Hi TD-Max,
Thanks for the clarification.
There are 3 “critical” areas or zones of a mattress that contribute the most to alignment. These are under the shoulders (mostly for side sleepers), under the recessed part of the lumbar spine (small of your back when you are on your back or waist if you are on your side), and under the hips/pelvis.
When your pelvis sinks too far into the mattress when you are on your back … it “tilts” which affects the natural alignment of lumbar curve. If the firmness under the pelvis and lumbar curve is suitable … controlling pelvic tilt will usually also control the natural alignment of the lumbar spine.
In some cases however … when there is more weight in the middle or for people who have a more curved spine or where the differential between the width of their hips and waist (usually in women) or between their shoulders and waist (usually in men) then even with firmer layers under the pelvis which controls the tilt it can be beneficial to have a slight increased firmness (or thickness) under the lumbar curve as well to prevent it from “sagging”. In most cases … a material like latex which has a high compression modulus (gets firmer more quickly with deeper compression) becomes firm enough with more shallow compression under the lumbar curve to provide the “secondary support” which helps maintain the alignment of the lumbar spine. In a few cases though, some extra firmness (relative to the pelvic area) under this area may be helpful for some people which is where zoning can be helpful.
The shoulders are lighter and typically wider so side sleepers tend to need thicker layers of softer materials so the shoulders are “allowed” to sink in enough for the torso to take up the weight of the upper body and relieve shoulder pressure as well as make sure the upper spine is in alignment. High quality materials such as latex get firmer with deeper compression so they have an inherent ability to be firmer or softer where needed depending on the amount of compression but zoning can add to the ability of a mattress to be firmer where it’s needed and softer where it’s needed.
In the case of your mattress and circumstances … the firmest support layers that maintained the soft upper layer would be (from top to bottom) X-Firm/Firm/Medium. This may be worth a try (you would have maximum control of pelvic tilt) before you consider zoning. It may also be useful to try the medium on top over X-Firm/Firm/Soft because this would be the firmest support layering possible with a medium layer on top. Removing the soft layer from the bottom and just having 3 layers as a test (with a loose cover in this case) would be slightly firmer yet but by the time the compression forces reach the bottom layer they are very spread out and much of them have been absorbed and dissipated by the mattress.
If one of these combinations still doesn’t bring you close enough your ideal … then it may be worthwhile to consider testing a simple zoning solution.
As a test case to “test” the theory … I would start with a layer of firm 1" polyfoam (around high 30’s or low 40’s ILD) cut to a length that you could add it under one of the layers in the middle third of the mattress. This could go under the first, second, or third layer depending on how much of a firming effect you wanted under the pelvis (deeper would have less firming effect). Other options that can test some zoning combinations would be felted carpet underlay or folded blankets (which would provide lighter zoning) or even wood strips under the mattress in targeted areas (you can see an example here).
If it turned out that it wasn’t pelvic tilt that was the issue but lumbar sagging … then you could cut this layer in half (or slightly less) and place it under the lumbar curve itself (not under both the pelvis and the lumbar curve) to improve the direct support under this part of the spine.
If one of these zoning solutions was necessary and worked for you … then depending on how you feel about having a thin layer of polyfoam in your mattress you could either keep it or replace it with an inch of X-Firm latex cut to the same size.
There is also more information about zoning in this article and in post #2 here and in post #7 here
Phoenix