity”, will all work together to contribute to how soft a topper feels so if you change one then you may also need to change another one to compensate to have a roughly equivalent feeling of softness.

For example if you have a material that has a compression modulus of 1.9 (which is quite common with conventional foam) and an ILD/IFD of 25 (which means that it would take 25 lbs of force for a 50 sq in round testing head to compress a 4" layer by 25% or down to a thickness of 3") then it would take about 47.5 lbs on the testing head to compress a 4" layer by 60% (down to 1.6"). If you have another material that has an ILD of 25 and a compression modulus of 2.5 then it would take 62.5 lbs of force to compress the material by 60% so it would be more load bearing at the same compression level and feel firmer even though the ILD/IFD would be the same. In this case you would need an ILD of about 19 for the second material to feel roughly the same if you compress the material by 60% when you sleep on it (19 x 2.5 = 47.5). In most cases you will compress a topper by more than 25% (which is the percentage compression that ILD/IFD is tested). Thicker layers will also become firmer more gradually as they compress than thicker layers (see post #14 here for more about the effect of thickness).

I would also keep in mind that density and compression modulus are independent of each other so you can have a high performance lower density foam with a higher compression modulus and you could also have a higher density conventional foam with a lower compression modulus so I wouldn’t automatically assume that a higher density foam that is 2.5 lb density or higher is HR and has a higher compression modulus because it may not.

Whether a thicker topper will be more or less “supportive” will depend on whether you mean primary or “deep” support or secondary or “surface” support. A thicker topper will put you further away from the deeper support layers of your mattress so it would decrease the primary support (be less supportive under the heavier parts of the body such as your hips/pelvis) but it will also be softer and you would sink in more so it may “fill in” the gaps in your sleeping profile more so it can increase the secondary support.

There is also more about primary or “deep” support and secondary or “surface” support and their relationship to each other and to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may be helpful in clarifying the different types of “support” and “pressure relief”.

Phoenix