Hi Johnnydm,
I didn’t answer this earlier because I wanted to let it “perk” a little while.
You are getting into what I call dominating layers which can be quite tricky. They are usually used to make a softer foam (or even an innerspring where a firm insulator is used) feel firmer, improve support, or reduce the amount a certain part of the body sinks into a mattress. It is more commonly used in certain areas as a form of zoning. It can have unpredictable results depending on the foam that is being used and how it interacts with the other layers, the depth that someone sinks into their mattress, and the sensitivity of certain areas of the body to pressure. Sometimes just the act of increasing the thickness itself, even with a slightly firmer foam, can allow for a net effect of the heavier parts sinking in more even though it is preventing it in the layer below it.
In effect … what happens depends on a combination of the ILD of the foam, the thickness of the foam, the point elasticity of the foam, and the compression modulus of the foam. Without knowing at least the ILD and density (so that the compression modulus can be estimated) it’s difficult to theorize about what may happen.
In general though … if the foam is anywhere from a little firmer all the way down to softer … putting it in between layers will add to the depth of the cradle and also reduce the support of the mattress.
If the foam is significantly firmer … then even if it has a lower compression modulus it will decrease the depth of cradle (reducing pressure relief) and improve the support.
There would also be a slight difference between a different layering arrangement of the 2" and 3" layers you have although this may be below the threshold of perception. A foam not only compresses under weight but it also “pulls” against the foam beside it which is “pulling back” against the compression. This is true as long as the foam is a single layer. If you have two layers that are stacked on top of each other, they will act softer than a single layer because when you compress into the second layer … the cut means that there is no foam pulling back against the compression that goes into the layer below. This also means that a 2" layer on top of a 3" layer of the same type of foam will be slightly softer than the other way around because a greater percentage of the compression will have foam “pulling back” against the compression.
So first of all … having the 3 inch layer on top will slightly firm up the mattress although this may not be noticeable. If the camping foam you have is significantly firmer than the latex (which it probably isn’t) … then it would reduce the pressure relief slightly and increase the support slightly. The increased support would be most noticeable in the areas of greatest weight (again assuming that the foam is significantly firmer than the latex).
If the camping mat is significantly firmer than the latex … or perhaps just because it is thin and will compress to it’s maximum more quickly and “bottom out” … it would create firmer support and would also lessen pressure relief. If it was only slightly firmer, or the same or softer … then the pressure relief may improve but because you would be “stopped” sooner because of the thinner overall foam … support would also likely improve.
So to relate this to your experience…
You noticed a decrease in stiffness which could mean that the mattress was “acting” softer on top (I’m inferring this because it seemed that the issues you mentioned before were connected to being too firm on top and even though you didn’t mention stiffness before it is what you meant by soreness). This would indicate to me that what you did actually softened the mattress and improved the pressure relief (you were sinking deeper into the foam on top) and filled in the gaps more which took up more of the weight and helped support the arch of the lumbar. Backwards logic also says that the camping foam may not be as firm as you thought or perhaps it is only a little firmer than the Dunlop. The support would also have improved under the heavier parts because there was not as much foam to sink into in total before you “hit the plywood” because the 2" layer was removed. In other words … it was both softer and firmer.
All of this is pure speculation of course because I don’t know exactly how you are interacting with the mattress and also don’t know the specs of the 1" camping foam but it would certainly fit the circumstances and your feedback.
Higher ILD Talalay and Dunlop do get “closer together” in compression modulus (comparable ILD’s would already be the same with 25% compression no matter how the densities compare) which would move the relative firmness beyond 25% compression closer together. This would be more true with all natural Talalay than with blended Talalay because blended is always lower density than all natural.
The firmest Dunlop usually goes up to about 95 kg/cubic meter which is about 6 lbs/cubic ft. The softest Dunlop (used for toppers) is usually around 65 kg/cu meter which is about 4 lbs/cubic ft. Blended Talalay goes down under 3 lbs cubic ft. and the firmest versions (44 ILD) are about 5 lbs/cubic ft. The natural Talalay is heavier so at the top of the scale it would be much closer to Dunlop than at the bottom of the scale.
So compared to your current setup (camping foam with 3" of 28-30 Dunlop over it)
This would increase the depth of cradle and pressure relief and how much your gaps were filled in compared to the Dunlop by itself with no softer foam over it.
I would take this 1 step at a time though because although the 1-2" talalay on top would soften the upper layers … putting the extra 4" on the bottom (3" Dunlop and the firm talalay) would also allow for more sinking in of your heavier parts compared to the previous support layer which was basically the camping foam and the plywood even though the Dunlop had firm talalay over it. It’s never as effective to use dominating layers as it is to actually have firmer layers underneath. It’s great for fine tuning if you have “room to spare” with pressure relief but it’s very difficult to firm up a mattress that is too soft in the support layers except a little bit. If there is softer foam under a firmer layer … it will still compress … just a little bit less. The firm foam doesn’t "“stop” the compression of the foam underneath it in other words … it just modifies it a little bit.
By adding the new softer layer first … then you can see how that works for pressure relief and then add the next step based on your experience with the last one. You may not even need the firmer talalay. Wherever possible … I would always use softer latex over firmer latex rather than the other way around and only use dominating layers for zoning of fine tuning (rather than larger changes).
With your weight, My “gut” says 2" of softer talalay would work OK over a medium transition layer. The “questionable variable” may be that you have a couple of inches of Dunlop that may need to be replaced for a firmer bottom layer although you may just get away with it. Once you have filled in the gaps a little more firmly and more of your weight is spread out over a larger surface area, Then I would decide if you needed a further step.
In all of this you haven’t mentioned anything about our shoulders and whether they are feeling pressure or “scrunched”. It might also help to know what type of side sleeper you are (top leg bent and directly on the mattress or more of a fetal or semi fetal position with both legs bent and one on top of the other). Where are your arms when you side sleep (one under your head or not) and what type of pillow do you have (thickness and softness looking at upper spine and neck alignment and position).
Phoenix