Hi mw913,
[quote]I am wondering which of these two components will be the first to wear out.
I am still curious, in general, if the HD core or latex top is more likely to wear out first. I am 5’9" 180 [/quote]
There is more about the many variables that can affect the durability and useful life of a mattress “as a whole” in post #4 here and the posts it links to.
The latex would be a more durable material than polyfoam if you were sleeping directly on both materials but once you put one on top of the other in a mattress then there are too many variables involved to know how much each one will soften or break down relative to the other one because it will depend on the specifics of each layer, the thickness of each layer, the firmness of each layer, the body type and sleeping style of the person on the mattress, and how sensitive you may be to the foam softening that leads to the loss of comfort and support and the need to replace a mattress. The upper layers of a mattress absorb more of the compression forces of sleeping will tend to soften or break down materials more quickly than the deeper layers but if the deeper layers are also a less durable material then that will also lead to more rapid softening and breakdown than a higher quality and more durable material as well even if it is being compressed less. In a perfect world the durability of both layers will be “balanced”.
In most cases neither layer will be “worn out” completely and will both contribute to different degrees to the loss of comfort/support that leads to the need to replace a mattress but how much each layer will contribute individually for any specific person isn’t really possible to predict with any accuracy.
It’s often the case that a DIY mattress won’t save much money (if it saves you anything at all) vs a good quality/value “finished” mattress that would be an “apples to apples” comparison. If you are attracted to the idea of designing and building your own 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).
While there is no way to quantify how long any mattress will last for any particular person because there are too many unknowns and variables involved that are unique to each person, if a mattress is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new and meets the minimum quality specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer.
You can see my thoughts about buying a mattress/topper combination in post #2 here. If you decide to go in this direction then I would make sure that there isn’t too much lower quality/density soft foam or other materials in your base mattress which could significantly shorten the life of the combination.
Once your mattress needs to be replaced then if the latex is still in good condition you would still have the option of mattress surgery where you could replace the polyfoam support core and add a new cover (see post #2 here)
Some of the better lower budget latex and latex hybrid options I’m aware of are listed in posts #3 and #4 here.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful quality/value comparisons between mattresses but PPP (Posture and alignment, Pressure relief, and Personal preferences) is always the most important followed by the durability and useful life of the mattress followed by all the other criteria that are the most important parts of your personal value equation.
Phoenix