Membrain technical note
Condensation, and how it differs from ingress
A practical way to distinguish surface condensation from water travelling through a wall, slab, joint or opening.
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The mechanism
Condensation forms when moist indoor air meets a surface cold enough for water to deposit on it. The moisture was already inside the room; it did not necessarily enter through the building envelope.
Typical pattern
- Diffuse, shallow surface dampness without a strong directional gradient.
- Concentration in corners, behind furniture, on reveals and other colder surfaces.
- Black fungal spotting as an early sign.
- Improvement with ventilation or changes in season and room use.
- Droplets on glass, tile or metal.
What argues against it
- A strong top-down or bottom-up gradient.
- Staining that follows a joint, opening or service route.
- Saturated material, active dripping or substantial plaster delamination.
- A clear relationship with rainfall.
A useful discriminator
Water that travelled from somewhere usually leaves a direction or route. Condensation follows temperature and ventilation patterns instead. Efflorescence also points toward water moving through masonry rather than surface condensation.
Limit
Condensation is a judgement about the whole room, not one close-up. It should be supported or excluded using room context, season, ventilation and pattern rather than used as a fallback explanation.
Technical boundary
This note explains diagnostic reasoning. It does not specify a product, quantity or application method. Repair systems should be selected against the confirmed site condition and the current approved manufacturer documentation.
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