I weigh 300 lbs.I have a very comfortable mattress. I have 2 latex toppers soft 20 ild 7 inches total. My core is 4 inch poly foam 2.6 density 35 ild. will this poly foam core soften with use. It feels great now. I was thinking about adding another 3 inches of 2.6 density 35 ild poly foam to my core. Does poly foam soften and for my weight is 7 inch core better than 4inch core.
Hi Sleepy103114,
A mattress or “sleeping system” will generally tend to soften and break down from the top layers down because ongoing foam compression and deflection is the primary reason that foam materials will tend to soften and break down over time. In an apples to apples comparison between the same types of foam … softer foam layers will compress more than firmer foam layers and layers that are closer to the top of the mattress will tend to compress more than layers that are deeper in the mattress so any softer foam layers or layers that are closer to the top of your mattress will generally be less durable than the same type of foam in firmer versions or that are deeper in the mattress. In other words it would probably soften to “some” degree over time but how much it would soften and how much any softening would affect you in terms of PPP (Posture and alignment, Pressure relief, and your own Personal preferences) will depend on how much the polyfoam core is compressing under your weight “through” the layers that you have on top of it and where you are inside the range between “I can sleep on anything” and “princess and the pea”.
The thickness of a mattress or any individual layers inside it is really just a side effect of the design and the design goals of a mattress and is also only one of many variables that can affect the feel and performance of a mattress relative to any particular person and by itself isn’t particularly meaningful (see post #2 here). In some cases higher weight ranges will sometimes do better with a mattress that is thicker than lower weight ranges (see post #14 here for more about the effect of thickness) but even this depends more on the specific design and combination of materials in the mattress and on how well your testing or personal experience indicates the mattress “as a whole” matches your specific needs and preferences in terms of PPP than anything else.
2.6 lb polyfoam is a good quality and durable material in any thickness but the “bottom line” is that the only way to know whether adding another layer to your sleeping system will be beneficial, detrimental, or neutral or will be a better “match” for you in terms of PPP will be based on your own personal experience when you sleep on the mattress because there are too many unknowns, variables, and personal preferences involved to be able to predict this based on specs (either yours or a mattress) or “theory at a distance”.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be helpful as well.
If you are sleeping well on your mattress now then I personally wouldn’t argue with success and would hesitate to make any additional changes unless you have a specific reason to do so. If you are adding additional foam layers to your mattress then you would also need to use a thicker mattress cover as well so if you have already purchased a mattress cover you would need to replace it with one that is thicker.
Phoenix
Thanks phoenix I am learning a lot. I automatically assumed that for my weight I needed a at least a 6 inch poly foam core because when I see foam mattresses on the Internet most of them have at least a 6inch core for support especially those advertised for my weight class. I have just learned that there a lot of other factors involved. The quality of my latex toppers and the quality of my polyurethane foam as well as my personal preference. You are right diy mattress is an adventure. I lost 200 dollars already because I bought a 50 ild poly foam core. It was way too firm. I gave it away. I learned that I like a medium firm mattress. 35 ild poly foam core is for me. I feel it is worth it because I am not going back to coil mattresses ever again. I woke up with painful ribs for the last 5 years. I am just going to go with a 4inch poly foam core for now. It feels great no pressure points. It is like sleeping on a soft but supportive cushion. It is a great feeling. 2.8 density foam is really good foam maybe that is all I need. Time will tell. Thanks for the beducation.I got a feeling I will be needing your advice again in the future.
Hi Sleepy103114,
You only had 1 “mistake” which isn’t bad at all … and congratulations on a successful DIY mattress and finding the combination of layers that works well for you 
I’m looking forward to your comments and feedback or any other questions you may have down the road.
Phoenix