There are a lot of variations in the chemicals and methods used to make memory foam which results in memory foams with different characteristics and feels even if two versions have the same density.
Besides the density, some of the variables include breathability, temperature sensitivity (the temperature range it softens or becomes firmer), recovery time, ILD (a measure of softness with lower being softer) , Sag factor (the rate a foam gets firmer with deeper compression), the type of polyols used (TDI, MDI, and plant based polyols), offgassing and smell, surface feel, cell structure, additives like charcoal and clay or others that can increase the foam density (sometimes creating a “fake” density without any of the benefits of increased density), the ratio of viscosity (like a fluid) and elasticity (like rubber) of each foam, and others including creep (which is the tendency of viscoelastic materials to get softer or internally relax more with sustained consistent pressure regardless of humidity or heat).
What this means in practical terms is that different memory foam formulations will feel different even if they have the same density. While density is the single biggest factor in foam durability, it is only one part of the story in terms of how a foam feels. There are a couple of general guidelines that you can go by however to some degree.
Lower density foams tend to be more breathable (more open celled) and faster reacting. They are less “stiff” initially (which is why people think they are softer) so they are less motion restricting but they don’t conform to the body quite as well as higher density foams that are more heat sensitive. This means that higher density foams are often thought of as firmer "because they take more time to soften and “resist” movement more because of their greater time delay but they are actually more pressure relieving once they have softened and conformed. They are also more restrictive in terms of the “sleeping in sand” feeling that many people talk about.
One of the reasons that 4 lb foams are being used as much as they are (like in the tempurpedic cloud series) besides the fact that in general they are less costly is because people seem to like the “softer” less restrictive feeling that they initially provide. While they are a little less durable than higher density foams of the same basic formulation, the tradeoff is worth it to many people. If I was heavier than average (starting at around 200 lbs or so) … I would begin to question the use of 4 lb memory foam because durability would be more of an issue or at least it would be important to know ahead of time that the feel I was looking for was worth the tradeoff.
All memory foam would be considered to be soft and most have an ILD in the range of about 9 to 15 (Tempur is on the higher end of the scale) and some go slightly higher into the upper teens. ILD is very misleading though with memory foam because it changes with heat and humidity so a stiff feeling foam can be very soft as it more slowly conforms to the body. It’s like the difference between slapping water or honey and slowly putting your hand into it. With fast movement they feel firm but with slower movement they feel soft.
Bear in mind too that a big part of how a memory foam mattress feels in terms of its “firmness” depends on the layers under the memory foam (usually polyfoam) and on the thickness of the memory foam. If a mattress had less than 2" or more than 4" of memory foam in it … I would begin to look even more carefully at the rest of the layering to decide on the suitability of memory foam layers that may either be too thick or too thin (both of which could cause issues in certain constructions and with certain people with different weights, shapes, and sleeping positions)
There are also different ways to make memory foam more breathable. These include different formulations to “open up” the cell structure and also mechanical methods to do the same. What happens with these is that the “window panes” or thin parts of the cells in the foam are broken while the stronger struts remain so the air can flow more easily. There is also a method of making memory foam that is used by Foamex (who makes Aerus and Sensus) called VPF or variable pressure foaming which can create more open celled foam which doesn’t require chemical or mechanical methods to create the more open cell structure. This is one of the reasons I like this particular foam but all of the American manufacturers (and most European and even some Asian manufacturers) are making better quality and more “customized” foams as their research and expertise improves. There are other ways that some manufacturers make more breathable or cooler foam as well such as punching holes in it or adding materials that are more convective and less insulating (such as gels) to the memory foam.
The Sensus is a denser slower reacting foam which is very high quality but not as breathable as the Aerus. Aerus also comes in 5 lb versions but this is much more difficult to find. I personally think that the combination would be a good idea for those who weren’t sure about how they would feel on slower reacting denser foam in terms of heat or freedom of movement and I personally would give up some durability for the feel of a layer of 4 lb foam (or a more breathable and faster reacting 5 lb foam) … although others may make different choices. The combination of 4 and 5lb foams would be somewhat like the Tempurpedic cloud series.
In terms of support … all memory foam is soft which is why it is never used in the support layers of a mattress. There is a difference between them though in how long they take to soften so some clearly feel “firmer” than others and some even have a higher ILD and support factor so they don’t sink in quite as far as others. In general … an ild difference of a few lbs is not really noticeable.
The different Tempurpedic foams tend to be a little less breathable, a little “firmer”, a little slower reacting, and a little more temperature sensitive than “average”. They are in the upper quality range for their density and have a wide variety of well thought out layering patterns but there are many people who prefer the feel of other memory foams. They are also one of the most expensive as I’m sure you know which is why I consider them to have lower value than many other memory foam choices.
I spent some time this morning updating the Nashville post that I linked to and talked with several of the choices on the list (mainly because I was having a hard time believing that Nashville really was such a mattress desert :)).
If you were looking in the direction of latex … I would suggest you test the Pure Latex Bliss and the Jamisons to get a sense of different layering that would work and then use them as a prototype for an online purchase.
If you are looking in the direction of memory foam (which it seems you are) I would be tempted to make a few calls to some of the outlets that carry alternative brands to see if they carry anything similar to what you are looking at as there are more local memory foam choices than there are latex choices.
I would do this on the phone and ask questions like …
Do you carry any memory foam mattresses that use a layer of 4 lb memory foam over 5 lb memory foam without any polyfoam over the memory foam?
Is your memory foam North American made?
Is your memory foam CertiPur certified*? (this is a testing organization made up of mostly American foam producers which tests for chemicals, offgassing, and durability to some degree. They did this to differentiate themselves from some of the poor quality asian memory foam that was flooding the American market).
What is the density of the polyfoam support layers under the memory foam? (it should be a minimum of 1.8 lbs and better yet if it’s 2.0 lbs or higher). The typical ILD of the polyfoam layers underneath will be in the range of about 28 to 36 so if they know this too it’s a bonus but it’s a support issue not a quality issue. This will give you a better idea of the firmness of the support layers.
How much does it cost in ____ size mattress only? (if they carry a mattress that sounds interesting).
What kind of cover (ticking) does it have? (stretcheable knit fabrics are the best as they can stretch to accomodate the form shaping ability of the memory foam better).
The advantage of making a few calls like this is that if there was something that sounded interesting at a reasonable price … you would have the chance to actually lie on it. I would do this on the phone though so I didn’t have to drive around and spend hours of my time. The outlets that won’t tell you what you want to know (including prices) aren’t worth your business anyway when you have good online options.
I would also take some time to talk with some of the online options I listed. They are mostly very knowledgeable and helpful especially about the “feel” of their specific mattresses.
So hopefully this will help a little. Now that you are looking online … even though you’re in a bit of a more “barren” area in terms of factory direct manufacturers … you still have the chance to get the best possible quality and value.
I hope I’ve covered most of the questions that are important to you but if not … feel free to keep them coming.