Hi Prof_Admiral_H_Nelsonm,
I think that sleepmommy’s comments are accurate and helpful (Thank you :))
I’ll try to add a few more details that may help you as well.
The first “rule” I would use is to buy a mattress or “sleeping system” with a configuration that you have tested. If you tested a mattress design with a solid non flexing foundation and it provided you with good PPP (Pressure relief, Posture and alignment, and Personal preferences), then adding a flexible layer underneath the same mattress may have unpredictable effects that in theory could be either helpful or detrimental. Adding unknown variables compared to what worked well when you tested a mattress would be risky IMO. In most cases … foam mattresses are designed to work best on a solid non flexing surface where the foam does the work of providing pressure relief and alignment.
Flexible slats will also have a different effect with different types of mattresses.
There are different types of innersprings and their feel and performance will each be affected differently with a flexible slat base (or a box spring) underneath them.
Innersprings in general don’t absorb the energy of compression which goes right “through” them so even with thicker mattresses an innerspring will compress the flexible slats or box spring underneath it (energy in the top equals energy out the bottom).
Pocket coils though act independently so these would be most suited to a flexible slat system because the slats will affect the area more directly under heavier compression so you can use them for more accurate “fine tuning”. Innersprings that are connected together with helicals act more in “groups” and the effect of a flexible slat system or box spring will be less specific in terms of the area they affect and because the lighter compressed springs will be “holding back” the springs that are bearing more weight … the effect of a flexible slat system will be less.
Latex on the other hand absorbs some energy with compression (about 20% - 30% hysteresisdepending on the type of latex) which means that the thicker the mattress the less energy comes “out the bottom” of the layer compared to the energy of compression on the top and the less effect the flexible slats underneath them will have. In general, because latex is so flexible, they will be effective up to a mattress thickness in the range of about 6" - 8" or so although this would also depend on the weight of the person and the compression forces on the mattress. Once the mattress/person combination becomes too thick … the effect of the flexible slats would be negligible. In Europe where flexible slat systems are more common … mattresses also tend to be thinner.
Polyfoam is “stiffer” than latex so the maximum thickness where a flexible slat system would have a meaningful effect would tend to be less.
In general though … I would stick with the design that your personal testing has shown you to be most effective and if you haven’t tested a specific combination then I would consider an “unknown variable” (or in your case two since you are considering both a topper and different foundations) to be somewhat risky and use the knowledge and expertise of the manufacturer or retailer of a component that you are considering to help you make a “theoretical” choice whether a flexible slat system may be helpful or detrimental in terms of PPP with the particular mattress design you are considering using on it.
I probably would personally limit going in that direction “in theory” to either pocket coils or thinner latex mattresses if I went there at all.
Phoenix