Hi mneal,
There are a couple of suggestions I can make that could give you some insights into how to make the best possible topper choice.
The first of these is that if your mattress is still in good condition and has a reasonably flat and evenly supportive surface (no significant impressions or soft spots) then a topper can work very well to improve pressure relief and what I call “secondary support” (the type of support that fills in the gaps in a sleeping profile and is connected to the comfort layers rather than the support layers of a mattress). If the condition or design of the mattress itself is part of the problem (other than comfort layers that are too thin or firm) and has either significant impressions or soft spots under certain areas of the body or if the comfort layers of the mattress are already too thick and soft … then a topper will only follow the dips and soft spots or will be even thicker and softer and will provide only partial or temporary benefits at best and in some cases can lead to even worse problems with alignment.
The first step is choosing the material that you prefer. Generally this will be a choice between memory foam, polyfoam, natural or synthetic fibers (including featherbeds), latex, or shredded latex. Each of these have a different type of feel and performance and distribute weight differently. Regardless of the type of material though … I would make sure that the material is higher quality which will be more durable and tend to perform better than lower quality materials (especially lower density foams which will soften more quickly). This is especially important with higher weights or in conditions or a type of usage that may cause more stress and wear on the topper or cause it to soften, compress, or degrade more quickly.
I would also tend to stick with materials that you have used in the past with good success unless you have a chance to test an unfamiliar material in person or you are confident that a material you have no personal experience with will be suitable for you based on your reading and research and perhaps most important or most importantly your conversations with knowledgeable people that you can talk to in more depth on the phone and who can give you some meaningful insights into how that material may feel and perform for you compared to others. Of course the exchange or return policy of the retailer or manufacturer can also lower the risk of a topper purchase.
Since latex and memory foam are the most popular topper materials there is more about the pros and cons of each of them in post #2 here.
There is also more about wool toppers in post #8 here.
There is also more about featherbeds and fiberbeds (with down alternative fibers) in post #2 here and the posts it links to.
The 3 ways to get the most accurate possible idea of how a particular material will feel and perform … in order of importance … are to test it in person on the mattress you will be using it on (or a very similar mattress), to have more detailed conversations with knowledgeable people who make or sell that specific material and are familiar with how it compares with other materials in terms of either feel or performance, and finally online research that can give you some insights as well. Just bear in mind that a great deal of the online information is also inaccurate or misleading and only “disguised” as being neutral or factual and meant more for marketing and selling something that a particular retailer sells than it is to actually educate and inform based on fact. Different people may also have very different perceptions and preferences than someone else and what is “perfect” for one person may not be suitable at all for the next.
In your case … if a 3" memory foam topper in the range of 5 lbs on the same mattress has worked well for you in the past … then I would tend to use the same thing again unless your mattress has also developed issues that would change the effect of the same type of topper or unless you had compelling reasons to use a different material either because of changes in you or because you were never particularly happy with the feel and performance of the topper you have been using in some way and strongly believe that a different choice would be an improvement.
If you are choosing a topper that is different in some way from what you are familiar with or where you are unsure of how the mattress/topper combination will work for you because one or the other are more of an unknown, because of unfamiliarity with either, or because the mattress itself has changed since a combination worked well for you … then there are some TOPPER GUIDELINES IN POST #8 HERE that you can use along with your own experience on your mattress to choose the thickness and softness that has the best odds of working well for you.
Because there is no “formula” that applies to any specific situation then these more general or “intuitive” guidelines based on your actual experience on your mattress are generally the most effective way to choose the most suitable topper that has the highest chance of success.
Post #2 here and post #2 here would also be worth reading.
Some of the BETTER SOURCES FOR TOPPERS I’M AWARE OF ARE LISTED IN POST #4 HERE although you would need to check which of these can ship to you. If you are uncertain it would also be a good idea to check for any exchange or return policies just in case the topper you choose is firmer or softer than you would like (see POST #4 HERE FOR SOME OF THE RETURN OR EXCHANGE POLICIES I’M AWARE OF).
This would be suitable for “averages” but like all generic guidelines they may not apply to any specific person because of differences in body type, sleeping positions, or personal preferences. In addition to this … when you are choosing a topper you don’t have the choice of which support layer to choose (which can affect the type of topper that works best with it … and with you) and you already have a comfort layer in your mattress which also can’t be changed (unless you open up the mattress cover and remove and replace it) so you are adding a topper to something which is already “fixed” which is very different from choosing a mattress that can use any combination of layers and isn’t building on or “fine tuning” something else.
Finally … it’s important to remember that the best use of a topper is with a mattress where the comfort layers of the mattress are too thin or too firm but are still flat and even and don’t have significant dips or soft spots that have developed over time. While it’s fairly simple to add some surface softness and pressure relief to a mattress that is too firm but otherwise still in good condition … it’s not really effective to use a topper to change the primary support for a mattress (which comes from the deeper layers not the upper layers) or to “fix” a mattress where the comfort layers have developed dips or soft spots because the topper will just tend to sink into or “follow” the dips or soft spots and could cause other issues such as alignment problems and back issues as well because you could end up with top layers that in combination with what is already in your mattress don’t allow you to sink in evenly or are too far away from the support layers or components of the mattress. Trying to make a mattress firmer or “more supportive” with a topper is not particularly effective and will usually have either partial or temporary success or in the worst case cause more issues than it solves.
If you are absolutely forced to add a topper in circumstances where support needs to be improved or where a topper normally would be only partially effective or ineffective … then I would first read post #4 here about improving primary support or adding toppers over a mattress or layers that are too soft or have developed soft spots or are sagging and then if a topper still seems to be worth trying then I would use a thicker topper (in the range of 3"-4") that is as firm or dense as possible and was a reasonable choice for your minimum pressure relief needs (based on body type and sleeping position) because this will at least have the greatest odds of evening out the soft spots or dips in the mattress surface to the degree possible (because of the thickness) and have the lowest odds of creating additional alignment issues (because of the firmness) which may be the most likely option to make the best of a bad situation where a mattress replacement or “mattress surgery” or other ways of fixing a mattress that has lost support isn’t a possibility.
Phoenix