Hi howardsbabygirl,
This would depend on knowing more about the details of the foam. Once they’ve provided this to you I’d be happy to share any thoughts I have.
Not necessarily. CertiPur is only one of the criteria I use for memory foam mattresses (see post #10 here). It tests for the relative “safety” of the foams. They mainly test for VOC’s and certain harmful chemicals in the mattress although they do have some minimal testing for durability as well. In terms of quality though … foam density is the most important spec … not CertiPur certification.
You can read about the main benefits of using thicker layers and thicker mattresses in post #10 here. An inch of difference though is too small to “quantify” in most cases and the other differences in the foam (such as firmness, density, or type) would probably make a bigger difference than an extra inch of thickness. With your lighter weights though … the odds are greater that a thinner mattress would be OK and have less risk of feeling like it is getting too firm too quickly. Thinner layers are actually more supportive in the sense of holding up your heavier hips/pelvis but they also run the risk of getting firm too quickly and not being as adaptable or comfortable with different weights and sleeping positions.
A 4" comfort layer in the “correct” density or ILD (or type of memory foam) could also work well (or even a 2" layer of 4 lb memory foam over a 2" layer of 5 lb memory foam (such as in the Tempur Cloud Supreme) but the problem is there is no way to really know for sure what is “correct” for you without testing the mattress or sleeping on it if you make a purchase without testing. I would think you are probably in the range of 3-4" depending on the type of memory foam and base layers used. I would also keep in mind that 5 lb memory foam will usually tend to feel firmer than 4 lb memory foam so an extra inch may be desirable.
For example … one manufacturer may make a mattress using 3" of 4 lb memory foam over a 5" layer of base foam with a certain cover that they would normally suggest for a certain group of people while another may make a very similar looking mattress that uses 3" a different 4 lb memory foam with a different type and firmness of base foam with a different cover and they may suggest this mattress for a different group of people (although with layering this similar the odds are better that at least the two groups would “overlap”).
Because of all the many variables of different types of foam or materials, there is no formula that can be used to know for sure the mattress layering that would work best for you or to do anything more than make approximations. If you are considering an online purchase … then the knowledge and experience of the online manufacturer that you are working with can be one of the most important parts of the purchase because they will have specific knowledge of the mattresses they make and the materials they use and have many customers with different body types and sleeping positions that they can use as a reference point so they will usually be able to use “averages” to help fit you to the model that has the best chance of matching your needs and preferences. When you are buying online … the knowledge and experience of the manufacturer or merchant is one of the most important parts of a mattress purchase.
It also may be worth keeping in mind that because they are so commonly available … many manufactures will have a fairly good idea of which of the Tempurpedic models will most closely approximate the mattresses they make. In these cases … you can use the Tempurpedic line to get a sense of what some of their mattresses may feel like.
Some of the better online memory foam sources I know of are listed in post #12 here.
Phoenix