John, because you have an inner and outer GRP moulding with foam in between you have a great insulation against the cold and condensation forming.
Now in the winter, Spring and Autumn a single
forms condensation on the inside if there is a temperature difference, particularly in the UK. Just like the kitchen windows at home steam up when the wifey is cooking
and its cold outside. This condensation forms from people inside the yacht, cooking
because gas cookers
give off water vapour and general damp from living in a soggy country. The answer on a yacht is ventilation, then more ventilation and did I say ventilation. However, how do you heat a yacht and ventilate at the same time?
So back to the Sadler, the two GRP mouldings with foam in between reduce this right down. You will still get condensation on hatches etc. If it has heating
installed then that will be so much more efficient too. Down side is a small loss of interior
spaces and a risk of water being trapped inside through damage or leaking underwater fitting, hence survey
Final advantage is a stiffer, stronger and presumably lighter hull
Our Moody has an inner moulding too (but no foam) which does help stop the warm air inside from reaching the inside of the cold outer moulding and then forming condensation. We also have lots of ventilation so keeps bedding, matresses clothes etc dry and reduces the risk of mildew and the yachtie smell that follows in a damp boat.
But, if water does get in to the foam you have a problem getting the foam out which is one reason why it isn't used now, plus the extra cost of course.