Diffusion through a material is a different process from air flowing through a gap beside a window or along a ventilation duct.

Diffusion matters when assessing moisture within the wall. Rooms need controlled ventilation to remove moisture and other pollutants generated during use. Even a material with very low vapour resistance cannot provide that air exchange. An airtight layer is compatible with a reliable supply of outdoor air through the intended openings or ventilation system.

Showering, cooking, drying laundry and even breathing release water vapour indoors. In a new or refurbished house, wet building processes can add construction moisture. The total varies throughout the day and with the occupants’ routines. A moisture estimate for one household may be quite different from another’s.

A roof leak, a leaking pipe or moisture rising from the ground can also cause damp patches. A fan cannot repair faulty waterproofing. If a patch is confined to one spot or grows after rain, investigate its source. Record where and when it appears and under what conditions. This helps identify which parts of the building need inspection.

Relative humidity depends on air temperature. As air cools at the same moisture content, its relative humidity rises. Water can condense where the surface temperature reaches the dew point. A cold corner or window reveal is therefore often more vulnerable than the middle of a well-insulated wall. Surface water alone does not identify the insulation material as the cause.

A thermal bridge, a gap in the insulation or an air leak can create a cold spot. Furniture pushed against a cold wall can keep warm room air from reaching it. Assess the detail and the conditions around it. Painting over the patch leaves the underlying cause in place.

An airtight envelope limits uncontrolled airflow through the building fabric. It also prevents warm, moisture-laden air from carrying water into colder layers. Continue the airtight layer across joints and around service penetrations. Two layers with gaps do not necessarily form a continuous air barrier between them.

Provide routes for incoming air, transfer between rooms and extraction. Natural stack ventilation and mechanical ventilation work on different principles. An extract grille needs an adequate supply of replacement air. Heat recovery can reduce ventilation heat losses if the system is correctly sized, balanced and maintained.

Insulation materials differ in thermal conductivity, moisture behaviour, fire performance and mechanical properties. PIR can achieve the required thermal resistance at a smaller thickness than many conventional insulation materials. Check the particular board and its facing. A facing with high vapour resistance affects the directions in which the wall can dry.

Mineral wool, EPS and XPS each need a suitable build-up too. A single property is a poor basis for ranking them. The design must account for every layer, driving rain, construction moisture and how the rooms will be used. Internal insulation and moist components enclosed between layers with low vapour permeability need particular care.

Record temperature and relative humidity at several times of day. Check whether the issue follows a shower, indoor laundry drying or a change in the weather. Inspect air inlets and grilles and check filters as directed in the system manual. A running fan can be heard even when it moves too little air. An airflow measurement may be needed.

If damp persists, have the affected building fabric, services and ventilation inspected. Match the repair to the cause, whether it is a leak, a cold junction, insufficient ventilation or residual construction moisture. Insulation, airtightness and ventilation can be designed together to give you warm walls and fresh indoor air.