Condensation

What is condensation?

Air always contains some water vapour. The amount of water vapour that air can hold depends on its temperature: warm moist air can hold more water vapour than cooler air.

Condensation occurs where the moisture in the air meets a cooler surface (for example, when water droplets form on the outside of your cold drink). This is because when air hits a surface cooler than itself, the maximum amount of water vapour it can hold will decrease, and some of its water vapour may condense out into liquid water. The temperature at which this happens is called the dew point.

You will often see references to relative humidity (%RH). This refers to the amount of water vapour held by the air, relative to the maximum amount it could hold at that temperature. For example, 50% RH means that the air currently holds half the water vapour that it could possibly hold at that temperature.

Why is condensation important?

Condensation in your home may occur on surfaces or inside construction systems like walls, ceilings or floors (called interstitial condensation). Interstitial condensation can lead to mould, mildew and decay. This can cause rotting of building materials and, if mould spores become airborne indoors, harmful effects on human health.

The act of breathing releases water vapour, so you add moisture to the inside of your home as you breathe. Household activities such as showering, cooking, washing and drying clothes also contribute to a high level of water vapour inside the home. The risk of condensation forming increases if cold surfaces are present (for example, single glazing or standard aluminium-framed windows and uninsulated walls).

Older homes with poor sealing allowed air flow and allowed water vapour-laden air to diffuse through the structure. As newer buildings become more energy-efficient, the interior environment is increasingly different to the exterior environment. Condensation problems can arise in your home when water vapour diffusion is not considered.

The balance of ventilation and airtightness in your home, and the choice and installation of insulation, can either eliminate or reduce the risk of condensation, or make it worse.

The National Construction Code requires that all new buildings are designed to manage condensation risk.

Condensation and mould risk in your climate

The dew point varies with the relative humidity (%RH) and temperature. For example:

• if the temperature is 0°C and the %RH is 75%, the dew point will be –4°C
• if the temperature is 10°C and the %RH is 80%, the dew point will be 7°C
• if the temperature is 20°C and the %RH is 80%, the dew point will be 17°C
• if the temperature is 35°C and the %RH is 80%, the dew point will be 31°C.

This means that the risk of condensation will vary according to your climate zone.

National Construction Code (NCC) climate zones and condensation risk

NCC Climate zone	Summer/wet season risk	Winter/dry season risk
1 Hot humid summer, warm winter	If air-conditioned, high risk of condensation forming on cold surfaces within the structure	Minimal risk
2 Warm humid summer, mild winter	If air-conditioned, high risk of condensation forming on cold surfaces within the structure	Some risk of condensation forming inside structure
3 Hot dry summer, warm winter	Minimal risk	Some risk of condensation forming inside structure
4 Hot dry summer, cool winter	Minimal risk	High risk of condensation forming inside structure
5 Warm temperate	If air-conditioned, high risk of condensation forming on cold surfaces within the structure	High risk of condensation forming inside structure
6 Mild temperate	Low risk	High risk of condensation forming inside structure
7 Cool temperate	Low risk	Certain risk of condensation forming inside structure
8 Alpine	Certain risk of condensation forming inside structure	Certain risk of condensation forming inside structure

Source: Jesse Clarke, Pro Clima

General principles

To minimise the risk of surface condensation and moisture accumulation in interstitial spaces, you should manage the amount of water vapour in your home.

The direction of travel of water vapour will depend on your climate and season. Water vapour moves from high vapour pressure to low vapour pressure:

• In hot humid climates, the flow is generally from outside to inside (except where night-time cooling of the roofing drops its temperature to the dew point).
• In cool climates, where relative humidity is generally low and internal vapour levels in an airtight building are high, the flow is typically from inside to outside.
• In mixed or temperate climates, flow can occur in either direction. Flow from the inside to outside is usually dominant in the winter when surfaces are at risk of reaching dew point. 
• Many climate zones with cold winters are also bushfire prone. It is important to manage both condensation risk and bushfire risk. Most commonly accepted materials and installation practices used in the past did not do this. It is important to always use best-practice systems and products from reputable manufacturers and installers.

To control the movement of water vapour-laden air:

• limit water vapour passage into the building envelope by using climatically appropriate vapour control layers and airtight materials on the inside or outside, depending on the vapour source. For more information refer to Building membranes in this chapter. 
• make the building envelope vapour permeable. The ‘exit’ layers (furthest from water vapour source) should be more permeable than the ‘entry’ ones (closest to water vapour source); this ensures that exiting water vapour will pass through the envelope
• reduce internal humidity levels by venting common sources to the outside (for example, bathrooms, clothes dryers, cooktops).

Minimise the risk of water vapour-laden air condensing on interior surfaces. This can be achieved by:

• maintaining internal surface temperatures above dew point by installing insulation, insulated glazing units and quality window frames 
• ensuring windows can be covered by insulating blinds or thick curtains with pelmets  (however in a new build, using insulated glazing units is the best solution)
• ensuring air circulates behind large furnishings on high risk external wall surfaces (for example, south-facing or uninsulated walls, corners where thermal bridging occurs)
• avoiding thermal bridges 
• consider design strategies such as allowing for a cavity within the wall space to ventilate water vapour and for condensation to drain out externally
• installing mechanical ventilation heat recovery systems.

Areas where mould is likely to form

Source: Ontario Association of Architects

Building membranes

There are various types of membranes used to control the flow of air and water vapour through the building structure. All new buildings should have some sort of membrane.

Building membranes and control layers are of 3 main types (as defined by Australian Standard AS/NZS 4200.1:2017 Pliable building membranes and underlays – materials):

• vapour-impermeable membranes that do not allow the transmission of water vapour 
• vapour-permeable membranes that allow vapour to pass through 
• smart membranes, or vapour control layers, with vapour permeability that varies with temperature and humidity; various proprietary brands are available.

Membranes that comply with bushfire requirements should always be specified and installed correctly – refer to commercially available best-practice systems and the manufacturer’s fire resistance data.

Choosing and installing a membrane

Different membranes have different vapour permeabilities. Permeability is a material’s ability to let water vapour diffuse which is measured in micrograms of water vapour passing through a material per Newton second of force applied (μg/N.s).

Membrane products are tested and rated according to Australian Standard AS/NZS 4200.1:2017. The higher the vapour permeability (μg/N.s) rating, the more permeable the material. Check with your supplier about the suitability of the product for your dwelling and climate as per Australian Standard AS/NZS 4200.1:2017.

Vapour permeability and resistance of typical materials

Source: Russell 2011.

Depending on your climate zone, the vapour control layer should be placed on the warm (vapour entry) side of the structure where they will be kept at above dew point temperature.

Best-practice roofing uses membranes that are vapour permeable and installed to direct any condensation that forms on the underside of the roofing, clear of the structure.

Smart membranes can be highly effective as internal air and vapour control layers where cold and warm sides are interchangeable.

Membranes in tropical climates

In tropical climates, external humidity levels are consistently higher than internal levels, and external surfaces are rarely cooler at night. Condensation risk increases if the building is air-conditioned.

If the building is consistently cooled, temperatures of internal linings may drop below the dew point of the humid outside air passing through external linings. This can cause condensation on the cooled interior side of the building envelope. In this case, vapour and air control layers should be located towards the outer layers of the wall and designed to drain water away from building elements.

If roof surfaces cooled by radiation to the night sky create temperature differentials, significant condensation can result on the underside of exposed impermeable surfaces such as metal roofing. The solution is a correctly specified and installed membrane that drains condensation clear of building elements without leaking.

Membranes in temperate climates

In temperate climates, lower internal−external temperature and humidity differentials often reduce condensation risk but do not eliminate it.

Water vapour movement can be in either direction depending on seasonal and diurnal fluctuations. External membranes should be vapour permeable and internal membranes, if installed, should control vapour and air entry.

Membranes in cooler climates

In cooler climates, water vapour usually moves outward because outside temperatures are generally lower than inside. The likelihood, extent, and cumulative time where surfaces will be at or below dew point increases as the weather becomes colder. Even though external humidity is generally lower, internal humidity (water vapour) rises as occupants spend more time indoors, and use mechanical clothes dryers or drying racks in front of heaters.

Building membranes installed on the cold external side of insulation must therefore be vapour permeable and designed to drain condensation away from building elements (for example, lightweight cladding systems should be installed on cavity battens). Membranes used on the warm internal side of the insulation should function as air and vapour control layer to limit vapour from the warm humid interior from diffusing through the wall.

Remember to check for commercially available best-practice systems and detailing from reputable manufacturers and installers when building in bushfire prone areas.

House under construction in Canberra using external brick and lightweight materials; the vapour-permeable membrane will be clad with timber and metal.

Photo: Light House Architecture and Science

Thermal bridging

Thermal bridges are pathways for heat and cold to cross from the inside to outside (or vice versa) through floor, walls and roof components. Thermal bridges can lead to condensation problems when surface temperatures reach the dew point.

Building frame

The building frame can act as a thermal bridge, in all climates, conducting heat inward or outward and allowing it to bypass otherwise effective insulation. Metal framing is a particular problem because of its high conductivity.

On a cold winter’s night with an outdoor temperature of 0°C, metal wall studs can conduct that freezing temperature straight through to the internal linings, which would drop to a temperature well below dew point and may cause condensation on the interior wall. Conversely, in hot weather each wall stud would act like a strip heater.

They are made of various insulating materials with rigid foam being a common choice. While the NCC contains requirements for mitigating condensation risk, this does not mean that the Deemed To Satisfy (DTS) approaches in the NCC will necessarily achieve that. Professional advice should be sought.

Thermal bridging across timber frame versus steel frame

Source: Envirotecture

Windows and doors

Windows and doors can also be thermal bridges, and are a common location for condensation. This is most common in the coldest climates, but can occur in temperate climates too.

Your choice of glass, and in particular whether you choose insulated glazing units, will affect how readily condensation will form on the glass surface.

Your choice of framing is even more important. Aluminium, which is one of the most common choices for window frames, is a very good conductor. Aluminium frames form a thermal bridge into your home and are prone to condensation. Less-conductive framing materials, such as thermally broken aluminium, uPVC or timber, may reduce condensation on your window and door frames.

Managing condensation in roofs

The risk of condensation in a roof space depends on the interaction of ventilation and insulation in that space.

Condensation can form on the underside of all roofing materials to varying degrees at all times of the year, whenever the surface temperature drops below dew point. This can happen in all climate zones. When condensation forms, it must be drained to the outside of the building to prevent moisture accumulation in insulation materials and internal lining materials.

Vented roof systems

Creating a purposeful fully vented roof space may solve the problem of condensation build-up, but 3 things must then be managed by careful design and correct material specification:

• preventing further condensation from forming under a primary moisture barrier (sarking) below the roofing 
• removing humidity within the roofspace
• providing adequate insulation (high R value) between the roofspace and the internal spaces below. 

If these aspects are managed properly, a very high-performance roof system can be achieved by using a vented roof. International research has shown that ventilating the cavity between the roofing and the moisture barrier (sarking) below not only removes moisture, but convectively removes heat during the summer. The darker the colour of the vented roof, the more convection that is induced – making the system self-regulating.

Particular care must be taken over the detail, so that the following ‘layering’ of materials and functions is provided:

• Roofing can be metal or tiles, but it should be assumed that condensation will form underneath; specify batten material that is resistant to rotting or rusting (careful specification is needed to avoid incompatible metals).
• A longitudinal spacer batten (counter batten) is provided under the roof battens, which runs on top of each rafter or truss. This is deep enough to provide a ventilation pathway from eaves to ridge, so is usually a minimum of 25mm deep (45mm is ideal), and will have bird and pest wire top and bottom. If your dwelling is located in a bushfire-prone area, refer to Bushfire protection for advice on protecting openings.
• Your roof pitch will determine the specification of the vapour-permeable or vapour-impermeable moisture barrier to be installed between the spacer batten and the rafter or truss, Bushfire Attack Level (BAL) rated if required.

With vented roofs, the roofspace is then considered to be outside the thermal envelope. Thermal performance modelling with a Nationwide House Energy Rating Scheme (NatHERS) simulation tool can provide information on different levels of ceiling insulation through the design process. For more information refer to the Building rating tools.

Managing condensation in walls

Vapour-permeable walls

Although walls obviously need to be waterproof, they can be vapour permeable at the same time. Microscopic openings in vapour-permeable membranes will allow individual water vapour molecules to pass through, but will not allow liquid through. This is because liquid water molecules are larger and the tight surface tension of vapour-permeable membranes prevent them from moving through.

Framed walls use layered components to achieve some degree of waterproof vapour permeability, but careful analysis is required to determine whether this will be adequate on its own. This type of wall will normally have bulk insulation in the frame (between studs), with vapour-permeable membrane over the frame exterior, plus a thin cavity batten between membrane and cladding. This cavity is essential to allow water vapour to escape.

Note that this system is the minimum allowable under the National Construction Code ‘Deemed to satisfy’ rules. Careful consideration must be given to climate and construction when specifying membranes, as an incorrect choice may cause a higher risk of condensation. Always check the manufacturer’s technical information and suitability with the supplier.

Selectively permeable intelligent membranes can control the rate of water vapour permeability, from impermeable to highly permeable. External conditions such as humidity and temperature cause the patented microstructure to open or close as required. When appropriately designed and built, this results in an extremely high-performing wall system. Seek expert advice when considering these systems to ensure you get the expected result.

Installation of a selectively permeable intelligent airtightness system to wall framing, with tightly taped joints

Photo: Pro Clima

Airtight walls

Making a building truly airtight can have advantages, airtight houses have better thermal control, but must have a reliable means of gaining fresh air and managing humidity build-up. Houses will benefit from a mechanical heat recovery ventilation system if they are to maintain energy-efficient control of internal temperature.

If you decide to make your house airtight, an airtight barrier membrane suitable for your climate can be installed either on the inside or the outside of the wall frame. In both cases, the wall frame structure and any insulation inside it must be allowed to dry. If the airtight barrier is inside, then a vapour-permeable membrane is generally also required on the outside. If the airtight barrier is located on the outside, a vapour control layer may not be required inside – the wall frame is free for water vapour diffusion through the interior linings and its moisture content is then managed as part of the interior. This can seem rather complicated – it is advisable to obtain expert advice from a design professional who understands the interaction of building components in your climate zone.

• Australian Building Codes Board, Condensation in buildings Handbook, Canberra.  
• Cole G (2011). Residential passive design for temperate climates, Acumen, EDG 66 GC, February, Australian Institute of Architects. 
• Edwards J (2019). Condensation management, ReNew, Melbourne, Victoria.
• Luther M (2007). Air leakage in buildings: review of international literature and standards, Environment design guide, TEC 23, Australian Institute of Architects, Melbourne. 
• Russell A (2011). Pushing the envelope: have we considered condensation?, Paper presented at ABCB National Conference 2011, Building Australia’s Future. 
• Turner L (2017). The key to thermal performance: Insulation buyers guide, ReNew, Melbourne. 

Learn more

• Read Insulation for tips on how a well-insulated house can reduce your energy costs
• Explore Ventilation and airtightness to find out how to make your home both healthy and energy-efficient
• Read Bushfire protection for details on building in bushfire prone areas

Authors

Principal authors: Andy Marlow, Dick Clarke 2020