Hi ucaggie,
Your Sealy Posturepedic Moran Firm as you said has a total of 1.5" of firmer Sealy foam and 1" of supersoft foam in the comfort layer which in combination with the innerspring creates a firm mattress. It does however already have some foam in the comfort layers so this needs to be taken into account in the thickness choice of your topper. How thick this should be will depend to some degree on how firm the mattress comfort layers feel to you in terms of pressure relief. Does it need just a “little bit” more to relieve pressure points on your side or is it clearly too firm for your side sleeping. Part of the difficulty with this is that Sealy doesn’t provide the details of the foams they use in the comfort layers.
Having said alll that … I would tend to go with a topper that was no more than 2" thick. More than that and IMO you may be risking good alignment on your back. In general, the closer you are to the support layer the better the alignment will be and a 2" topper would give you a total of 4.5" of foam over the innerspring.
As far as what type … this would depend on your preferences and the types of material you prefer.
Your most common choices will be either slow recovery viscoelastic foam (memory foam) or quick recovery elastic foam (polyfoam and latex). Regardless of which material you prefer, I would use the highest quality you could because the top layers of a mattress or a topper will wear more quickly than the lower layers and higher quality materials not only perform better but last longer. There is some information here about the pros and cons of memory foam and here about the pros and cons of latex.
If you prefer the feel of memory foam … I would choose a 5lb or greater foam for durability or at the very least a 4 lb form if you prefer the slightly more breathable and quicker response that is more often found in this density range and are willing to give up some durability for your preferred feel.
If you prefer the more lively and instant reacting feel of latex (easier movement on the mattress) and it’s greater breathability, then I would choose either blended Talalay latex or 100% natural dunlop latex. Both are high quality products but Dunlop is a firmer feeling and less lively version of latex. Talalay is also more consistent in firmness across the surface of the topper and has a wider range of firmness levels available than talalay.
A “typical” choice in a talalay topper is in the range of 19-24 ILD with lighter people sometimes going even softer at 14 ILD while heavier people or those who prefer a firmer surface feel (or who don’t sleep on their sides) may go higher in the range of 28-32 ILD. Dunlop is typically in the mid 20’s but will have a greater variance across the surface. There are some types of Dunlop which are rated even lower in terms of ILD but this is somewhat misleading because of how ILD is tested and because Dunlop gets firmer faster as it compresses than Talalay and the ratings are only taken at 25% compression. Softer latex foam is very point elastic meaning it can take on the shape of the body very well which means like memory foam it is very pressure relieving but it has a more “on” the mattress feel rather than the more “in” the mattress feel like memory foam.
High quality polyfoam is much more difficult to find and is more often available at a foam shop or specialized outlets online. It is lower cost than latex but somewhat similar in feel (instant reacting) and if the density is 2.5 lbs or higher, it has a compression modulus of 2.4 or higher, and has a resiliency of 60% or more … then it can accurately be called HR foam and would be very high quality and durable and somewhat similar to the feel of latex. Most polyfoam that is sold is much lower quality, far less durable, and of course much cheaper than higher quality polyfoam. While these can also work well, they will not last nearly as long.
So overall I would choose the thinnest and firmest topper that adequately provided the pressure relief you needed with either the fast or slow recovery feeling and other characteristics you prefer.
Phoenix