The cushions get the most wear of almost any other part of the furniture. It is only natural that after years of use the filling should be replaced. One of the most common and cost efficient replacements is polyfoam.
This article describes two of polyfoam's main characteristics: density and firmness or ILD (Indention Load Deflection)
Although different foam manufacturers may use different numbering or rating systems, one of the most useful numbering system is composed of four digits. For example, 2535. The first two digits specific the density of the foam. The second two digits specify the ILD or firmness of the foam. In the previous example, the 2535 number is broken in half, 25 - 35. The first two numbers, 25, signify that the foam is a 2.5 lb foam. The second two numbers, 35, signify that the foam has a ILD (firmness) rating of 35, indicating that it is a medium firm seat foam.
Density: The density means how much foam material is in a given size. Foam is rated by how much a cubic foot of foam weights. For example, a piece of foam with a 1.8 lb rating means that a cubic foot of that foam weighs 1.8 pounds, and a 2.5 lb foam weights 2.5 lbs.
- 1 lb to 1.5 lb foam is a low quality foam, intended for very low use. It will break down quickly under moderate to heavy use.
- 1.5 lb to 1.8 lb foam is a medium quality foam intended for low to medium use. Typical use is for RV cushions or possibly vacation cabins that don't get much use.
- 1.9 lb to 2.4 lb foam is an intermediate quality foam, good for moderate use.
- 2.5 lb to 3 lb is a high quality for, intended to extensive use.
Firmness - ILD (Indention Load Deflection) The ILD measures how much weight or downward pressure it takes to compress the foam to 75% of it's original size. Using the 2535 foam as an example, it would take 35 pounds to compress 4" foam to 3" in height.
Information for this article take from 40 years experience as an upholsterer and from these companies' websites.