Why not just fit a horizontal board under the gunwale to close the gap between hull and cabin linings? What you are proposing narrows the cabin space. 40mm of sprayfoam (if it is indeed 40mm thick everywhere) should be more than enough insulation. But if you do need more, add it to the existing and leave the back of the lining boards open to the air. Condensation is prevented by insulating between the warm moist internal air and the cold steel shell.

No doubt her partner insisted on it.