The fundamental properties o f air and moisture mixtures are complex. The Chartered Institution of Building Services Engineers (CIBSE) publish the main UK guides to the fundamental theory relating to the properties o f humid air, water and steam and to moisture transfer and condensation in buildings. Guides Cl and C2 (CIBSE 1975) contain psychrometric formulae for determining the basic properties of humid air, water and steam. Formulae are provided for the determination of percentage saturation, relative humidity, moisture content, enthalpy, specific volume, vapour pressure and saturated vapour pressure. Guide AlO (CIBSE 1986) considers the theory of the behaviour o f water in buildings and provides useful information regarding material hygroscopicity, equilibrium moisture content, evaporation and condensation at plane surfaces, vapour diffusion in materials and condensation in structures by calculation and by the graphical method.
The American Society of Heating Refrigeration and Air conditioning Engineers (ASHRAE) publish the main North American design guide for moisture in building and construction (ASHRAE 1989). The guide describes the fundamental theory behind moisture movement and vapour migration by diffusion. It also describes methods for predicting surface and interstitial condensation in buildings.
A simple steady-state moisture model used to predict the internal vapour pressure in buildings based upon the external vapour pressure, moisture production rates and the ventilation rate was proposed by Loudon in 1971 (Loudon 1971). The same steady state moisture model is used as the basis for the moisture calculation incorporated into the current British Standard for condensation in buildings (BSI
1989). This simple model does not account for moisture admitted into materials or diffused through them. The same steady state moisture model has been attached to the BREDEM-12 model and forms the basis of the NHER Evaluator condensation analysis (NES 1994).
The BRE have developed a moisture admittance model that accounts for the absorption and desorption o f moisture (Jones 1993). An absorption coefficient (a) and a desorption coefficient (P) are added to the basic steady-state model introduced by Loudon. Values for a and P can be determined under equilibrium conditions for any room. The paper acknowledges the basic steady-state moisture addition to the BREDEM model (Boyd et al 1988) and proposes an admittance formulae that could be incorporated into the BREDEM model to improve its prediction accuracy. Further validation o f the BRE admittance model and the determination of the a and p coefficients has been carried out by monitoring relative humidity under real dynamic conditions (Serive-Mattei et al 1993). The moisture admittance model is discussed in more detail in section 4.2.3.
2.5.3 Combined thermal and moisture models
The last 20 years has seen substantial developments in combined thermal and moisture models for predicting environmental conditions in buildings. There are empirical and theoretical models ranging from the relatively simple to the very complex.
Although most o f the combined models are theoretical there are some which are based entirely on observation and experiment. An empirical model called the Moisture Assessment Prescriptive Procedure (MAPP) was developed to assess the appropriateness o f energy conserving measures for Canadian dwellings (Platts 1988). The model assesses two impacts; air leakage and moisture production of
occupants, which are used to predict an internal relative humidity. A critical humidity is determined based upon the regional climate and the construction of the house. By comparing the predicted humidity with the critical humidity each dwelling is given a positive or negative safety margin which determines the most appropriate retrofit measures.
There exists a large number o f combined thermal and moisture models which it is appropriate to separate into steady-state and transient versions. Of the steady state models there is a further division since some of the models are basic whilst some are able to account for issues such as the absorption and desorption o f moisture by the building fabric and the furnishings.
The steady-state thermal and moisture models first introduced by Loudon (1971) form the basis o f the current British Standard relating to condensation in buildings (BSI 1989). The same formulae have been introduced into the BREDEM-12 energy model to produce a version able to perform condensation analysis for a variety of different heating scenarios (NES 1994).
Other steady-state models assume a critical relative humidity as a starting point and assess how this can be achieved by adjusting input parameters. Optimum ventilation rate and minimum energy input are common in these models. A model developed by Finbow (1982) determines the optimum dwelling ventilation rate at which the heat input necessary to prevent the internal relative humidity rising above 70% is at a minimum. A similar model which examines the ventilation requirement in dwellings to prevent condensation from occurring has been developed (Meyringer 1985), and another model has been used to examined the role of ventilation systems with heat recovery as a means to reducing condensation in existing housing (Awbi & Allwinkle 1986).
It has been recognised for some time that the absorption and desorption of moisture by the fabric of the building and the furnishings is significant in accurately predicting internal relative humidity in buildings, particularly when
State models that account for moisture admittance. An American model has been developed for calculating indoor humidity in dwellings which takes into account the absorption and desorption of moisture by the building fabric (Tenwolde 1988). The model incorporates a single storage parameter which is positive during absorption and negative during desorption.
An extensive review of the current methods developed for predicting internal relative humidity which account for the effect o f the absorption and desorption of moisture by the building materials and furnishings has been carried out (Jones
1995). Seven methods are described including the BRE admittance method (UK), TEA annex 14 methods (Europe), the Tsuchiya method (Japan), the IBP method (Germany) and the EDF and CSTB methods (France). The conclusions of the review indicate that there is no single method appropriate to all types of humidity calculation.
There exists a large number of transient combined thermal and moisture models. These models are by their nature complex and in most cases require computers to determine the outcome of the calculation because o f their dynamic nature. Most models have been developed for very specific applications and therefore they are not always relevant to all situations.
Apart from models developed for very specialised applications, there are a number of transient models developed for use in domestic situations. Simple dynamic models, requiring relatively few input parameters, have been developed which assess the impact of ventilation and moisture production rates on the air moisture content (Letherman 1988) (Taylor 1997). Other more sophisticated models account for a large number o f input parameters including moisture admittance, air movement between spaces, indoor evaporation and condensation and internal moisture generation (El Diasty et al 1992) (El Diasty et al 1993a). There are models that examine the limiting conditions for avoiding mould growth (Letherman 1989) and some which have been developed for specific types of buildings including air-conditioned buildings (Isetti et al 1988) and dwellings (Kusuda 1983) (De Wit 1990). Strathclyde University has developed a model
which predicts mould growth using the ESP-r transient thermal model (Rowan et al 1997).
Other transient models have been developed which account for many input parameters including moisture stored in materials, moisture brought in with outside air, moisture from groundwater through basement constructions and condensation to, and evaporation from, windows (El Diasty et al 1993b) (Barringer & McGugan 1989).
More complex transient models have been developed which use computational fluid dynamics (CFD) (Gan 1996) and finite element methods. Cunningham (1990) has developed a finite element difference model for heat and moisture transfer which has been validated by experiments carried out using different types of flat roof construction. Kerestecioglu et al (1989) also describes the development o f a model that is capable o f solving various transport phenomena using finite element methods. The authors admit that although the programme offers extensive capabilities the input file preparation is tedious and is intended for users with an extensive numerical analysis background.
Table 2.22 indicates the combined heat and moisture models that have been reviewed.
Group M odel type Developed by
Empirical models MAPPS Platts 1988 Steady-state models Basic models Loudon 1971
Finbow 1982 Meyringer 1985
Awbi & Allwinkle 1986 Boyd et al 1988
BSI 1989 lEA 1991 Stum 1992 Lombardi 1995 Moisture admittance models Tenwolde 1987
Jones 1993
Serive-Mattei 1993 Transient models General transient models Kusuda 1983
Letherman 1988 De Wit 1990 Cunningham 1990 Pederson 1991 El Diasty et al 1992 Gan 1996 Rowan et al 1997 Taylor 1997 Air conditioned buildings Isetti et al 1988
El Diasty et al 1993
Multiple zone buildings Barringer & McGugan 1989
Table 2.22 - Theoretical combined heat and moisture models
A review of computer based combined heat and moisture models for building analysis has also been carried out (Trechsel 1994). The models were reviewed during the production of IE A Annex 14 (LEA 1991a) and altogether twenty-eight models from Europe and North America are described and arranged into seven
groups according to model dimensionality. The models and their country of origin are shown in Table 2.23.
Group Name Organisation and
country
ID heat and moisture transport models Wand Belgium
Glasta Belgium HAMPI Canada MATCH Denmark TMB France V30 France WFTK Germany JOKE Germany CONDO Germany HYGRO Netherlands P1200A Sweden FKUT74:6 Sweden BREC0N2 UK MOIST USA
2D heat and moisture transport models CHEoH France
TONY France
V320 France
WUFIZ Germany
VADAU Sweden
2D heat and air transport models NatKon Belgium
ANHCOMP Sweden
3D heat and air transport models WISH-3D Netherlands ID heat, air and moisture transport models WALLDRY Canada 2D heat, air and moisture transport models EMPTEDD Canada
TCCC2d Canada
TRATM02 Finland 3D heat, air and moisture transport models Konvek Belgium