Water in various forms is a major source of deterioration in buildings, and its control is a prime responsibility of the designer. Vapour diffusion is one of the mechanisms whereby water can find its way into the components of a building and actually damage materials. It may sometimes be of less importance than other mechanisms such as rain penetration or air leakage, however it must still be a major design consideration.
In some climates, the temperature of internal surfaces can fall below the "dew point" of the air, leading to condensation of moisture vapour within the air when contacting that surface. This is most noticeable on single glazed windows in bathrooms, where the relative humidity can be very high. Condensation on wall surfaces often promotes the growth of mildew which can stain the surface and trigger ailments such as asthma.
More serious cases of condensation can even damage the fabric of a building. There are instances of this even in dwellings designed by reputable architects. The most serious condensation problems tend to occur in well-sealed, but under-insulated buildings.
Humidity within a space can be offset by additional heating (which raises the temperature therefore lowers the relative humidity), or by ventilation which removes air humidified by moisture given off in the use of the building (human respiration, cooking etc.). Both of these methods will tend to increase the energy consumption of a building.
The reduction of humidity by the use of dehumidifiers (which usually use refrigeration to create a local low temperature region within the dehumidifier) can have beneficial effects in reducing the ill effects described above. There is also a small additional benefit in that the condensation of moisture in a dehumidifier releases latent heat of condensation.
Each litre of water condensed gives about two-thirds of a kWh of heat to the room, in addition to the energy used to drive the unit, which is also released to the room.
Placing insulation moves the temperature profile within the space between the interior and the exterior towards the cold side of the insulation. In the case of highly permeable materials like fibreglass, this can (at least in theory) lead to condensation within the insulation itself, negating its effect, or even wetting cold side woodwork and leading to deterioration of the structure.
Once vapour pressures on the two sides of the building envelope are known and a selection and arrangement of the building materials has been made, the vapour flow calculation is carried out in a manner similar to that used for heat flow. There is, however, one important difference owing to the ability of the vapour to condense. The initial calculation is based on the premise that there is a continuity of flow and that all the vapour entering the envelope on the high vapour pressure side will emerge on the low vapour pressure side. If, on its passage through the building envelope, the vapour is cooled to below the dew-point, condensation will occur and the basis of the calculation is upset. Even so once the plane of condensation has been established the method can be applied to calculate the flow of vapour to it and away from it. The difference between the two gives the accumulation of water within the envelope.
Condensation occurs when the temperature of the air, as it passes through a cold material, falls below the dew point. If this occurs halfway through a material, then some of the water vapour in the air will condense into moisture droplets within that material. This can cause many problems.
If the material is a fibrous insulation, the presence of moisture will seriously reduce its insulative capacity by allowing heat to conduct straight through it (water is a reasonable good conductor of heat). If it is a wood or fibreboard construction, the presence of moisture will promote rot which can affect both the visual and structural performance of the material. In very cold climates, the expansion of this moisture when it freezes can even cause major structural damage to masonry constructions.
In many cases, some condensation can be tolerated within a structure, the amount depending on the water-holding capacity or water tolerance of a particular construction under particular conditions of use. If some condensation is to be permitted, then the materials must have adequate opportunity to dry. The effect of frost action on wet materials, the sudden release of accumulations of ice, dimensional changes produced by changes in moisture content, and many other factors must be considered before this solution is adopted.
Only a few building materials (glass, metal and some plastics) can be regarded as completely impervious to the flow of water vapour. Most commonly used building materials will put up some resistance, but this will only limit the flow not eliminate it. A vapour barrier is therefore a thin sheet of impervious material interposed within a composite structure in order to prevent vapour flow. This is different from a damp-proof membrane, which is intended to stop the flow of liquid moisture through capillary action in a structure. As a vapour barrier must prevent moisture vapour as well, it must be airtight and moisture-proof. This means that it cannot have any unsealed perforations as this would render it useless. Thus, great care must be taken during both design and installation.
When used together with insulating materials, vapour barriers are best placed on the 'hot' side of the insulation to minimise access of water vapour to the cool regions where it might condense. Such measures need to be considered when insulation is to be installed. Some of the references below provide an excellent discussion on vapour barriers.
- Vapour Diffusion and Condensation, CBD-57
- Vapour Barriers
- What are they? Are they Effective?, CBD-175: http://www.nrc.ca/irc/cbd/cbd175e.html
- Vapour Barriers in Home Construction, CBD-57
- Water and Building Materials, CBD-30