Density is the most commonly talked about attribute of a memory foam mattress. In general, higher quality beds are made from higher density foam and shoppers look to density as a way of assessing quality in a mattress. High density memory foam beds are more expensive because they have more foam per bed, but they offer better support and more durability.
Measuring high density foam involves weighing a sample of foam, measuring its dimensions, and using the data to calculate a value expressed in pounds per cubic foot (pcf). Memory foam beds range in density from 2.5pcf to 6.0pcf. It is common in the market place to express density simply as a number of pounds (lb or lbs). For example a 5lb high density foam mattress indicates a foam density of 5 pounds per cubic foot.
Foam density is widely regarded by foam manufacturers as the main factor determining the durability of a high density foam mattress. Durability is the ability of foam to retain its original comfort and support characteristics. In general, higher density beds are more durable. They are also more expensive to produce due to greater material content.
Density and Firmness
The most common misconception about memory foam mattresses is that density is relative to firmness. In fact density is not directly dependant on firmness. For example, a 5 lb density memory foam mattress can be softer than a 3 lb density memory foam mattress.
Foam firmness is expressed as an ILD (Initial Load Deflection) rating. ILD ratings range from 10 - 16, higher numbers being more firm.
The Perfect Density for a Memory Foam Mattress
Foam is made up of thousands of tiny cells. The cells are formed by a polyurethane wall and filled with space (air). Higher density foams have thicker cell walls and less space inside. Lower density foam has thinner walls, greater space. If foam were made too dense, its cell walls would be too thick / stiff. It would be hard to the touch and would not conform to the body properly.
It costs more to make dense foam, and at some point it can be made too dense. So there is a balance that needs to be struck between performance, cost, and durability. The marketplace today has gathered around a 5 lb memory foam mattress as the optimal choice for bedding. While more expensive than lower density foams, it provides better, long term contouring support.
Memory Foam Mattress Base Foam
Many people mistakenly think that memory foam beds are made with all memory foam. But only the top layers are made of memory foam, the base layers are made of traditional closed cell, polyurethane foam. The purpose of the base foam is to add cushioning support to the memory foam layers. Base foam ranges from 1.8lb to 2.5lb, but because it is a different type of foam, it is usually more durable than the memory foam it is paired with. It is less dense than memory foam, yet much firmer, ranging from 20ILD to 40ILD for the various base layers.