WildIris, the problem you’re having with your DIY mattress is the same issue most of us are struggling with as well and that is how to best balance spinal alignment and comfort at the same time. If you use single sheets of (latex/poly/memory/etc) foam that go across the entire bed then ultimately you will likely end up trying to reach an acceptable compromise between the two. In order to actually get the best possible outcome you would need to effectively tailor each area on the mattress it to your specific height/weight/shape/feel/position/etc. I know you are asking about just adding/subratcing a layer or making one softer/firmer but most people don’t really stop to think about what’s actually happening in their design to make this work well for them. If you are a side sleeper then stand straight up and have someone measure you across the shoulders, waist (smallest part) and hips. Now take all these measurements and divide by two. Typically the difference between the shoulders/hips and your waist will be quite distinct and this is what you are asking your mattress to account for and support while not providing too much pressure on the other areas which creates pressure discomfort. It’s also why sleeping position matters because if you are a side, back AND stomach sleeper this causes alot of issues trying to get the mattress to respond well to all those different situations. This is why folks that are lighter and less curvy find the exercise of looking for a bed far less challenging than folks that are heavy like me with wide shoulders, big legs/bum with a relatively smaller waist. This is also why we talk about BMI (which is kg/m2 or even easier I just use lbs/inch height so the conversion is easy to compare) when looking at mattresses. The one issue with applying this idea of BMI or lb/inch across the entire mattress is that the distribution of people’s weight is not linear so for instance the hips/trunk likely carry alot more of the weight per inch than say your feet or head and so the foam you use needs to account for this difference too. Obviously people aren’t shaped like cylinders either so we need to account for these differences when designing a mattress that works for us.
Ideally you are trying to get something close to this in terms of spinal alignment (hers is not perfect but likely quite comfortable, minus the pillow)
The reality is that most of the time you end up somewhere between these two since some areas of your body carry more weight (per inch of height) than others.
So bottom line, if you use springs as a base layer and you lie on those springs with no additional comfort layers then those springs will cause you to have a certain spinal alignment (likely not correct because otherwise you could just put a soft latex layer on top and be done with it). What you are asking the comfort layers to do is to make up the difference to create the perfect alignment you need and provide pressure relief (no small task). When dealing with an all latex mattress, then all those layers are working together to provide both alignment and pressure relief. It’s also helpful for it to be progressive (meaning it gets firmer the deeper you push into the mattress) so that’s why you will often see the firmer latex layers at the bottom and the softer ones toward the top. Now I’m not sure how curvy you are but you have to take into account the difference between the shoulder and waist at a minimum to create the support you need then add additional comfort layers as needed to give you pressure relief to keep those protruding parts (shoulders/hips) from getting sore while sleeping. You may be able to achieve this with just changing out layers for firm or soft and stacking (increasing overall thickness) will help with both alignment and comfort but it also adds to the number of possible combinations involved (which can be expensive and frustrating). As you build the mattress up layer by layer I would be checking your spine alignment at each level so you know if adding the last layer made things better or worse.
Another way to potentially solve this alignment issue with less layers is to use zoning where you put different firmness in different areas depending on what support/relief characteristics you need specifically. This can be combined with a flexible slat base to help take up some of the conforming you need for proper alignment. I wish there were more vendors that sold latex layers that were precut into strips to make it easier for us DIY folks to try multi-zoning within layers but I don’t know of any offhand (maybe Sensei or Phoenix do?)
Multi zoning of layers like this design (Flobeds) can help achieve both alignment and comfort (top comfort layer is missing in this photo)
You can see the design can get complicated on how wide the sections are and what firmness to use in each
Good thread on how to create zoning if you’re interested.
https://forum.mattressunderground.com/t/zoning-measurements
Anyway there are plenty of options to get a solution that works for you but it all requires patience and perseverance to make it happen. If it were me I would start with trying to get it perfect with single layers first but don’t dismiss the idea you may need to move on to more elaborate designs if none of the simpler options work for you. I know it’s alot to take in but hopefully it will help you figure out what direction best to go from here in terms of strategy and layer choice for your specific situation.