Would a 6 inch thick 2.4# polyurethane foam core that is 35 ILD feel the same (in terms of firmness) as a 6 inch thick 1.8# polyurethane foam core that is 35 ILD? Obviously the 2.4# would be more durable, but is there a difference in the feel, even with the same ILD numbers? If there is a difference, which would feel firmer?
Hi a2dox,
There is more about the different specs that can affect how soft or firm a foam feels in post #4 here. The two most important of these would be the ILD and the compression modulus although there are also other foam properties that will affect the “feel” of a foam material as well outside of just its softness/firmness that are more subjective.
Depending on how deeply it was compressed … a polyfoam layer that has a higher compression modulus would start off feeling softer and then become firmer faster as you compress it more deeply than another layer of polyfoam that was the same thickness and ILD/IFD and would tend to feel firmer if it was compressed more than 25% (which is the percentage compression that ILD/IFD is measured) and would also feel softer if it was compressed less than 25% of its thickness. Higher density polyfoam will tend to have a higher compression modulus than a lower density polyfoam…
Phoenix
I had read that before but didn’t fully understand it. So let’s see if I get it this time.
Theoretically, the 2.4# foam would have a higher compression modulus than the 1.8# foam of the same thickness and ILD. So if it’s the core layer of my mattress, and I’m only sinking into it a little, it would seem softer than it would with 1.8# foam, but if I’m sinking into it a lot, it would potentially feel firmer than the 1.8# foam. Is that a correct interpretation (assuming that all other things are equal)?
Hi a2dox,
The compression modulus of a foam material (AKA sag factor, comfort factor, and support factor) has nothing to do with its thickness and is one of the properties that is chemically formulated into the foam. It is defined as the ratio between the IFD (indentation force deflection) of a foam material at 25% compression and the IFD of a foam at 65% compression. A foam that takes twice as much force for a 50 sq in compressor head to compress a 4" thick layer by 65% of its thickness as it does to compress the same layer by 25% of its thickness would have a compression modulus of 2.
While higher density foams tend to have a higher compression modulus than lower density foams … this isn’t always the case because it depends on the specific formulation of the foam.
A foam that has a higher compression modulus would feel softer if it was compressed less than 25% but would “firm up” faster as you compress it more deeply and would feel firmer if you compress it more than 25%.
Phoenix