Foam Quality & Density
Foam Come in a variety of qualities and densities. By quality, we are generally referring to the density. Density means, how dense is the foam = How much foam material is contained in a given amount of space. For practical purposes, the density of foam is measured by weight. For example, a low quality foam might weigh approximately one pound a specific size, while a high quality foam for upholstery might weight about 2 1/2 to 3 pounds for the same size.
In our shop we don't stock polyfoam, but we do have some foam samples in the various qualities and softnesses. On the sides of the samples, as shown the picture, the grade numbers are marked. The grade number consists of two specifications. the first two digits denote the density. The second two numbers denote firmness. For example, a specific size of the 2535 foam will weigh 2.5 pounds and will have a compression ratio of 35 pounds. (It takes 35 pounds to compress that foam a specific amount.)
A question was asked recently, on another board, about how to create a crown (or crest) on a flat foam cushion. There are probably many ways to do this, here is one way.
Now, about getting a "crown" on the cushions.
Making a little bit of a crown:
After cutting your foam to size, you can just wrap or glue about 1" bonded dacron wrap onto each side of the foam may give you the look you want. It gives a little bit of a crown.
When this is stuffed into the cushion, it will give a rounded effect. (which I use for most of my jobs.)
Getting more of a Crown:
If you want more of a crown than that (see drawing below), to give a higher crown on a foam cushion (see A in Drawing), you can start by cutting a piece of 1/2" foam about 2/3 the size of the foam (B in drawing). Be sure to taper (cut at a slant) the edges of the dacron. Then glue it to the center of the top and bottom of the foam. Then, for the layer of Dacron (C in Drawing), cut 2 pieces the same size as the foam and glue one on each side of the foam, on top of the smaller pieces of Dacron.
Then, as papasage said (on that other board), you can cut the cushion side boxing narrower. For instance, if you are using 5 inch foam, you can also cut the boxing about 5 " wide also. After it is sewn, the boxing will be 4" wide (It will cause the top and bottom of the cushion to come down over the side a little, which, when combined with the dacron wrap, will increase the "appearance" of a higher crown.)
I hope that helps.