: Okay, I get it, but if not minicell then what foam did you use?
: Make sure it is closed cell or it will absorb water. Nice seat
: back and thigh braces. Will they be padded later on?

It is definitely closed cell foam. I think this particular example is sold as a yoga mat or camping mattress.

I have used similar foam in the past and it shows no sign of problems after seven years. I have always found laminating layers about an inch thick is cheaper than using a single thick piece. In this case it was way under one third the price. Several times in the past I have been able to get offcuts, which also helped keep the price down.

I usually pad the backrest, but after a couple of hours paddling yesterday, I felt no need to add any padding, because it is the right shape. However that may be partly because I spent years paddling TK1s and canoes with no backrest, so I don't rely on it much. As I built this kayak for my daughter, I may have to relent and add padding to the backrest.