Insulation materials open to diffusion (e.g. those based on minerals or plants) make sense here, and for plasterwork, materials open to diffusion such as clay or lime plaster. The point is to avoid moisture collecting and mould developing within the insulation, since moisture can escape.

If materials not open to diffusion are used, an airtight vapour barrier has to be installed on the inside.

A vapour barrier is a foil with a defined resistance to water vapour diffusion, which prevents the moisture content of indoor air from penetrating thermal insulation in a building.

The airtight layer must be inside the interior insulation.

Because, in contrast to external insulation, the external wall is not heated from the inside, the dewpoint is shifted further inwards and is now located between warm insulation layer and cold external wall. At this dewpoint there is a risk of moisture condensing and mould developing in consequence. That is why, where interior insulation is employed, an experienced specialist should be consulted about the physics of the building.

Among other things, air flow around the insulation must not be possible; instead, the insulation materials must adhere to the entire surface of the external wall. Above all, junctions between intersecting ceilings, interior walls etc. must be handled appropriately. Intersections must be planned in detail and implemented accurately! Wall intersections and reinforced-concrete ceilings should be provided with flanking insulation, e.g. with insulation wedges, in order to reduce thermal bridging towards the external wall.

As an additional measure it may help to supply warmth from within to the room along the full length of the internal insulation, so that the temperature of vulnerable areas is raised somewhat.

It also makes sense to keep indoor humidity at a low level with fan-based ventilation systems.

With timber beam ceilings it is necessary to seal the beam ends within the insulation layer airtight, so as to avoid outward convection of moist air.

If the basement ceiling receives additional insulation on top (instead of underneath – the usual arrangement), interior insulation and basement ceiling insulation form a continuous layer.

Basically, convection is the main threat to interior insulation as regards structural damage, because if warm, moist indoor air passes through the structure and cools down on the way out, water vapour condenses out.