Moisture in buildings
Moisture is the presence of water, often in small or even trace amounts. Moisture can be found in water vapour, condensation, and in or on the fabric of a building and can cause damp resulting in problems such as staining, mould growth, mildew and poor indoor air quality, and so on.
The common sources of moisture in buildings include:
- Condensation.
- Penetrating damp.
- Leakage from pipes, tanks, drains, and so on.
- Rising damp.
- Building defect, e.g. lack of adequate roof space ventilation, faulty retrofit installation, application of paint or plaster that affects the breathability of the building element, and so on.
- Indoor moisture sources, e.g. cooking, bathing, washing, hot tubs, indoor swimming pools, and so on..
Moisture levels can be reduced through a number of measures:
- Natural or mechanical ventilation.
- Use of de-humidifiers or air conditioning units.
- Insulation of cold surfaces, such as pipes.
- Increasing air temperature.
- Removing sources of moisture such as drying clothes and ensuring vented tumble dryers are appropriately vented to the outside.
- Mending leaking pipes, wastes and overflows.
- Eliminating rising damp and penetrating damp.
- Introducing moisture barriers such as vapour barriers, damp proof membranes, and so on.
NB Technical paper 35: Moisture measurement in the historic environment, published by Historic Environment Scotland in 2021, defines moisture as: ‘Water that is freely mobile as a liquid, gas (vapour), and solid (ice). All moisture is water, but not all water is moisture, as it is not necessarily freely mobile.’
It defines moisture content as: ‘The quantity of moisture present within a material.’
Assessing risks in insulation retrofits using hygrothermal software tools, Heat and moisture transport in internally insulated stone walls, by Joseph Little, Calina Ferraro and Beñat Arregi, published by Historic Environment Scotland in 2015, defines moisture as: ‘Water in its solid, liquid or gaseous/vapour states.’
It defines moisture content as the: ‘Mass of water (in any state) per unit volume of a material, often ex-pressed in kilograms of water per cubic metre (kg/m3).’
Moisture convection as the: ‘Movement of water vapour through a space via air currents or bulk air movement (as opposed to by diffusion).’
And moisture diffusivity (Dw) as: ‘…the capillary transport of moisture in the liquid phase, which is the predominant moisture transport mechanism in capillary porous materials. In the context of building physics it is sufficiently accurate to regard the liquid transport in the pore spaces as a diffusion phenomenon (although it is basically a convective phenomenon). WUFI measures it through two liquid transport that depend on both material proper-ties and boundary conditions.’
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