Water reaches the wrong place in a building by several quite different physical routes. They are often muddled together as 'damp', but each behaves differently and demands a different response. A competent moisture diagnosis is essentially the work of identifying which mechanism (often more than one) is responsible before specifying any remedy.
The five mechanisms at a glance
| Mechanism | Driver | State | Typical control |
|---|---|---|---|
| Vapour diffusion | Vapour-pressure gradient | Vapour | Vapour control layer / sd-value design |
| Air convection (leakage) | Air-pressure difference (wind, stack, fans) | Vapour in moving air | Continuous air barrier |
| Capillary transport | Surface tension in pores | Liquid | Capillary breaks, vapour-open detailing |
| Gravity / bulk water | Gravity (rain, leaks, flooding) | Liquid | Weathering, flashings, drainage, DPC/DPM |
| Built-in / construction moisture | Wet trades, rain during build | Liquid/vapour | Drying-out time, monitoring |
1. Vapour diffusion
Water vapour diffuses through materials from high to low vapour pressure — typically outward in a UK winter. It is slow and distributed, governed by the materials' μ- and sd-values. It is real and matters for interstitial condensation, but it moves far less water than air leakage. Controlled by designing the vapour profile of the build-up (covered in the vapour diffusion article in this guide).
2. Air convection (air leakage)
Moist indoor air carried bodily through gaps in the envelope by pressure differences (wind, the stack effect, extract fans). This is the heavyweight: it transports vastly more moisture than diffusion — often the dominant cause of interstitial condensation — yet it is the mechanism most often overlooked. Controlled by a continuous air barrier, which is why airtightness is a moisture-safety measure as much as an energy one.
3. Capillary transport
Liquid water wicking through the fine pores of a porous material by surface tension — the same effect that draws water up a paper towel. It is the true mechanism behind genuine rising damp (groundwater wicking up porous masonry) and behind moisture spreading sideways from a leak. Materials with fine, connected pores (some brick, stone, mortar) are strongly capillary-active; capillary breaks (a DPC, a cavity, a capillary-passive layer) interrupt the path. Note that hygroscopic salts left behind by capillary moisture then attract more water from the air, sustaining damp readings long after the original source is gone.
4. Gravity and bulk water
The simplest and often largest: liquid water arriving in quantity and running downhill — wind-driven rain on a wall, a leaking gutter or downpipe, a failed flashing or parapet, a plumbing leak, or flooding. This is penetrating damp. It is managed by the building's weathering details: render and pointing, flashings, cavities, drainage, overhangs, DPCs and DPMs. Diagnosis usually means finding the external defect, not treating the internal symptom.
5. Built-in (construction) moisture
New construction starts wet — wet plaster, screeds, mortar and concrete, plus any rain absorbed during the build — and can take months to dry to equilibrium. Seal it up too soon or finish it with a vapour-closed layer and that moisture is trapped, causing early-life condensation and mould that gets mistaken for a defect. It is controlled by allowing realistic drying time and, on critical projects, monitoring moisture content before closing up.
Hygroscopic sorption — the buffering effect
Overlaying all of this, many materials are hygroscopic: they adsorb moisture from humid air and release it when the air dries, buffering swings in indoor humidity. Timber, lime, clay plasters and natural insulants do this well; foils and plastics do not. Hygroscopic buffering is generally beneficial (it smooths humidity peaks), but hygroscopic salts in masonry are the troublesome side of the same physics — they pull moisture from the air and hold it, producing persistent damp patches and false-high meter readings.
Why diagnosis must come first
Worked example — air leakage versus diffusion
The relative scale of the mechanisms surprises people, so it is worth putting rough numbers to it. Building-science studies of cold-climate walls have repeatedly shown that the quantity of moisture carried into a construction by air leakage (warm, humid air passing through a small gap under a pressure difference) can be tens to hundreds of times greater than the quantity that diffuses through an equal area of intact, vapour-permeable material over the same period. A frequently-cited illustration found that diffusion through a square metre of vapour-retarder-protected wall over a heating season amounted to a fraction of a litre, whereas the air leaking through a small gap in that same wall carried many litres of moisture into the cavity over the same period. The lesson for diagnosis is blunt: chase the air leaks first. A continuous air barrier prevents far more moisture damage than any vapour-retarder upgrade.
Wind-driven rain — the dominant bulk-water load
On exposed elevations, wind-driven rain is by far the largest moisture load a wall ever sees — orders of magnitude more water than diffusion or even air leakage deliver. The UK's wet, windy west and exposed coastal and upland sites are classified for driving-rain exposure (BS 8104 and the exposure maps), and a solid masonry wall on a severely-exposed elevation can absorb substantial quantities of rain that then has to dry back out. This is precisely the load the steady-state Glaser method ignores, which is why internal insulation on a rain-exposed solid wall must be assessed with transient (WUFI) analysis using the correct local driving-rain exposure. Managing bulk water — sound pointing and render, working gutters and downpipes, flashings, overhangs and a rain-shedding external finish — is usually the highest-value moisture intervention on an exposed building, before any internal measure.
The hygroscopic salt problem in depth
Hygroscopic salts deserve their own attention because they sabotage both diagnosis and remediation. When capillary or penetrating moisture has been present — whether from genuine ground water, an old leak, or de-icing salt splash — it carries dissolved salts (chlorides, nitrates, sulphates) into the masonry and plaster and leaves them behind as the water evaporates. These salts are hygroscopic: they actively pull moisture out of humid room air and hold it, so the affected plaster stays damp and reads sky-high on a resistance meter long after the original water source is gone. They are also highly electrically conductive, which is exactly why a pin meter over-reads on salty plaster and produces so many false 'rising damp' diagnoses. Genuine remediation of a salt-contaminated wall usually means removing and replacing the contaminated plaster (often with a salt-resistant or sacrificial render system), not injecting a chemical DPC into a wall that was never rising-damp in the first place.
Diagnosing the dominant mechanism
In practice, identifying the mechanism is detective work that reads the pattern as much as the meter:
- Timing: damp that worsens within hours of rain points to penetrating (gravity) water; damp that worsens on cold nights and in winter points to condensation; damp that is roughly constant year-round may be capillary or salt-held.
- Location and shape: a defined tide-mark low on a wall suggests capillary rise; damp high on a wall or around openings suggests penetrating water or a thermal-bridge condensation point; damp in cold corners and behind furniture is condensation.
- Surface temperature: thermal imaging shows whether the damp area is a cold spot (condensation) or not.
- True moisture content: carbide or gravimetric sampling distinguishes real liquid moisture from salt-held atmospheric moisture — decisive for ruling rising damp in or out.
- External evidence: an external survey finds the failed gutter, cracked render or bridged DPC behind a penetrating-damp pattern.
