[yobbo]

The problem is air escaping from the hot side through "cracks" or holes and causing condensation as it reaches the cold side.

In nordic standards, there must be an air-column between the insulation and the outer "cold" layer, to ventilate out any moisture that might get trapped there. There must also be a moisture barrier between the "hot" side and the layer of insulation, typically a PE-sheet, to prevent air from leaking into the insulation.

Since the wood expands/contracts with changes in temperature and humidity, filling compartments in wood constructions with foam does not guarantee air tight barriers.