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Monday, 21 October 2013 03:12

CONDENSATION IN RESIDENTIAL BUILDINGS Part 1:

ABSTRACT

Condensation in buildings has become a concern, as thermal insulation is increased to improve energy efficiency. European standards for assessing condensation risk focus on winter conditions. Simplified psychrometrics used in the European steady-state condensation risk-assessment standard make it unsuited to warm humid Australian climates. The American standard can be used in warm humid regions, as it has been designed to accommodate locations such as Florida and Hawaii. Water vapour transmission data for Australian building materials, based on tests at in-service temperature and humidity conditions, are needed. Australia needs a standard for assessing condensation risk in buildings to establish a consensus on appropriate input data and analysis and assessment procedures  for conducting hygrothermal analyses of Australian construction.


Keywords: houses, condensation, hygrothermal, insulation, energy-efficiency, ventilation, standards.

Recent incidents of mould growth inside Australian houses have been published in the ABCB Condensation Handbook (ABCB, 2011). A few decades ago mould on interior surfaces of houses due to condensation was rare. Australia has followed Europe, the UK and the USA in requiring new houses to be more energy-efficient by increasing thermal insulation in the building envelope. There has also been an increasing utilisation of air conditioning of houses in the humid tropical and subtropical regions of Australia. This paper addresses the impact of these changes and other matters such as the ventilation needs of energy-efficient homes and their indoor air quality and energy and operation costs (NHBC Foundation, 2009; LBNL, 1998) not addressed in the Condensation Handbook.

Water vapour is water in its gaseous form. Its relatively slow movement by dispersion through porous building materials is driven by differences in vapour pressure. In cold winter conditions in heated and ventilated buildings the vapour flow is from indoors to outdoors. In summer conditions in air conditioned buildings the vapour flow is from outdoors to indoors. Unvented cavities in construction with still air allow faster movement of water vapour, as still air offers little resistance to water vapour transmission.
Ventilated cavities allow even faster movement of water vapour. The rate of water vapour transport in ventilated cavities is determined by the ventilation rate. Typically, more water vapour in building fabric is moved with air flow than occurs by dispersion through porous materials.
In contrast, movement of water in liquid form is relatively slow. Water flow can be driven downward by gravity through porous construction materials or in any direction by capillary forces. These water flows are controlled in building construction by drainage planes using sarking, dampcourses and flashings, and by draining accumulated water to the exterior of the building.

Early studies of indoor AIR Quality and Ventilation in energy-efficient houses.

A Building Research Station publication (Loudon, 1971) written during the Middle East oil embargo explored the interrelationships of indoor heating, average insulation of the building envelope, ventilation rate and relative humidity on the risk of condensation and mould growth in a UK terrace house. The risk of condensation was found to be low, with standard envelope thermal insulation and ventilation rates. Comparisons were made with equivalent European houses that have higher insulation and lower ventilation rates. The equivalent European house    had    an    indoor    air    temperature    of    12→C    obtained    with    a ventilation rate of 0.5 air changes per hour, and an indoor water    vapour    input    of    7.2    kg/day    while    achieving    indoor    relative    humidity below 70% RH. These conditions were considered not ideal but acceptable to many people in the UK. The Ministry of Housing and Local Government (MH&LG, 1961) suggested an air temperature of 18→C    in    living    rooms    and    12.8→C    in    kitchen    and    circulation    space,    and bedrooms. Note that the focus was on energy efficiency. There was no mention of indoor air quality, which has now become a major issue in energy-efficient houses. A recent worldwide review by the Building Research Establishment (NHBC Foundation, 2009) of indoor air quality in energy-efficient houses notes that there have been problems, and suggests less than 70% RH and a minimum of 0.5 air changes per hour are needed for acceptable indoor air quality.

With measurements of air leakage by fan-door pressure tests and caulking to achieve low leakage, mechanical ventilation was found to be essential to achieve indoor air quality. Australian standard AS 1668.2 (Standards Australia, 2007) recommends fresh air ventilation rates for indoor spaces much higher than 0.5 air changes per hour for bedrooms, but they are not mandatory
The 70% relative humidity upper limit to deter mould growth can be achieved by controlling ventilation fans using relative humidity sensors.

Reliance on occupants opening windows for natural ventilation was not considered acceptable, as surveys (NHBC, 2009) showed that less than 20% of occupants in new energy efficient houses ever open the windows. A detailed study of the relative performance of a range of mechanical ventilation systems for energy-efficient houses was conducted by Lawrence Berkeley National Laboratories (LBNL, 1998). This study includes relative system and operating costs. The more expensive systems cost US$2,000 and had annual operating costs around US$900 in 1998 dollars.

Multi-port supply systems with filters, heat and moisture exchangers, and indoor air pressurised to 10 Pa above outdoor air pressure, were recommended for energy-efficient houses  in hot and humid climates. Multi-port supply plus single- port exhaust systems with filters and heat exchangers, and indoor air pressurised to 10 Pa above outdoor air pressure  were recommended for energy-efficient houses in cold climates.

Hygrothermai Analysis

There are steady-state as well as hour-by-hour hygrothermal analysis methods for assessing risk of condensation in construction (examples are provided in Part 2 of this paper). The latter method, developed over the past decade, requires substantial input data on environmental conditions as well as construction material properties not currently available in Australia. This restricts its use in Australia. While this method can model accurately, time-dependent water vapour dispersion through solid materials, computation of water vapour transport in air-filled cavities is problematic due to lack of knowledge of potential ventilation in cavities. The simpler steady-state method (Glaser, 1958) is widely used and considered conservative because it ignores the safe moisture storage in materials such as bricks, timber and plasterboard. It is much less conservative for construction that has little moisture storage capacity, such as lightly clad steel-frame construction. In cold climates, expanded polyethylene foam R0.2 thermal break strips are provided between the steel studs and external claddings to avoid cold thermal bridging and prevent pattern staining by mould on the surface of interior plasterboard.

Brickwork can safely store substantial amounts of water and release it during warmer dryer periods of the year. From research in the US (Lstiburek, 2002), brickwork in a modest 186 sq m brick veneer house can safely absorb and release 1892 litres; timber 189 litres (113 litres in Australia as less sheathing board is used); and plasterboard 19 litres. However, damp bricks can encourage growth of moss and lichen or suffer damage when freezing and thawing occurs.Hygrothermal analysis methods in various countries use different calculation methods and assessment criteria. Each method is likely to achieve a different result. This is why a consensus standard is needed to suit Australian construction and conditions.

Choosing Material Properties used in Hygrothermal Analsis

When choosing material properties to use in hygrothermal analysis it is important that the properties closely match those of the actual materials used. This is easier said than done. Very few Australian building material manufacturers currently publish the vapour-resistance properties of their products. Past condensation problems in buildings in the UK and the US resulted in tables of vapour resistance data for local building materials but these do not necessarily reflect those of materials used in Australia.

For example, vapour resistance for aluminium foil is shown as typically    1000    MN.s/g    in    Appendix    C    of    the    ABCB    Condensation    Handbook based on information from the British Standard BSI EN ISO 13788:2002 (BSI, 2002). The aluminium foil materials used as vapour barriers on studs of brick veneer walls in Australia    conform    to    the    Australian/New    Zealand    Standard    AS/ NZS    4200.1:1994    (Standards    Australia,    1994).

Typical Australian aluminium foil sarking used in walls would have a medium vapour resistance classification of around 300 MN.s/g.    However,    hygrothermal    analysis    requires    specific    vapour- resistance    vales    for    materials    under    in-service    conditions.    The    AS/ NZS    4200.1:1994    standard    cites    ASTM    E96    (ASTM,    2005)    as    the    standard to be used for determining the vapour transmission of pliable membranes used in Australian buildings.

To complicate the issue, the Glaser steady-state method is not the only method of analysis available. Some Australian practitioners are trying to use the popular dynamic hour-by-hour WUFI software (ORNL, 2012) without a complete input data set. There is also a wide range of practices in determining outdoor and indoor air temperature and humidity conditions.
Mean monthly data are used in the UK, while in the US moving- monthly means, and means over three months, are used. Ventilation rates are often overlooked in analysis. Also, different criteria such as dew point and a 70% RH limit over specified periods of time are used to assess the relative risk of mould or condensation.

Read 2609 times Last modified on Monday, 21 October 2013 04:13

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