When designing residential or commercial structures, Building Designers should be aware of the potential of condensation and the risks that condensation may pose to the structural integrity of a building, and to the building’s inhabitants.
What is ‘condensation’?
Water exists in three forms – liquid (water), vapour (moisture) and solid (ice). Condensation is the process by which water vapour changes, or condenses, into liquid water (condensation is the opposite of evaporation).
Water beading on the outside of a cold glass of water is an example of condensation, as is dew on the leaves of grass or on your windows. Condensation on a window is one of the least dangerous forms of this phenomenon. When the window cools overnight, the inside ambient air alongside the window loses heat and the water vapour molecules in the air lose energy. They start to cling together until they form water on the glass.
The “dew point” – for which a constant barometric pressure can be assumed – is the temperature at which condensation is relative to temperature and humidity.
Factors contributing to condensation
- The level of steam, evaporation and occupancy in a given room;
- Relative humidity, temperature and pressure both inside and outside (which varies all the time);
- The structure and composition of a wall, floor or ceiling (sloping roofs above ventilated lofts are rarely a problem);
- Internal and external wall surfaces e.g. external cladding, internal linings, paint type etc.
Modern buildings are more susceptible
Modern building techniques can exacerbate the creation of condensation. The goal of modern building practices is to reduce the air gaps and openings into a structure, which can prevent the moisture that does accumulate, to dissipate easily.
Most modern buildings are air conditioned, and have thicker insulation. Air conditioning can lead to condensation, and because of the thicker insulation – the condensation simply continues to collect rather than harmlessly evaporate.
Condensation within structures
Condensation that forms on the inside of a building structure – a wall, floor, or the roof – is
called ‘Interstitial Condensation’. Excessive amounts of interstitial condensation can be detrimental to a building’s structure, and therefore is of most concern to builders and homeowners.
Interstitial condensation occurs when water vapour travels through porous building materials or apertures and subsequently the vapour cools sufficiently to induce condensation.
If effective measures to address the risk of condensation have not been implemented, and the rate of moisture accumulation is higher than the rate of moisture diffusion, then the moisture content of the building material increases which exacerbates the situation.
Interstitial condensation explained further
Condensation within a building can form as visible surface condensation. This of course can be easily wiped off.
Interstitial condensation is of greater concern. Interstitial condensation forms on surfaces within the building fabric. In cold weather, interstitial condensation is caused when water vapour inside a building is able to move outward via diffusion through permeable building fabrics or air movement, and reach a surface within the building cavity that is below the dew point. When building materials become saturated with moisture, rot and rust can occur which can affect the structural integrity of the building.
Minimising the risk of condensation within buildings
Because there is no one solution to suit all Australia Climate Zones, guidelines for preventing condensation must be broken down into two segments:
- Building Structure and;
- Climate Zone.
The direction of the flow of moisture through building elements, and the permeability of these elements, is of most concern.
Moisture diffusing through the permeable building elements goes from the high vapour pressure side to the low vapour pressure side.
This is called the ‘Vapour Drive’ and it follows the same direction as the flow of heat. During winter, the direction of diffusion is out of the building and in summer it diffuses into the building. Once the direction of the Vapour Drive has been ascertained, it is necessary to consider the vapour diffusion through the building elements.
Hot and humid climates
With hot and humid conditions, the moisture drive is into the building. When building in hot and humid conditions such as Darwin, where humidity can reach up to 72%, the goal is to stop the moisture from entering the building and condensing on the interstitial building elements cooled from the air-conditioning.
An impervious wall wrap on the stud exterior is required. It is essential that all joins and penetrations are taped with a vapour seal tape.
For cold climates, the moisture drive is out of the building. It is therefore necessary to manage the vapour through the interstitial building elements out of the building, or until it hits a building element which can handle the moisture.
Wall cladding such as metal sheeting and composite aluminium sheets are designed to handle water condensing on their interior sides, and then directing the moisture out of the building. This is an acceptable method for most light-weight wall constructions. However, if there is an impervious building element within the interstitial wall, or a thick building element with high vapour resistivity, the recommended solution is likely to differ. It is recommended that design experts are consulted prior to construction to determine the most suitable products for your construction.
Condensation mitigation principles
Reduce moisture in the interstitial construction
- Duct all vents to the outside
- Ensure no bridging of façade cladding
- Fix any water leakages
Ensure ventilation flow
- Ensure nothing is blocking ventilation flow
- Ensure ventilation flows in and out of cavities
- Ventilation out of the building
There are a range of Fletcher Insulation products that can assist in mitigating the risk of condensation, the five most common being; Sisalation® Vapawrap™, Sisalation® Tuff Wrap, Sisalation® Multipurpose, Permastop® Building Blanket and Tyvek® HomeWrap .
The correct selection and use of these products is very important and will be dependent on a number of factors including but not limited to; geographic location, application and the overall system design.
Fletcher Insulation is able to assist in recommending the most appropriate products for use in your design construction. For further advice, contact Fletcher Insulation’s Technical Services Team on 1300 654 444.