Boat Forum / Building / March 2006

Just to the waterline is absolutely standard.  You can't insulate
against the cold from the ocean.  But you can insulate under
floorboards with rigid foam too, to keep the cold at bay.

Evan Gatehouse

Insulation always makes sense whenever a temperature differential is
present.

Going below waterline(with the insulation) is beneficial unless other
concerns are present(bilge water will be absorbed by styrofoam ,even
Urethane foam eventually if submersed for long durations,or mechanical
access etc).

The type/properties/dimensions of fluid (Air inside/water outside),
materials in crossection, and condition of surface movements can be
calculated into a transfer number which has a profound influence on
heattransfer (in or out).
Insulation or not can make a significant difference.

Of course there is the added expense, but if you cruise in waters temp
beyond of 68 to 74 F or so insulation will make boat more comfy.

You will loose some of the dehumidification effect as air moisture is being
condensed at cooler below waterline surfaces and but have less condensate in
the bilge.
Good ventilation and dehumidifier is a plus to have.

ETAP is a French boat manufacturer where foam is sandwiched between outer
and inner hull.