Memory Foam Mattress Grading
Viscoelastic memory foam is temperature sensitive; it softens in warmer temperatures, and firms up in colder conditions. This explains why it feels firm when you first lie down, but within 4-5 minutes it begins to soften and conform to your body. Memory foam mattresses are actually graded based on viscoelasticity, firmness, durability, resilience, and strength.
The density of a mattress is what most people look for first when evaluating a mattress. It also happens to be one element of the grading system. Some might be under the impression that density tells you how firm a mattress is, but it actually is just a reflection it's overall weight. A heavy mattress is therefore more dense, and contains more materials and chemicals. The result is that it will be more viscoelastic.
The best density is thought of as anything above 4 pounds, though some customers have reported that they prefer as low as 3 pounds. For those who want the best combination of comfort and support, 5 pounds or more is considered best.
The ILD rating indicates how hard the mattress is. The formula for this rating is figuring out much weight is needed to make a 25% indent into the mattress. For example, if the rating was 15, this would tell us that 15 pounds was needed to compress it by 25%. So the higher the rating, the more firm the foam is. Most mattresses have a rating somewhere between 12-16.
The top layer will certainly be softer to provide relief from pressure points, while the base will be harder to provide support. Some manufacturers couldn't tell you what rating their foam has, and others refuse to disclose the information publicly. This rating really doesn't tell you much about the quality of the foam.
Resilience is another element that is measured. It's tested by dropping a steel ball from a particular height, and then measuring how much the memory foam recovers. Ideally, the top layer should actually have low resilience. That way, it won't bounce back against the body and result in pressure points. The base however should be resilient so enough support is provided. Also, a highly resilient base will make it that much more durable.
Tensile strength is the other factor involved in memory foam grading. This involves the amount of force required to tear the mattress by stretching it apart. Quite frankly, this really doesn't provide any insight into the quality or durability of memory foam.
Manufacturers tend to reveal as little as possible about their mattresses, so finding out all of this is virtually impossible. Sometimes they won't even tell you the density.