Hi bghouse.
[QUOTE] I’m a stomach sleeper - but I hate sinking into the mattress. I like it firm with then a softer layer on top.
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There is no doubt that being a stomach sleeper presents some challenges. Your first priority when sleeping prone is to make sure to avoid hyperextension and the swayback position that can cause lower back issues if the mattress is either too soft/thick comfort layer or too soft support layer. In the general stomach, sleepers need the thinnest comfort layer, (one inch or at the most two) and side sleepers need the thickest comfort layer. It is rare that anyone … even a side sleeper … would need more than 4" in a comfort layer.
It looks like you are planning to get an online mattress purchase shipped at a particular USA location and then cross the border into Mexico. There are tens of thousands of BIB mattresses available on line that fit this criterion (just too many for anyone to be able to track) which is why I suggest that you narrow things down a bit based on your personal value equation and what is important to you.
Both innersprings and a firmer latex or polyfoam core can be used as a support layer and each has very “different” characteristics but the most important differences are the ones you can feel and that you personally prefer. Both can be softer or firmer depending on design so a pocket coil could be firmer than a latex core or the other way around they could be zoned or not all depending on the specifics of the components you are comparing. There is more about this in post #10 here and more detailed information about innersprings vs latex support cores in post #2 here The choice between an innerspring support core and a good quality polyfoam support core is really a matter of personal preference and not a “better worse” choice. The weak link of a mattress is generally in the comfort layers above the innerspring and not in the innerspring itself. You can read more about foam base vs innerspring base in post #28 here
Innersprings absorb less energy than a polyfoam support core, which means they are more resilient. This means that an innerspring mattress will be more “springy” than polyfoams and those who have tried both will validate that the feel between them and how they each react to motion is very different. Some prefer one while others prefer the other. Both can make high-quality support layers.
There is also a property called “point elasticity” which is the ability of a material to compress without affecting the areas beside it. Because of its elasticity … there are millions of points that can compress individually and which only affect small areas around it. Polyfoam will not be as point elastic as latex. Innersprings on the other hand have less “point elasticity” than both of those items. Their ability to take on a body shape and isolate motion is more limited to the number of coils and how the coils are connected to each other. Pocket coils with a high coil count would be the most “point elastic” of the innerspring types.
An innerspring unit will tend to be more durable than a high-density polyfoam support core, but neither is usually the weak link within a mattress.
All of this of course can be modified by the layers both above and below a particular component or material.
There are so many differences between them that it is really impossible to answer these types of “apples to oranges” questions in any other way than through very general comparisons that may not represent the differences between two specific support materials. There are those whose “ideal” mattress is latex over innersprings for example and not only that, it may be a particular type of innerspring. There are others who will only sleep on latex using a polyfoam support core. Others prefer all latex.
When you mentioned the “inherited Zinus” RV mattress … is this to be used as a reference to approximate the mattress in terms of thickness, components, and specs?
If so, I’d place a caution in terms of the durability and useful life of this mattress (see the mattress durability guidelines here ). Personally, I am not a fan of visco-elastic foams in RV use because of the wide swings in temperature and humidity and this can cause premature wear and even permanent damage to memory foam, especially if they’re exposed to a below-freezing environment and then used before attaining room temperature. Add to this moisture or condensation that may become trapped in your mattress which would encourage the growth of dust mites, mold, and mildew.
This said, there are quite a few boxed bed mattresses that only contain polyurethane foam (or even a little latex). Depending on your BMI range you’d generally want to choose their firmer options for stomach sleeping. Also as a stomach sleeper (again depending on your BMI) you don’t need that much thickness to get both the support and comfort that you need.
Another alternative would be to use a lower profile spring unit, and then combine this with as much comfort material in a higher density or hardness that you desire and can be rolled packed for shipping. Many manufacturers produce thinner innerspring mattresses, but most of them do tend to be quite low quality so I’d make sure to find out the mattress information listed here so you can compare the quality of the materials and components to the mattress durability guidelines here
Since you liked your 6" firm Dunlop mattress but the overall weight is prohibitive you could also look into a latex polyfoam combo, with a firmer feel.
You certainly can find options, both innerspring and foam, that will be able to support you in the thickness you desire, but it will involve a little bit of online searching or visiting of stores in your area as there are far too many Boxed beds and innerspring variety to list.
Hopefully, that helps you out a bit.
Phoenix