The mattress was previously on a european posture slat base but we have since switched to a fixed wooden slat upholstered ensemble base (split king). The overall feel is now completely different, and although I prefer firm versus my partner who likes soft, we can feel the stiffness of the base which I worry will increase over time.
I was wondering whether I could apply the reverse approach to a mattress topper. For example, adding a layer of latex padding on top of the base to go under our mattress. If this is a feasible solution, I wanted your advice on the height and firmness that layer should be, taking into consideration compression etc.
Like any “active” component in a sleeping system that responds to weight and pressure, the flex in a flexible slat support system under a mattress can certainly change the feel and performance of the mattress and affect whether a specific mattress would be a suitable choice in terms of comfort, firmness, and PPP (posture and alignment, Pressure relief, and your own Personal preferences).
“How much” of an effect it will have and how much you will feel the effect will depend on the flexibility, stiffness, and response of the the slats and on the specifics of the mattress and it’s thickness (you may feel the flex of the slats less with thicker mattresses than with thinner mattresses) and on your weight and sleeping positions. It may be beneficial for some people, detrimental for others, or neutral for others yet but if the slats are flexing when you are sleeping then they can certainly change the feel and performance of a mattress compared to using a rigid and non flexing support system underneath it.
While the difference may be noticeable … now that you have made the change there would be no reason for the mattress to continue to get firmer.
It would be difficult to try and duplicate the amount of flex and the stiffness of the slats with a layer of latex and it may take a fair bit of trial and error in testing different latex layers to find the firmness and thickness that would most closely approximate the slats that you were using previously.
If you just need some additional softness and pressure relief in your sleeping system though then the risk would probably be lower and your chances of success would probably be better if you added a softer topper to your mattress although it would also have a different “feel” than the slat system or even an additional layer of latex under your mattress. Deeper layers will contribute more to the primary contouring support of the mattress that can help keep your spine in good alignment although it can certainly “feel” somewhat softer as well while upper layers will contribute more to the “comfort” and pressure relief of the mattress.
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress and it’s still in good condition (which would likely be the case with your mattress) then a good quality topper can be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination (or any change to your sleeping system) is a good “match” for you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the combination. If you can’t test the mattress/topper combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice for any specific person on any specific mattress.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
Because of the uncertainty involved with purchasing a topper where you can’t test the combination in person … a good exchange/return policy can also reduce the risk of an online topper purchase so I would also make sure you are comfortable with the options you have available after a purchase to exchange or return the topper and any costs involved just in case a topper you choose also doesn’t turn out as well as you hoped for.