Hi Mike77,
Unfortunately there are too many variables involved so there isn’t a formula that can predict which combination of layers or which design will work best for any specific person so your own trial and error and personal experience will really be the only way to know whether any specific design will be a good “match” for you in terms of PPP.
Unless you have a great deal of knowledge and experience with different types of mattress materials and specs and different layering combinations and how they combine together and can translate them into your own “real life” experience that can be unique to you (which would only be a very small percentage of people) … I would avoid using individual specs such as layer thicknesses or ILD numbers or compression modulus or other complex combinations of information or specifications to try and predict how a mattress will feel or perform for you or how it will compare to another mattress and focus more on your own actual testing and/or personal experience and if this isn’t possible then on more detailed conversations with a knowledgeable manufacturer or retailer.
I would also keep in mind that every layer and component in a mattress (including the cover) will have an effect on the feel and performance of every other layer and component above and below it and specs such as ILD are only one of several variables or “specs” that will determine how soft or firm an individual layer or a mattress “as a whole” will feel to different people (see post #4 here). In addition to this the ILD of different materials or different types and blends of latex also aren’t always directly comparable to each other (see post #6 here) so using the ILD of a particular layer or combination of layers as a reliable indication of how any mattress will “feel” or how firm will feel to you compared to another mattress with a different combination of layers can be more misleading than helpful.
While knowing the specs that can affect the quality and durability of the layers and components in a mattress is always important … when you try and choose specific layers or components based on complex combinations of “comfort” specs that you may not fully understand then the most common outcome can often be “information overload” and “paralysis by analysis”. Even the best mattress designers in the industry are often surprised at what a mattress they design “should have felt like” when they design it and what it “actually feels like” when they test out their new design.
I would tend to take a “one step at a time” or “bottom up” approach and purchase layers and components one at a time and then you can base your choice of each new layer using your experience on the previous combination as a reference point until you reach a final design that is “good enough”.
While there is no way to quantify how long any mattress will last for a specific person or predict exactly when you will decide to replace it because it is no longer suitable or comfortable for you (because this is the only real measure of durability or the useful life of a mattress that really matters) because there are too many unknowns and variables involved that are unique to each person … if a mattress is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new (see post #2 here) and you have confirmed that it meets the minimum quality/durability specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer and the chances that you would have additional “bonus time” would be higher as well.
There is also more detailed information about the many variables that can affect the durability and useful life of a mattress in post #4 here.
If you are spending unusual amounts of time in bed (or if you are in a higher than average weight range) then I would tend to use higher density and more durable materials than the minimums in the guidelines and would probably use 2.0 lbs as a minimum density for polyfoam (and higher would be better yet) and 5 lb as a minimum density for memory foam or any type or blend of latex which are all among the more durable foam materials.
Phoenix