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.