Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings 3.7 Material Responses to Relative Humidity There are many building materials which are commonly said to absorb water. Building science discussions make a distinction between adsorption and absorption. Some suggest that adsorption is a fourth state of water, along with ice, liquid water and water vapour. Adsorbed water is, in effect, water vapour whose isolated molecules have become attached to the microscopic surfaces of porous materials without clumping or clustering into liquid water. The materials which attract and capture water vapour molecules are considered hygroscopic. (“Hygro”, instead of “hydro”, indicates that water vapour is involved rather than liquid water.) When a hygroscopic material has adsorbed all the water vapour it can, it is still able to take in liquid water by capillary suction (wicking) or absorption and store it in the pores and cracks of the material. Wood, for example, can increase its moisture content up to about 25 or 30% at 98% relative humidity simply by capturing (adsorbing) water vapour molecules from the atmosphere onto its pore walls. Wood which is fully saturated by liquid water can hold two to four times that amount of moisture in its pore spaces. Hygroscopic materials respond to relative humidity rather than to absolute humidity. They take in more water vapour when relative humidity is high and release it when relative humidity falls. Since relative humidity rises as temperatures go down, cooler temperatures will increase water vapour capture and storage in hygroscopic materials. This response to relative humidity can provide a useful hygric buffer or safety margin when water vapour levels are rising or temperature is falling. Figure 3.11 shows the approximate temporary storage capacity of the three main components of the external walls of a modest brick veneer dwelling. Figure 3.11 – Storage capacity of materials in the external walls of a 185 m² brick veneer house Brickwork buffering capacity : 1,100 litres Timber frame buffering capacity : 150 litres Plasterboard buffering capacity : 15 litres DRAFT HANDBOOK Australian Building Codes Board Page 25
Information Handbook: Condensation in Buildings Safe storage capacity in the building fabric can immobilise water vapour when relative humidity is rising and release it when drying conditions prevail. The interior of the house in Figure 3.11 would have about 4 litres or kilograms of water vapour in the air when the indoor temperature is 20°C and relative humidity is 50%. Adding one more kilogram would take relative humidity up to the 70% threshold for mould growth. Four more would bring relative humidity to 100% or dew point. The interior plasterboard lining, in first contact with this airborne water vapour, can take up some of it by adsorption and help to stabilise relative humidity at less problematic levels. Taking account of the timber frame behind the plasterboard would increase the buffering capacity of the wall assembly more than ten times. Brick veneer for the external cladding increases it by more than 80 times. The comparative storage and buffering capacities of lightweight and high mass walls are illustrated in Figure 3.12. This sort of variation is highlighted in Chapters 4 and 5 as an important consideration when selecting envelope construction for buildings in locations with greater risk of condensation or with higher indoor water vapour loads. Figure 3.12 – Indicative range of moisture storage capacity in masonry and light framed walls Brick cladding Concrete block Steel frame Steel cladding Plasterboard DRAFT HANDBOOK Relying on the hygric capacity of the building envelope construction is not a panacea for avoiding problems with water vapour. Some of the capacity will be needed to deal with periodic wetting by rain, surface water or rising groundwater, and some for construction moisture in the building fabric itself (such as water in concrete slabs or steam cured concrete blocks). Chapter 5 considers the demands on the building envelope and the measures needed to ensure that water (as liquid or vapour) is excluded as far as possible and does not accumulate faster than it can dry. Australian Building Codes Board Page 26
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