Hi dodo,
[quote]HD foam has higher ILD numbers than HR foam to describe the same level of firmness. Why is that? Is it because of the higher density/weight of HR foam?
Thanks again for your help and all the excellent articles on this site.[/quote]
All polyfoam is measured in the same way for ILD but ILD (often called IFD with polyfoam) only measures the resistance of the material when a 4" layer is compressed by 25% (see post #6 here about ILD). There are also several other specs besides ILD that contribute to the perception of firmness/softness in a material which are compression modulus, and point elasticity both of which (along with the thickness of a layer) can affect the perception of firmness/softness (see post #4 here). HR polyfoam has a higher compression modulus than HD polyfoam which means that even though it would be the same firmness if you only compressed the material by 25% … it will become firmer faster than HD polyfoam if you compress it more than that (and would be softer if you compress it less than 25%) and since most people don’t sink into a mattress by exactly 25% … their firmness ratings would probably be based more on what people actually feel on different materials “on average” than they would on ILD alone.
Because firmness and softness depend on many variables (not the least of which is the person themselves) … comfort or firmness ratings can vary depending on the manufacturer, the person, and by the type of material that is being described and there is no consistent standard between different manufacturers, different materials, or between different people about what a soft, medium, or firm material really is, especially if it’s only based on one of several variables that can affect what people “feel”.
Phoenix