A previous paper analysed a mathematical model of a non-condensing cavity. This paper extends the analysis of the first paper to analyse the seasonal moisture behaviour of a condensing building cavity. Climate statistics are used to calculate the duration of the winter wet-up period, and a rate of condensation formula is integrated to give total winter condensation. Although engineering design calculations cannot yet be attempted, some illustrative examples are given based on field data. The results give preliminary verification of the model analysed in both papers.
This paper, the first of two, presents a conceptual model of moisture concentrations in a building cavity. The model is comprehensive and general considering air infiltration, vapour diffusion and material hygroscopicity under non-steady state conditions. The resulting linearised coupled differential equations are analytically solved to study the case of long term cavity moisture behaviour. Dimensionless parameters and algebraic formulae are presented describing all important moisture performance parameters for a non-condensing cavity.
Evidence of the importance of air infiltration in moisture control in building structures has been steadily accumulating. A general model of moisture behaviour in structures has been built up including for the effects of cavity air leakage, for the hygroscopic behaviour of timber, for the effects of condensation and various geometric factors.
Eleven countries are cooperating to establish guidelines for minimum ventilation rates which are sufficiently large to meet the demand for outdoor air in buildings without unnecessarily wasting energy. The most important pollutants have been identified as: carbon dioxide, tobacco smoke, formaldehyde, radon, moisture, body odour, organic vapours and gases, combustion products and particulates. To a certain degree some of thesesubstances can be used as indicators for acceptable air quality to establish minimum ventilation rates.
Moisture enters an attic both from the house and from the ventilation air. It has been assumed that when the roof sheathing temperature cools below the attic air dew point, condensation occurs on the roof sheathing. If this were true, then increased attic insulation levels would require increased attic ventilation rates. Results from an experimental study are presented which show that in fact the roof sheathing is in dynamic equilibrium with moisture in the attic air, and that several hundred pounds of water can be stored in the attic wood without ill effects.
Treats the causes of deterioration in buildings, thermal bridges, the indoor climate, data for the design and execution of buildings and living conditions in rooms. Section headings are The formation of moulds, Humidity in buildings, The temperature factor, tau, as a criterion of the thermal quality of thestructural elements, Conditions of occupation of buildings, Thermal bridges, Natural ventilation of buildings, Conclusions, Advice.
Examines the causes of condensation problems and ways of reducing or eliminating them. Deals with diffusion through the building envelope, mechanical dehumidification and ventilation. Considers condensation inside cavities and roof spaces.
For the transient analysis of the thermal and moisture conditions in multilayer constructions a numerical algorithm and a computer program based on the Crank-Nicholson method and quasi linearisation are formulated. Temperature and moisture content are used as transport potentials. In energy balance equations and conditions, convention and accumulation of moisture, the diffusion flow of water vapour, the capillary and surface diffusion flow of liquid water and the viscous flow of humid air and water are considered. The boundary layer and interfacial balance equations are derived.
The model of moisture concentrations in a building cavity containing hygroscopic material presented in earlier works is extended to allow for evaporating surfaces within the cavity (eg soil, water tanks) and fluctuating external climatic conditions. Linearized coupled differential equations are solved for three cases - 1. Steady state 2. Step function 3. Periodic climate driving forces. The third case gives formulae predicting the cavity moisture contents at any time of day or year, and shows that the steady state approximation is adequate for all but the tightest cavities.
Examines CO2-controlled ventilation for a variety of buildings. A theoretical study shows that the modification of the ventilation rate which can be obtained by the control of a 2 speed fan or by variation chimney cross-section enables the ventilation rate to be independent of external conditions (wind, temperature) and to produce annual energy savings of the order of 1500-2000 KWhr for a 100m2 house.
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