5) What does the 'density' of a memory foam mattress really mean, and why does it matter?

The density rating of any foam mattress generally refers to the density of the top layer of visco-elastic memory foam, not the entire mattress - as the supporting base foam layer of a true memory foam mattress uses a 'high resilience' or 'impact resistant' polyurethane foam. This should never be confused with visco elastic memory foam. It's important to understand that the top layer of visco-elastic memory foam is the most costly to manufacture, and ultimately the most important component in any memory foam mattress composition.

The density rating is a measurement of how much visco-elastic memory foam actually resides within the foam layer of the mattress. The lesser the density, the lesser the raw material content actually making up the memory foam layer, and ultimately the bigger the cells or 'air cavities' as a result. The higher the density of the memory foam layer, the heavier the memory foam will be. And most importantly, the cells or 'air cavities' in the memory foam will be smaller, more tightly packed, and present in greater numbers. This makes them both harder to break down and ultimately indicates the muscular / skeletal support and 'life expectancy' you can expect from the mattress. It is clinically recognised that the higher the density of the memory foam layer, the better support the memory foam will provide.

A high-density memory foam mattress will feel more full-bodied and sumptuous in comparison with lower quality imitations. However it is a common misconception that this dictates the firmness of the memory foam. Advanced manufacturers of true high-density visco-elastic foam are able to manipulate firmness ratings regardless of density. Many manufacturers producing low-density variants, such as 50kg/m3, will often make their memory foam feel firmer in an attempt to pass it off as high density. Inevitably the cell structure of mattresses with lower densities will break down much quicker and ultimately cause the memory foam to sag over a relatively short period of ownership and use.

As a rule of thumb, only companies supplying true high-density memory foam mattresses offer an extensive, confident guarantee or warranty period. Many lower density memory foam manufacturers will only offer as little as 12 months.

The density rating is calculated by weighing a cubic meter of the memory foam, which is why you see ratings expressed by weight, i.e. 85kg/m3. For those more comfortable using imperial measurements, the same calculation applies in that you weigh a cube of memory foam measuring 1 foot in each direction. The density is then displayed in lbs - i.e. 5.3lb.

The Ergo Flex™ memory foam mattress uses a luxury 9cm deep 3rd generation high-density memory foam layer, with a density rating of 85kg/m3, or 5.3lb.