EWPAA Structural Plywood and LVL Design Manual - Engineered ...

15 Condensation & Thermal Transmission15.1 IntroductionThe main objective of this chapter is to also provide, under the one cover, some basic information pertainingto condensation and heat flow in habitable type buildings be they domestic, commercial or industrial.It is imperative the designer gives due consideration to the question of heat flow and ventilation at anearly stage of the design process. Early attention to such detail will eliminate the need for later, costlyrepairs and inconveniences.Again, the purpose of this chapter is not to attempt to convert readers into being thermo-fluid experts, butrather to make them more aware of the problems that exist and to assist in their identification and solution.15.2 Condensation – CausesCondensation causes mould growth in houses and rot in the timber framing of the house thus threateningits structural integrity. Thermal insulation, whose function it is to prevent surface condensation, if notinstalled correctly, can cause it.Terms and DefinitionsOnly those terms and definitions considered relevant to the topic are presented here.• Dry-bulb temperature – The temperature of the air as registered by an ordinary thermometer (t).• Wet-bulb temperature – The temperature registered by a thermometer when its bulb is covered by awetted wick and is exposed to a current of rapidly moving air (t’).• Relative humidity – Ratio of the partial pressure of the water vapour in the mixture to the saturatedpartial pressure at the dry-bulb temperature, expressed as a percentage.pR wh = x100(15.1)psNote:If the air is completely saturated, the partial pressure will be the vapour pressure of water at the dry–bulbtemperature, i.e. at saturation t=t’.• Dew-point temperature (DP) – Temperature to which air must be reduced in order to causecondensation of any of its water vapour.The above terms will be required if the moisture content of timber was to be determined using FIGURE 15.1Such a situation may arise where, during a wet period, water has ponded under a house without a vapourbarrier. This can cause the underside of the timber floor to take up moisture which in turn can result inbuckling of the floor if the underside is unprotected and the top surface has been coated with say apolyurethane finish.15.3 Condensation – An ExplanationAir can retain water as vapour provided the temperature of the air and the amount of water arecompatible. The ratio of the water in the air relative to the amount which the air can hold is by definitionthe relative humidity.Warm air can hold more moisture than cool air. This means if air at a certain temperature is saturated,this corresponds to 100% humidity. If this air is then cooled water must condense out. This will occur asa fog of liquid droplets if the air is cooled en mass or as a condensate if cooled in contact with a surface.The temperature at which some of the moisture condenses as dew is the dew-point temperature.Problem Areas:231