Most foam grades are identified by a two part numbering system (i.e. 1938). The first two numbers identify the Density of the foam in pounds per cubic feet (pcf). The last two numbers identify the compression or the firmness of the foam. A higher density equates to a better piece of foam. Similarly, a higher compression means a firmer piece of foam.
The Importance of Density
Density is the key indicator of the foam's performance with regards to comfort, support and durability. It will also determine the cost of the foam. The denser the foam, the more material there is in the cushion or mattress to provide support for your weight.
It is important to remember, though, that the firmness of the foam is independent of the foam's density. High density foams can be produced to be very soft. Low density foams can be made to be very firm. Therefore, there is no such thing as a "hard" or "firm" density.
The Definition of Density
Foam density is not just weight. It's actually a measurement of mass per unit volume. Density is a function of the chemistry used to produce the foam. It takes into account the materials, both virgin and additives, used to increase density.
Better foams contain no additives or cheap fillers which increase the weight of the foam but do nothing for the physical properties including support and durability.
Flexible polyurethane foam is available in a broad range of densities from as low as 0.8 pcf to as high as 6 pcf. Most common foam applications, for seating or mattresses, use foam in the 0.9 to 2.5 pcf density range.
Firmness of Foam
What is the difference between firmness and support? Firmness is a measurement of the surface feel of the foam. Support is the foam's ability to "push back" against weight and prevent the foam from "bottoming out". Higher density foam helps prevent the feeling of having the foam collapse beneath body weight. Both firmness and support affect foam comfort.
Believe it or not there is even a measurement to determine the foam's ability to provide support. This measurement of support factor is calculated by measuring the firmness (IFD) of the foam when compressed to 25% of its original thickness (i.e. 4" piece to 3") and then again when compressed to 65% of its thickness. The ratio of the 65% IFD divided by the 25% IFD is the foam's support factor.
Foam will support weight better with a higher support factor. Foams with support factors of 2.0 lbs. per cubic foot or above are better suited for load bearing applications like seat cushions.
Durability of Foam
Density also translates into foam durability. The more polymer material used to produce the foam, the better the foam retains its original properties. Research into foam durability by measuring flex fatigue, or loss of foam firmness after flexing the foam a predetermined number of cycles shows that as polymer density increases, firmness loss is lessened. Less firmness loss means that seat cushions still feel "new" and mattresses retain their original "feel". Higher density foams also better retain their ability to provide support.
Other factors used to gauge foam durability also improve as density increases. One of these is compression set, or the tendency of foam to lose height as it is used. As density increases, height loss decreases dramatically. This means that fabric on furniture cushions stays tight and mattresses don't get body impressions. In fact, some of the more reputable mattress manufacturers have increased the densities of their topper foams to reduce instances of body impressions on innerspring mattresses. To better understand the durability of a mattress topper one must know the density of the foam.