So where reflectix is a bonus, why not use just the attic reflective on top of the foam, cheaper, thinner. Does it's job without trying to do the foam's. Save the expensive reflectix for where I can't use the other stuff.
I am thinking 2 layers foam, 2 reflective, same as you basically, just in thinner layers and without the bubbles. total thickness 1/2-3/4"
I wonder if it would be more efficient to have a layer of reflective on the inside, since it would reflect the energy before it warms the insulation and turns to conducted energy instead.
The attic reflective is probably as effective as Reflectix as long as it is used with foam. As I posted a while ago Reflectix has increased in price since last year - I paid about 89 for the largest roll at Home Depot.
The foam I have and will use is Ethafoam in 1/2" thickness. If you are using 2 layers to get the thickness why not use a thicker foam - it is easier to apply.
I don't think you would gain anything by adding foil on the inside. I would make it the first layer.
A friend lives on a Spencer 35 that was insulated with 1/2" Ethafoam over 20 years ago. It is the warmest boat I have been on in cold weather. It has held up well. The only problem is that it can get dirty and in his case the cat has scratched it in a few places. I think the best solution (if you have a cat or not
) is to line the insulation with something that can be wiped down easily.
My V-berth area has vertical beams glassed in 16" apart so the easy solution is to insulate between them to their 1" thickness and then line with something. I will probably use varnished wood strips here. In the lockers I plan to epoxy in strips the same thickness as the insulation and line with something like the plastic sheets that are about 1/16" to 1/8" thick and easy to keep clean.
Below the waterline I will not insulate I don't think. Under berths and seats one good solution is plastic carpet runner that has the protrusions on the back to grip a carpet. These will keep it off the hull and allow some air flow.