Hi mg517,
The ILD of latex is usually measured differently than the IFD of polyfoam so the testing results aren’t comparable between the two materials.
While there are different methods used to test the firmness of polyfoam … the IFD of polyfoam is usually a measure of the force it takes to compress a 20" x 20" x 4" piece of polyfoam by 25% of it’s thickness (1").
The ILD of latex is a measure of the force it takes to compress a 6" core of latex by 25% of its thickness (1.5").
Because it takes more force to compress a material by 1.5" than it does to compress the same material by only 1" … each type of testing would produce different results for the same material.
IFD/ILD is also only one of the factors or specs that can affect how soft or firm a foam material feels because it’s only measured at a specific depth of compression and the thickness of a foam layer, the compression modulus of a foam layer, and the point elasticity of a foam layer can also affect how soft or firm it feels (see post #4 here).
If you take all of this together … if a polyfoam layer has the same IFD rating as the ILD rating of a latex layer and they are both the same thickness … the polyfoam will generally feel noticeably firmer than the latex so it would be normal that a 20 IFD layer of polyfoam would feel firmer than a 20 ILD layer of latex of the same thickness.
the “30N” wouldn’t be related to the IFD of the polyfoam.
Phoenix