Hi pinupchick,
The comfort layers section and the support layers section of the site has all the information about the different qualities of different foams. In general though … polyfoam is the most commonly used material that will have issues with foam softening in certain densities and in certain types of construction (for example … 1.5 lb polyfoam in thinner layers in a two sided mattress may be quite durable even though if they were in thicker layers in a one sided mattress they would be much less durable). What is over and under each layer will also make a difference (the foams closest to the sleeping surface are the most subject to softening). Lower density polyfoam (less than 1.8 lb) in the upper layers especially would be one of the most important “warning signs”. Lower density memory foam (less than 4 lbs depending on body type) will also present durability problems unless they are very thin layers used for example in a quilting layer in the mattress. The key is to use the best quality materials that are possible in your budget range.
I would be willing to bet that the person who was claiming that it was “super high quality” probably had no idea about the type or quality/density of all the layers in the mattress so these were probably just “words” meant to sell a mattress and had little basis in fact. King Koil does make some mattresses (their Xtended Life series) that use higher density polyfoam but finding all the information you really need to know is very difficult and sometimes just not possible. If the factory doesn’t provide this information to the retailer … then the retailer can’t provide it to the consumer … even if they would otherwise want to.
the odds are pretty good that the foam is the problem and you may be surprised that if you took the mattress apart that the springs may still be fine. If foam softens … then adding toppers can’t fix the problem except perhaps very partially or temporarily. Toppers only really work for a mattress that is too firm. There’s more about some of the “not so great” options that may help a mattress that is sagging in post #4 here. As you discovered … a thicker wool topper is one of the most effective possibilities but even this is only a partial solution at best.
Unjfortunately this has almost become the “norm” in the industry today … at least with the larger brands that are sold in most of the mass market outlets. Even premium mattresses that cost thousands of dollars can have too much lower quality polyfoam in the upper layers and may last even less than the 4 years you had from yours. the key to avoiding this is to know the quality of every layer in your mattress so you can predict how it will perform in a year or more down the road.
Comfort is what you feel when you lie on a mattress. Support is what you feel the next morning (and with care this can be tested fairly accurately in a mattress showroom). Durability though is about how a mattress will feel and perform in a year or a few years down the road. This can’t be felt in a showroom because you can’t feel quality and even the lowest quality materials can feel great for a while or in a showroom. Quality/durability is the third part of the “mattress shopping puzzle” and is just as important as comfort and support. It can only be known if you know the details and “quality specs” of every layer in your mattress.
Phoenix