Condensation: Energy efficiency’s unwanted companion
Recent attempts to increase the energy efficiency of Australian buildings have resulted in tighter sealing and greater thermal insulation of external building envelopes, resulting in unprecedented risk of condensation build-up. DR RICHARD AYNSLEY explains the dangers of this rising threat.
In the past, unintentional ‘leakage’ in buildings allowed airflow to remove excess water vapour from internal spaces. Now that buildings are more tightly sealed, unwanted water vapour inside buildings has les opportunity to escape to the outdoors. This can lead to condensation and associated dampness, mould (Brennan and Burge, 2005), health issues, corrosion, decay and in extreme cases structural collapse. Where there is significant mould contamination the consequence is usually demolition.
We have all seen condensation form on the bathroom mirror after a shower during the cooler months of the year. This is H20 going through condensation phase change from a gas as water vapour to liquid water (CSIRO, 1995).
When H20 is in its gas phase as water vapour it obeys the Gas Laws. The water vapour responsible for condensation in buildings is only one of the gases that make up air in the Earth’s atmosphere, but each gas in air acts independently according to the Gas Laws. At a given atmospheric pressure, the temperature at which water vapour in the air condenses is called the dew point temperature. The science of psychrometry provides various equations that allow characteristics of water vapour in air to be calculated (Gatley, 2005). Graphically, dew point temperatures can be read off a psychrometric chart for standard atmospheric pressure along the saturation line for any combination of humidity and air temperature. Computer software is also available to perform the relevant calculations to determine dew point temperatures.
Surface condensation occurs when the temperature of a surface is at or below the dew point temperature of water vapour in the air adjacent to the surface. Condensation can also occur within porous building materials where the temperature of the material is at or below the dew point temperature of water vapour present. This is referred to as interstitial condensation.
In the past, simplistic straight-line graphs of steady-state temperature and vapour pressure profiles through construction were used to estimate locations where condensation may occur. Sophisticated computer software is currently being developed to dynamically model temperatures and vapour pressure and dew point temperature throughout a construction as they vary with time.
SOURCES OF WATER VAPOUR
There are two sources of water vapour to be considered with regard to potential condensation. The most obvious source is from the atmosphere as rain or just high humidity. The other source is from activities within the building, such as the water vapour we exhale as well as washing and cooking with hot water.
Traditionally, condensation in buildings is controlled by ventilation and providing drainage planes (Lstiburek, 2006). For example, the inside face of the external brickwork in a cavity brick wall is a drainage plane. Any moisture seeping through the external brickwork drains down the wall to flashings, which direct it out through weep-holes near the base of the wall. The cavity is open at the top of the wall and ventilating airflow passes into the cavity through the weep-holes at the bottom of the wall, coursing up through and out the top of the cavity. Water vapour is transported through building construction by air movement and differences in water vapour pressure. Where cavities are ventilated, air transports more water vapour than differences in vapour pressure (Lstiburek, 2002 and 2007).
Most tall commercial buildings have thin curtain walls to maximise internal rentable area. Some of these curtain walls have drainage systems; others attempt to provide an impervious barrier (Lstiburek and Carmody, 1994). Thin sheet materials on exterior surfaces change temperature much more quickly than heavy concrete or masonry materials and pose greater risks of condensation if seals leak. Where materials such as fibrous insulation or gypsum wallboards in walls need to be protected from condensation by water, vapour barriers are installed during the construction process, otherwise serious mould contamination can occur (LBL, 2009); see photo above. The LBL newsletter (LBL, 2009) suggests that 21 percent of current asthma cases in the US are attributable to mould and dampness-related conditions in homes with a national annual cost of $3.5 billion. Evidence also suggests that exposure to mould and dampness in offices and schools result in similar health impacts.
Inadequate insulation of air-conditioning ducts located in non-conditioned spaces or air-conditioning plenums are also potential causes for condensation in buildings (Lstiburek, 2007).
It is important to ensure that the vapour barrier is installed in a position on the warm side of where the dew point temperature is expected to occur. That may sound like a simple procedure, but the dew point temperature location within the construction moves over time and according to the seasons. The warm side of a construction can also reverse within 24 hours in arid desert climates and with seasons in more temperate climates. Contemporary designs underpinning water vapour control are complex, based on dynamic computer analysis, and they utilise vapour barriers often of different vapour permeability or smart vapour barriers that can change their permeability.
CONTROL CONDENSATION WITH AIR MOVEMENT
The thin stagnant air film adjacent to solid surfaces offers significant thermal resistance to heat flow between the solid surface and the main body of air near the surface when there is no air movement. Air movement across a surface can significantly reduce the thermal resistance of air films.
This reduction in the thermal resistance of air films due to air movement can be used in some situations to reduce the occurrence of condensation on indoor surfaces. In many cases the minimum temperature of the air film, at the solid surface, is only a degree or two below dew point temperature. This means that if air movement disturbs the stagnant air film, then the temperature of the moving air adjacent to the cold surface can be above dew point temperature.
One example of the application of air film disturbance to control condensation is seasonal condensation on concrete slabs in contact with the ground, as often occurs in warehouses. During springtime, the average air temperature begins to rise. In locations near large bodies of water, this increase in air temperature results in an increase in water vapour in the air. At the same time the temperature of the concrete slab on the ground lags about 30 days behind the increasing air temperature due to the thermal inertia of the ground. The combined effect is that the surface temperature of parts of the concrete slab on the ground can be below the dew point of water vapour in the air. The water vapour in the stagnant air film adjacent to the floor slab condenses.
A second example of the application of air film disturbance to control condensation is on the underside of large un-insulated, metal-roofed plant rooms in the humid tropical city of Townsville, Queensland. Each night radiation to the night sky results in the temperature of the roofing iron falling to around five degrees Celsius below the ambient outdoor air temperature, and below the we point temperature of the very humid air. There have been occasions when it literally ‘rained’ inside the roof space. The large roof space was subsequently fitted with ceiling fans blowing upwards. These ceiling fans disturbed the air film on the underside of the metal roofing and continuously circulated warmer air in the roof space, effectively raising the temperature of air moving adjacent to the underside of the roof. Condensation was controlled. This type of condensation is evident in the morning on the underside of metal verandah roofs.
The condensation under the roof would not have arisen if sarking or anti-con insulation had been installed underneath the metal roofing (AS 1562.1, 1992). the cost of retrofitting sarking or anti-con insulation after a roof has been installed, however, is usually prohibitive.
LIMITATIONS: BUILDING CODE & AUSTRALIAN STANDARDS
Recent changes to the Building Code of Australia call for increased R-values of thermal insulation to increase the energy efficiency of buildings. In a cold climate, when the R-value of thermal insulation located near the warmer inner surface of the building envelope is increased, the temperatures are decreased in cavities near the cold outer surface of the building envelope. This increases the probability of condensation occurring in such outer cavities.
Australian Building Codes and standards lack detailed guidance on control of condensation over the wide range of climatic conditions across the country. British Standards (BS, 2002), European Standards (ISO, 2002) and American Standards (ASTM, 2009), on the other hand, have recently been updated to help stem the increasing frequency of substantial damage to buildings due to condensation. British and European Building Standards use SI units, but relate to a relatively narrow range of climatic conditions. Condensation Control Standards from the US are likely to be more relevant to Australia, as US climates encompass alpine, temperate, hot, arid and warm humid tropical climates. Standards from the US would need to be converted from imperial units to SI units.
There is serious lack of authoritative advice on designing buildings to avoid damage from condensation. For example, explanatory information provided for clause 3.12 of Volume 2 of the Building Code of Australia (2010) on thermal insulation provides a caution regarding significant damage that arises when condensation occurs. There is no detailed guidance on how to determine when, where and how often it is likely to occur.
Another example is provided in Appendix A of the Australian Standard AS 1562.1 (1992) on design and installation of sheet roof and wall cladding. While the information in Appendix A on roof ventilation, water vapour and condensation is well-intended, it is in an informative section of the Standard. When a building specification states that all relevant work on a project must be done in accordance with a particular Australian Standard, judges in New South Wales have interpreted this to mean that informative sections do not need to be followed. This has effectively limited the liability of building contractors, but does not identify where liability for condensation damage rests.
Serious damage to buildings from condensation is increasing in Australia as building envelopes are tightened and thermal insulation is installed to increase energy efficiency. There is an urgent need for Australia Codes and Standards to be updated to provide detailed guidance to building designers for condensation control across all of Australia’s climatic conditions.
Dr Richard Aynsley recently retired as a director of Big Ass Fan Company at Loganholme, Queensland. He joined the Big Ass Fan Company full-time in Lexington, Kentucky, in 2003. Prior to that, he was a building science consultant to the company on the design and performance of large industrial ceiling fans while serving as Dean of Engineering, Technology and Management at Southern Polytechnic State University in Marietta, Georgia, US. He holds a bachelor degree with first class honours in architecture, as well as a doctorate in building aerodynamics from the University of New South Wales. His master of science (Arch/Eng) degree was gained from Pennsylvania State University at State College, Pennsylvania. While working as director, Research & Development, at Big Ass Fans he secured three US patents for Big Ass Fan designs of airfoil fan blades and winglets.
ASTM (2009) MNL 18 Moisture control in buildings: The key factor in mould prevention, 2nd edition, American Society for Testing and Materials, West Conshohochen, PA, US, 620 pages.
BCA (2010) Building Code of Australia, Vol 2, Clause 3.12, Australian Building Codes Board, Canberra.
Brennan, T and Burge, H (2005) ‘Assessing mould in buildings’, ASHRAE Journal, Jan, pp.158-164.
Brennan, T, Cummings, J and Lstiburek, J (2002) ‘Unplanned airflows & moisture problems’, ASHRAE Journal, November, pp. 44 -52.
British Standard (2002) BS 5250:2002 Code of practice for control of condensation in buildings, BSI, London.
CSIRO (1995) Condensation, Building Technology File, Number 5, August, CISRO Division of Building, Construction and Engineering, Highett, VIC, 4 pages.
Gatley, Donald P, (2005) Understanding Psychrometrics, 2nd edition, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc, Atlanta, GA. 382 pages.
ISO (2002) EN ISO 13788:2002 Hygrothermal performance of building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation, International Standards Organization, Geneva, Switzerland.
LBL (2009) Summer Newsletter Vol 7, No 3, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, http://eetdnews.lbl.gov/nl26/eetd-nl26-2-mold.html.
Lstiburek, J (2002) ‘Investigating and diagnosing moisture problems’, ASHRAE Journal, December, pp. 36-41.
Lstiburek, J (2006) ‘Understanding drainage planes’, ASHRAE Journal, February, pp. 30-35.
Lstiburek, J (2007) ‘The hollow building’, ASHRAE Journal, June, pp. 56-58.
Lstiburek, Joseph and Carmody, John (1994) Moisture control handbook: Principles & practices for residential and small commercial buildings, John Wiley & Sons, NY, 210 pages.
Standards Australia (1992) AS 1562.1-1992 Design and installation of sheet roof and wall cladding, Standards Australia, Sydney, p. 13.