Because memory foam doesnt provide any seating support in a sofa, thats why its only used as a topper; usually only the first 1 to 2 inches.

rmanbike,

Thanks for the input. Yes it is not a solid memory foam cushion. If it was solid memory foam it might be too soft? IDK I Opened up the Charles cushion and it was 7" of white foam with blue gel memory foam wrapped around the core.

The sofa I have now is considerably more comfortable, it is like a Tempurepedic sofa. I have a bad back and the few inches of memory foam helped relieve a lot of pain and discomfort. I am just wondering if I had even more memory foam it would make my back feel even better.

At this point in my life, having dealt with pain for so long I am going to try every non medical avenue I can. I have a feeling that if I was 100lbs lighter the cushion would be perfect, so i guess I might not need twice the thickness but twice the density? If only this was a foam expert forum. lol

PotatoDigger, have you tried like a gel-pad placed over the seat cushion and also memory foam pads is good at relieving pressure points.
Not knowing the source of your pain, im just wondering if you in any physical therapy for your back problems?
I find that keeping my strength up in the back and legs, along with a bit of physical exercise= pedal or elliptical bike does wonders for relieving pain and discomfort.

Here is a link that might be useful: good technical information on foam