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Monday, 21 October 2013 03:21

CONDENSATION IN RESIDENTIAL BUILDINGS Part 2:

Abstract

Condensation in buildings becomes more of a concern as thermal insulation is increased to improve energy efficiency. Examples of steady-state hygrothermal analysis of Australian brick veneer wall construction in temperate winter conditions are provided, as are reinforced concrete masonry in the tropics. A standard is needed for assessing condensation risk in buildings to establish a consensus on appropriate input data and analysis, and assessment procedures for conducting hygrothermal analysis of Australian construction in all climate zones.

Introduction

Australia, with relatively warm climates compared to Europe, has experienced fewer problems of mould in houses resulting from condensation in winter. Over recent decades a common form of wall construction, brick veneer, has undergone changes in timber sizes and use of insulation for increased energy efficiency. With a history of little risk of condensation and mould in brick veneer walls in the past, builders and designers paid little attention to hygrothermal analysis. In hindsight, the changes in brick veneer wall construction over a few decades certainly influenced the risk of condensation.

Hydrothermal Analysis

The following examples of steady-state hygrothermal analysis follow procedures described in UK Building Research Station Digest 110 (BRS, 1969) for heated and ventilated houses, and are based on the Glaser method (Glaser, 1958).
Melbourne and western Sydney, BCA climate zone 6, was chosen as representing major suburban areas where brick veneer walls were used in the 1970s. The Bureau of Meteorology’s Melbourne mean monthly climate data indicates the lowest mean minimum monthly air temperature as 6°C in July. No coincident relative humidity is provided for this temperature in the data set. However, coincident mean monthly air temperature and relative humidity of 8.7°C at 79% RH is provided at 9am in July.


Assuming that there is no sudden change in moisture content of the air, between say 5am and 9am, the water vapour pressure at 9am can be used to calculate the equivalent relative humidity at the mean monthly air temperature of 6°C at 95% RH, with a water vapour pressure calculated as 0.89kPa.
The method used in UK Building Research Station Digest  110 (BRS, 1969) for estimating indoor condition in heated and ventilated houses, at normal ventilation rates, is to take the outdoor condition of air and use a psychrometric chart to determine what the water vapour pressure and mixing ratio of the air is after it has been raised to the indoor air temperature proposed for thermal comfort, say 20°C. To this condition, 0.0034kg/kg is added to the mixing ratio to allow for indoor sources of moisture such as washing, bathing, breathing,  and the like, to estimate the indoor mixing ratio and water vapour pressure. In the example, hygrothermal analyses below-water vapour pressures were calculated using psychrometric equations.  The resulting indoor condition for Melbourne in July was 20°C at 62% RH or water vapour pressure of 1.43kPa. The typical ventilation rate at the time the Building Research Station Digest 110 (BRS, 1969) was written was one air change per hour.

Winter Hygrohermal Analysis of Brick Veneer Walls

In WesteRn sYdneY And MelBouRne In the 1970s brick veneer walls typically consisted of an external skin of 110mm brickwork (density≈ 1690 kg/m3), a 50mm cavity, reflective foil sarking with an antiglare surface (≈ 0.3) and reflective surface (ԑ≈ 0.03), 100mm timber stud frame, and 10mm gypsum plasterboard. At that time additional thermal insulation was rare. No widespread mould problems were experienced with these brick veneer walls over a few decades in Melbourne or western Sydney, BCA Climate Zone 6. It can be seen, from the graph in Table 1, that indoor vapour pressure is higher than outdoor vapour pressure, so vapour flow is from indoors to outdoors. The temperatures through the 1970s brick veneer wall do not fall to, or below, dew point anywhere through the construction when indoor air is heated to 20°C. This suggests that under these conditions there is only slight risk of condensation from indoor or outdoor mean minimum monthly air temperature or relative humidity conditions in July in the 1970s brick veneer wall.

Please down load the attatchent to view data and graphs / conclusion.

 

 

 

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