What is it? 

Memory foam is the common term for visco-elastic polyurethane foam, a product developed by NASA in the 1970s. Memory foam is sensitive to both pressure and heat; different versions of the foam possess these characteristics to varying degrees. Memory foam enthusiasts often say that the mattress is both soft and supportive at the same time. 

Memory Foam Ratings 

Visco-elastic/memory foam is sensitive to both pressure and temperature. It is rated along the following parameters: 

Weight: Weight of memory foam material is determined by its density in pounds per square foot. The relative density of different foams is determined by the amount of chemicals used in the composition of the foam: mattresses that have more chemicals per square toot will have a higher density. Density will not determine hardness, but it is an important overall variable to understanding the composition and behavior of your mattress. 

Resilience: Resilience measures foams springiness by measure the percent rebound of a steel ball dropped from a height of 36". The higher the resiliency, the better the mattress is likely to perform with compression forces. The term H.R. is a mark of quality, and refers to foam that is highly resilient and makes the steel ball  rebound to a relatively great height. 

ILD Rating: The ILD rating stands for Indentation Load Deflection, and measure how hard or soft the material is. Specifically, ILD measures the number of pounds required to achieve a 25% compression of a 50 square inch indentation on a 4" thick piece of memory foam. The lower the ILD, the softer the mattress. 

Tensile: Refers to the extent that foam can be stretched before it ruptures. The measurement is in pounds per square inch and percent of stretch prior to rupture. However, this rating is not that important to memory foam mattresses, as the stresses they are submitted to do not resemble stretching. So, focus on weight, ILD, and resilience as the most important measurements. 

Ratings for buyers to target 

The composition and behavior is a result of an interplay of all the materials that go into the foam influenced by the manufacturing techniques. The ratings above are an attempt to predict how the mattress will feel to sleepers and how it will wear over time. Which are most important and what should you look for? Look for an ILD rating of 10-16, realizing that most shoppers will be most comfortable in the 12-14 range. Below 10 and the mattress may not provide enough healthy support. Above 16 and the mattress may be so firm that it does not provide the comfort for which memory foam is famous. Look for a weight that is between 4 and 5, as indicators of quality materials and a durable product. 

"Core" mattress issues 

Memory foam is too soft and unsupportive to be used as a material for an entire mattress. Tempur-Pedic popularized the use of memory foam in mattresses. Their method was to use visco-elastic foam on top and polyurethane as the mattresses core. The benefits of molding to and supporting the entire body are delivered by the top of the mattress. Deep, overall support are delivered by the mattress core. If you want the comfort and memory foam but value the traditional bounce of an inner-spring mattress, or if you'd rather combine a memory foam top with an air mattress with variable controls for two sleepers, manufacturers are now making memory foam tops attached to all types of mattresses. A word of advice: you need at least two inches of memory foam on top for the mattress to function as intended, though buying a mattress with at least three inches of memory foam is a good idea, particularly if the sleepers are large-sized people. Remember that the lower the ILD rating, the softer the mattress and the thicker the top you may need.