Hi brass,
When you are building your own design from individual components then the most reliable way to predict the final outcome is to use a reference point of a mattress that you have actually tried and then build the exact same design with the same type of latex, layer thicknesses, ILD’s, and a very similar cover. Once you begin to make changes to your reference design then the results become much more uncertain and the changes will be felt in different ways by different people because of the many differences between body types, sleeping positions, and perhaps most of all by the different levels of sensitivity between different people. I have seen some people compare two very different mattresses and don’t seem to notice any difference at all and others who compare two mattresses that notice major differences in their sleeping experience from from seemingly very small changes that would be insignificant to most people. It all depends on where you are in the “I can sleep on anything” and the “princess and the pea” range of sensitivity. Even the most experienced mattress designers with years of experience can be very surprised at the outcome of a new design compared to how it was “supposed” to feel and perform in theory compared to how it actually does in real life.
If you have two mattresses that only have one relatively small difference between them (such as one layer being an inch thicker or thinner, a single layer change in materials, a smaller increment change in one layer, or two different covers) then it’s easier to predict the most likely outcome of the single change but the effect of even a single smaller change in design can still be surprising for some people because of all the variables involved and once you have larger changes or multiple changes between a particular design and a known reference point then the only way to really know how they will affect any particular person is through trial and error and your own sleeping experience.
If you are working on theory alone without using a specific reference point then it’s usually best to take a “bottom up” approach where you start with the deeper layer or layers and then “leave room” to build additional softness on top of it based on your actual experience on the deeper layers. This can reduce the chances of buying layers that you don’t need or that don’t work well in your final design.
The Nutrition has a dominating layer design with a firmer layer of latex over a softer layer which gives you a firmer sleeping surface that some people like (probably not as much with most side sleepers) with some additional softness underneath the top layer. It has a different “feel” from a more traditional progressive type of layering.
Assuming you tested the mattress without the 4" latex foundation … then the safest approach would be to use the exact same layers in the same design with a similar cover. There will be some differences between glued and unglued layers but these probably wouldn’t be that significant for most people (although the unglued layers in the top layers would make a bigger difference). If you change the core to a softer ILD then it would certainly make a difference and the mattress would be softer but you won’t be able to predict how much softer or the specific effect on your body of a softer support core until you actually try it. While thicker layers of latex on top of the core would modify the effect of the softer core layer … it would still be different and in some cases you may notice it more when you wake up in the morning either with or without discomfort or pain in your lower back than you would “feel” it when you were going to sleep at night. Again all of this will depend on how sensitive your body is to changes in a mattress.
The Spa Sensations is a completely different design using completely different materials and even if you did know the firmness of all the layers (which they don’t provide so its a moot point) … firmness ratings don’t translate between different materials anyway so you would still be in the dark about how this would compare to a mattress that uses completely different materials. There are just too many differences and unknowns between the Spa Sensations and what you are building to be able to use it as a reference point. You can see more about comparing ILD/IFD numbers between different materials (if you even have them in the first place) in post #6 here. ILD’s with memory foam are mostly meaningless when they are compared to other materials because ILD testing produces different results with slow response materials compared to fast response materials and they also change with response time, temperature sensitivity, and humidity and with how long they are compressed. Different memory foams of the same density can also have different ILD’s and softness levels.
You can read a little more about the differences between Dunlop and Talalay ILD’s in post #6 here. They also have a different compression curve (see post #49 here). In some cases Dunlop can feel softer (at compressions less than 25%) and in some cases it can feel firmer (at compressions more than 25%) so for the most part in layers that are generally compressed more than 25% Dunlop will feel firmer for most people. I also think that Talalay that is one increment firmer (about 4 ILD or so) would be closer to Dunlop in most applications but how this will play out in real life also depends on the specifics of the design and how much each layer is compressed. You can see some of the complexities involved and why the “feel” and performance of a mattress can be so difficult to predict based on “rough math” or “theory” in posts such as post #26 here and inpost #2 here.
Again … all of this is so complex and will vary so much between different people, body types, sleeping positions that it’s almost impossible to accurately predict how you will feel when you are comparing two different designs that have more than relatively small differences between them and this is just an unavoidable part of the trial and error, risk, learning curve, and hopefully rewards of the DIY process.
Phoenix