Foam is sold by Density
(determines the general quality), and Compression
(from soft to hard), stated in pounds per cubic foot.
I recommend a 2.1 - 2.25 pound density, with a 34 - 35 pound compression
mildew-resistant foam for bunks/berths & inside seat foams. The cheapest foam is only 1.5-pound density.
For sleeping, you want a minimum foam thickness of 4" (10cm), or even more, if you have the berth height. Where headroom
is limited, use a better quality 3" (7.6cm) foam with higher compression
Settee Backrests normally have the same foam as the bunks or seats; but a super-soft foam could be an option (as long as it doesn't comprise part of a pull-out convertible bunk mattress).
For seating you should get the thickest foam that doesn’t raise your bum up too high (feet comfortably on floor) - best test & measure, rather than assume.
Upholstery foam is often cut with a serrated double bladed saw
, rather than a hot wire knife (which is more often used on rigid foams, like Styrofoam). An electric kitchen carving knife
can function as an upholstery saw. A razor utility knife, with the blade extended, is adequate for smaller jobs.
Compress the top of the foam, with a scrap of plywood
(whatever), when cutting.