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Putting the layers together - progressive
The common factor in all progressive constructions is that they use a comfort layer that is THINNER than the recessed areas or "gaps" in a sleeping profile. If for example you need a pressure relieving cradle that is 3" deep (average side sleeper), then a progressive construction could have a comfort layer that was about 2" thick and the top 1" of the support layers would be used to form the rest of the cradle. If you needed a cradle that was 2" thick (common for back sleepers) then a progressive construction could have a comfort layer that was 1" or 1.5 " thick. In other words, no matter how deep a cradle you may need for your particular sleeping position or body profile, a progressive construction would use a comfort layer that is thinner than the depth you need and is designed to let you sink through it into the layer below. When you sink through it and into the support layer below, the qualities of the support layer "take over" and you can "borrow" its strengths to make up for some of the weaknesses of the thinner comfort layer above. For example you may wish to use memory foam in your comfort layer which is excellent at distributing body weight and relieving pressure but not so good with resiliency and holding up the lumbar. If you needed a 3" cradle and used 2" of memory foam, and put it over a highly resilient support layer such as latex or certain innersprings, then you would increase the resilience and lumbar support of the comfort layer while still being able to benefit from the pressure relief of memory foam. The deeper into a lower layer you sink, the more of its qualities you "borrow". This is why cheaper poor quality polyfoam can be used in the upper few inches of a mattress and by borrowing from the higher quality layers below, still feel good in the store and for the first little while.... until they wear out and lose their qualities. Sound familiar?
The biggest advantage of Progressive layering is that it can use any type of pressure relieving material in the comfort layers and can make up for any weaknesses in that material by borrowing from the layer below as long as it is thin enough to allow you to sink through it. This construction opens up the possibility of using materials in the comfort layers that may not work as well all by themselves and using them in thinner layers. Memory foam is a common example of this as memory foam has excellent pressure relieving qualities but often does not have the progressive resistance and resilience that is necessary in a thicker layer to support the lumbar area. By putting memory foam over a more highly resilient material which has a higher resilience or progressive resistance such as latex, pocket coils, or offset coils, the pressure relieving advantages of memory foam can be effectively used without the disadvantage of insufficient lumbar support (which needs resiliency) or allowing the hips to sink in too far (which need a higher sag factor). In the same way a comfort layer that in a 3" thickness doesn't have enough progressive resistance to stop the hips from sinking in too deeply, can be used in a thinner layer and put over a support layer that has higher progressive resistance. This would allow the use of HD foam for example which has lower progressive resistance to be used in a budget mattress if it was used over a lower cost innerspring that was soft on top and also had higher progressive resistance (such as a softer bonnell innerspring) and still be comfortable... at least until one of them wore out.
Its second advantage is that because it uses "combinations" of layering more than differential constructions and "combination" layers can have a wider variety of properties than any single material or any single ILD, it can be more "fine tuned" and a progressive construction can often fit each unique person and set of circumstances more accurately.
This biggest disadvantage of progressive construction is the difficulty of "getting it right" because you are dealing with the properties of layer combinations (which "borrow" qualities from other layers) rather than layers that function more independently of others. A comfort "zone" (the area that forms your pressure relieving cradle) with a single layer is easier to predict both in initial feel and in how it will feel in a few weeks, months, or years. The most common "do it yourself" constructions of standard thickness layers that progress from soft to medium to firm on the bottom for example are suitable for the majority of people but will not allow variation in layer thickness (which determines how much they "borrow" from the layer below) which makes the initial choice of the top layer thickness in particular more important. Once chosen, layer thickness usually can't be changed... only layer firmness/softness. Because layer thickness and layer softness work together, for those who fall outside "average" types of layering in their needs and preferences, incorrect layer thickness (especially on top but to some degree in the lower layers as well) can lead to endless "adjustments" that can be much more complex in their effect than the customer trying to adjust the mattress may initially believe... even with high quality materials.
Progressive constructions also rely on more of the qualities of the support layers other than simple "firmness". This means that the choice of a support layer is more restricted to those that have these other qualities as well. Because they have high degrees of many qualities, Latex, Pocket coils, Offset coils, and HR polyfoam are usually the best choices here and other types of support layers that are often used (Bonnell and continuous coils and HD polyfoam) are far less advisable. In other words, because the comfort layers are "borrowing" some of its qualities, the support layer needs to be particularly good at the quality that is being "borrowed".
Finally, the lower layer needs to be closer enough in ILD to the upper layer's more fully compressed ILD (not just its 25% ILD) so that a sudden "transition" is not uncomfortable. For example if you needed a 3" cradle for pressure relief and you had 1" of very soft foam over a very firm 2" foam layer, then you would quickly sink through the 1" and most of what you feel would be the firmer layer. Overall this layer would likely feel much too firm and cause pressure points. On the other hand if you had 2" of the same very soft foam over the same very firm 2" foam layer, it may be perfect as the slightly firmer 2" layer would mean that you would not be sinking in to the deeper layer underneath quite as far and borrowing less of its "firmness". The upper 1" of the firm foam may be soft enough to form an excellent cradle and feel perfect. You could accomplish a similar effect by using a softer 2" layer that would be closer to the 1" layer in ILD. Changing layer thickness and changing ILD are equally important parts of adjusting a progressive construction mattress.
Natural fibers are a special circumstance. While they can provide good pressure relief in certain circumstances after they have "broken in" and formed a semi permanent cradle in the comfort layers for effective pressure relief, they always need the help of high quality conforming support layers to help with pressure relief. These include Latex, HR polyfoam, and good quality Pocket coils or Offset coils. These mattresses also need special construction techniques to prevent compression of the fibers beyond the point that is needed for pressure relief and can be more expensive because of this. Budget versions of mattresses like this may not keep their qualities for long and with rare exceptions are usually not the best choice. They are also not as effective for different sleeping positions with a larger difference in profile (such as stomach/side sleepers) or unusual body shapes as the semi-permanent cradle that they depend on may not be appropriate for their range of sleeping positions or body shape.
Just to complicate this a little further, if a comfort layer is the same thickness as the gaps, then even though it would be a differential construction, the comfort layer itself may use a progressive approach such as 1" of latex over 2" of memory foam (or the other way around). In the same way the support core of a mattress can also use progressive techniques even in a differential overall construction. In other words, a progressive approach can be used within a comfort or support layer regardless of the construction of the overall mattress.
If this section has left you with the impression that choosing or adjusting a progressive layering can be a very complex process that has many moving parts, then you have an accurate understanding of the complexity of this method of construction. It is as much an art form as it is a science. While it can produce an amazingly comfortable mattress, it can often be much more difficult to get there than the more simple differential construction.