Hi kymist,
There is more about the different symptoms that most people experience on a mattress and the most common reasons for them in post #2 here and while it would be less common … it’s certainly possible that a mattress that is too firm can cause lower back issues as well (sleeping on the floor for example can cause lower back issues as well as pressure points). Anything that puts your lower back out of alignment can cause lower back issues (see the diagram here).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
This really isn’t possible to know because there are hundreds of different latex mattress designs that use different types and blends of latex, different layer thicknesses, different firmness levels, in some cases different types of zoning, or that may have other components that are different as well (such as the type of cover or even a different support system under the latex such as an innerspring) and some of them may be a perfect match for you and others wouldn’t (see post #13 here and the posts it links to for more about the difference between innerspring/latex hybrids and an all latex mattress). This would be the same as any category of mattresses such as an innerspring mattress where you may try many mattresses that don’t work for you but there are some that do.
All that can really be known with any certainty is that the specific combinations that you’ve tried aren’t a good “match” for you in terms of PPP.
You are well outside the “averages” of most people’s experience and with 3 layer component latex mattresses the majority of people will be well inside the comfort/support range that is suitable for them either with the original layering or by rearranging the layers that they have. There are also a smaller percentage of people that may need a layer exchange because the combinations that are possible with the layers that they have available aren’t working well for them. Beyond this there will always be a very small minority of people where nothing they try with the layers they have available seems to work and with this small percentage of people it can be very difficult to “diagnose” the reasons why and in some cases it can involve issues that aren’t even related to the mattress itself (such as physiological issues or pillow issues for example) or it could be that they are much more sensitive to very small changes and would do best on combinations that aren’t possible with the layer thicknesses and firmnesses that they have available.
While I don’t have any specific suggestions outside of the ones I’ve made in my previous replies … I would make a more general suggestion that I would try any new combination for a few days at least so that you can identify a “pattern” of experience rather than what can sometimes be an “anomaly” for a single night and it can also take some time for your body to “catch” up to the changes you are making and you may still be experiencing symptoms from a previous combination. Even the process of “change” itself can cause issues (regardless of the specifics of the change) if you don’t try each combination for “long enough” for your body to catch up. Slow and incremental changes where you spend “enough” time to identify how your symptoms changed in comparison to the combination before it is usually the most effective approach so that you can “learn from” how your symptoms changed with each combination compared to the one before it.
A more detailed conversation with Neal would also be a very good idea so you can take advantage of his knowledge and experience from hundreds of his customers that have provided feedback on his mattresses and all the possible combinations that are possible and he may have experience with other customers that have slept on their mattress that are similar to your own and be able to make some suggestions that may be helpful.
While anything is possible … you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components would be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own personal testing or sleeping experience (see mattress firmness/comfort levels in post #2 here). Without a reference point of a specific mattress or combination of layers that has worked well for you to use as a reference point the only way to know will be based on your own personal experience.
ILD’s are often not directly comparable even between different types of latex so when you are comparing polyfoam to latex it would depend on the type of latex you are comparing it to and in addition to this the methods for testing ILD/IFD are different with polyfoam than they are with latex so they aren’t really comparable generically either (see post #6 here).
In very general terms … IFD with polyfoam is tested on a 4" layer and latex ILD’s are tested on a 6" layer so when you are comparing a polyfoam layer with a latex layer and assuming that the testing method that was used with each of them are the same then a polyfoam layer with the same IFD rating as the ILD rating for a latex layer would be firmer (see post #6 here).
I would also keep in mind ILD/IFD by itself is only one of several factors that can affect the softness/firmness of a material or a mattress as a whole and there are several specs besides just ILD/IFD that can affect how soft or firm a material or a mattress feels. In many cases using ILD/IFD by itself as the only point of reference for firmness/softness can be very misleading (see post #4 here and post #2 here).
Phoenix