MOISTURE CONTROL AND PROBLEM ANALYSIS OF · PDF fileThe funda-mentals of moisture control and the application of hygrothermal models for problem analysis referring to heritage constructions

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MOISTURE CONTROL AND PROBLEM ANALYSIS OF HERITAGE CONSTRUCTIONS

Hartwig M. Künzel* [email protected]

Andreas H. Holm† [email protected]

Abstract Buildings belonging to the cultural heritage are valuable assets and must

be preserved for future generations. Therefore it is important that damaging temperature or humidity conditions caused by inappropriate construction measures are avoided. Even if the construction remains unaltered, changes in building operation may lead to moisture problems within the structure. In or-der to assess the effects of renovation or rehabilitation measures it is useful to study the moisture behaviour of all building components concerned. This can be done by monitoring the relevant conditions with sensors or by hygrother-mal simulation with transient calculation tools. While monitoring will only provide the current status, hygrothermal simulations can also predict what will happen when the construction is altered or the indoor climate changed.

Effective moisture control has to respond to the exterior as well as the in-terior moisture loads acting on building constructions. The paper explains the fundamentals, required input data and obtainable results of hygrothermal simulation models. It demonstrates how moisture problems may be investi-gated and eventually prevented by hygrothermal analysis. Finally, examples of European rehabilitation guidelines whose moisture control requirements were developed by hygrothermal analysis are given.

Keywords: Moisture Control, Hygrothermal Analysis, Climatic Impact, Rehabilitation Guidelines.

1 Introduction

Moisture control has become a world wide issue because building opera-tions and construction practices have been changing. In the industrialized countries the need to save energy has resulted in better insulated and airtight envelope systems which are more sensitive to moisture problems than the tra-ditional, poorly insulated constructions. The demand for higher thermal com-

* Dr.-Ing.. Head of Department Hygrothermics, Fraunhofer-Institute for Building Physics. † Dr.-Ing.. Head of Department Indoor Climate, Fraunhofer-Institute for Building Physics.

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fort increased the number of heating and air-conditioning systems installed in buildings. However, if the building envelope has not been designed to handle the newly imposed temperature and vapour pressure gradients, mould growth or condensation may occur. On the other hand, heritage buildings can only survive if properly maintained. This means capital is required which will only be available if the building provides suitable indoor comfort conditions.

Therefore, appropriate moisture control is a prerequisite for energy effi-cient and damage free design of restoration and rehabilitation measures for ex-isting buildings. However, good moisture control design depends on a variety of parameters such as climate conditions and construction type which changes from region to region. Therefore it is almost impossible to establish general rules that apply everywhere and for each construction. This is the reason why considerable scientific effort was committed to the development of hygro-thermal models that help to predict the transient temperature and humidity conditions in building envelope components like walls and roofs. The funda-mentals of moisture control and the application of hygrothermal models for problem analysis referring to heritage constructions are described in this paper.

2 Moisture loads

The main function of building constructions is the protection of an en-closed space from natural weather. In Figure 1 the hygrothermal loads acting on building envelope components are represented schematically for the exam-ple of an external wall. Generally they show considerable diurnal variations at the exterior surface which are propagated only to a minor extent to the interior surface of the wall. During daytime the exterior wall surface heats up by solar radiation which leads to the evaporation of moisture from the surface layer. Around sunset when the short-wave solar radiation ceases the long-wave (in-fra-red) emission may lead to overcooling (cooling down below ambient air temperature) and condensation water may appear on the façade. However, the highest moisture content is usually found when a wall is exposed to wind-driven rain. In general several load cycles are overlapping like summer/winter, day/night and rain/sun. Therefore, a precise analysis of the expected hy-grothermal loads should be carried out before starting to remodel an existing building.

2.1 Exterior and interior conditions

Ambient temperature and humidity or partial vapour pressure are the boundary conditions influencing both sides of the wall. The exterior conditions depend on the weather which shows repetitive patterns whose impacts on the

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building may be assessed by selecting representative meteorological data pro-vided for the particular location of the building.

Figure 1: Schematic representation of the hygrothermal effects and their alternating diurnal or seasonal directions acting on an external wall according to ASHRAE [1].

The indoor climate conditions depend on the purpose and occupation of the building. They are controlled to keep the interior space comfortable for human beings and/or suitable for furnishings and artefacts. However, for the building envelope they represent an important load that can be more severe than the ex-terior load especially when the indoor moisture generation is high. In museums and art galleries temperature and humidity are controlled by HVAC systems whose set-points are normally well defined. The interior conditions in residen-tial buildings are influenced by the occupant’s behaviour. In an average house-hold approximately 10 litres of water are evaporated every day. This moisture must be removed by ventilation or air-conditioning in order to assure comfort-able and hygienic conditions. Some heritage buildings like churches or castles are only temporarily occupied which means that the interior conditions may be close to the outdoor air conditions. In this case monitoring is the best way to determine the real conditions.

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2.2 Rising damp and construction moisture

When ground or surface water is wicked up in porous walls by capillary action this is called rising damp. This phenomenon my be a sign of poor drain-age or waterproofing of the building’s basement or foundation. However, there are other phenomena which show similar moisture patterns as rising damp. If the wall is contaminated with salts - a frequent problem of historic buildings - there might be a substantial amount of moisture in the wall due to increased hygroscopicity resulting from the hydrophilic properties of salt crystals. An-other reason for the appearance of rising damp may be surface condensation in massive buildings during summer time.

Construction moisture is a moisture load that is often disregarded. In Europe building damage as a result of migrating construction moisture has be-come more frequent because tight schedules leave little time for building mate-rials to dry out. Construction moisture is either delivered with the building products or it is absorbed by the materials while stored or built up at the con-struction site. Cast in place concrete, stucco, plaster, spray-applied fibre insu-lation and “green” wood are examples for materials that contain a lot of mois-ture when applied. Other porous building materials such as clay brick or natu-ral stone may take up precipitation or ground water when left unprotected dur-ing storage or when the building leaks.

2.3 Air pressure differentials

Unbalanced mechanical ventilation systems, wind and stack effects may cause air pressure differentials over the building envelope. In contrast to wind, the stack effect is a permanent load during the heating or cooling season that may not be neglected. To make matters worse stack pressure is generally act-ing in the same sense as vapour pressure; from inside out during the heating season and in the inverse direction during the cooling season. Therefore, air-flow through cracks, imperfect joints or air permeable assembly layers may cause interstitial condensation just like vapour diffusion. However, condensa-tion due to stack induced airflow is likely to be more intense and concentrated around leaks in the building envelope. Air flow through cracks and joints may also transport rain water into the building envelope. In order to avoid moisture damage air flow through the building envelope should be prevented by at least one continuous air-tight layer (air barrier).

3 Hygrothermal simulation models

In civil and architectural engineering as well as for the purpose of heritage preservation there is an increasing demand for calculative methods to assess and predict the long-term heat and moisture (hygrothermal) performance of

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building envelope components. Controlling the temperature and humidity con-ditions in a particular wall or roof assembly is a critical task to prevent damage or premature aging of building materials. The need for better hygrothermal calculation tools has been provoked by numerous moisture related failures. Since there is no doubt that moisture transfer has an important influence on performance and service life of building components, the prediction of the hy-grothermal behaviour becomes very important. In the past these predictions were largely based on experiments, practical experience and simplified calcu-lation tools, such as the steady-state dew-point method.

There is a variety of transient calculation models available (e.g. Freitas [2], Mendes [3] Trechsel [4]) which provide reliable results. In order to be consis-tent hygrothermal simulations should meet certain requirements which have been specified for the first time in the WTA Guideline 6-2 [5] (WTA = Interna-tional Association for Science and Technology of Building Maintenance and Monument Preservation). The German version of this guideline was issued in 2002 because existing calculation methods were unable to predict the hy-grothermal consequences of rehabilitation measures or indoor climate changes in the case of heritage buildings. Due to its international importance an Eng-lish version of this guideline was published only two years later. The most re-cent WTA Guideline concerning hygrothermal simulations deals with the evaluation of calculation results in order to predict the risk of mould growth on building materials [6].

In the meantime the demand for hygrothermal simulation tools for design and forensic investigation purposes has increased considerably. As a conse-quence the European Standard EN 15026 [7] which is largely based on WTA Guideline 6-2 has been issued in 2007. This proves that the need to preserve heritage constructions has contributed substantially in pushing forward new technologies not only in the field of restoration and conservation but also in general building science. In order to comply with WTA 6-2 or EN 15026 tran-sient hygrothermal simulation tools have to include the following transport and storage phenomena:

— heat storage of the dry building material and of the contained moisture;

— heat transport by thermal conduction with moisture-dependent thermal conductivity;

— latent heat transport by vapour diffusion with phase change (vapour evaporation/condensation);

— moisture storage by water vapour sorption and capillary forces;

— water vapour transport by diffusion;

— liquid transport by surface diffusion and capillary conduction.

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As an example for the implementation of these phenomena the differential

equations employed for the well-established WUFI® model [8] are presented here: Moisture balance (1) Energy balance (2) where: φ relative humidity, t time, T temperature, w moisture content, psat saturation vapour pressure, λ thermal conductivity, H enthalpy, Dφ liquid conduction coeff., δp vapour permeability, hv latent heat of phase change.

The left hand side of the moisture balance represents the moisture storage

which is proportional to the derivative of the water retention curve (∂w/∂φ). The transport terms on the right hand side are described by the divergence of liquid and vapour flow. While vapour pressure (pv = φ·psat) which is the driving force for vapour flow strongly depends on temperature (saturation pressure psat ~ exp(T)), liquid flow is governed by capillary forces which are a function of relative humidity φ only (Kelvin equation).

The divergence of vapour flow multiplied by the specific heat of evapora-tion represents the latent heat term on the right hand side of the energy bal-ance. The sensible heat flow is represented by the divergence of heat conduc-tion. The storage term on the right hand side of the energy equation – de-scribed as derivative of total enthalpy – contains the heat capacity of the dry material plus the enthalpy of absorbed water which depends on its state (solid, liquid or gaseous phase).

( )( )satp pDt

w φδφφ∂φ∂

φ ∇+∇⋅∇=∂∂⋅

( ) ( )( )satpv phTt

T

T

H φδλ∂∂ ∇⋅∇+∇⋅∇=

∂∂⋅

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Figure 2: Flow chart for hygrothermal simulations in prEN 15026 [9]. This chart has been dropped in the final version of this standard En 15026 [7] only because charts

are discouraged due additional efforts required for translation into the different Euro-pean languages.

3.1 Input and output data

The execution of hygrothermal simulations including inputs and outputs is best described by a flow chart from prEN 15026 (2004) in Figure 2. As first input, composition and exposure (orientation and inclination) of the building component under investigation are required. Then the material parameters for all construction layers have to be selected from a material database. The neces-sary material properties include:

— Bulk density ρ in kg/m³, serves to convert the specific heat by mass to that by volume;

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— Specific heat capacity c in kJ/(kgK);

— Thermal conductivity λ in W/(mK) of the dry material and its moisture-dependence;

— Porosity ε in m³/m³, which determines the maximum water content wmax;

— Moisture storage, i.e. sorption and suction isotherms w = f(φ) in kg/m³ that give the equilibrium moisture content of materials as function of relative humidity in both the hygroscopic and the capillary water (over-hygroscopic) range (see Figure 3);

— Vapour permeability in kg/(m²sPa) which may dependent on the ambi-ent air humidity;

— Liquid diffusivity Dw in m²/s both for water uptake and redistribution res. drying of materials as a function of moisture content. Multiplying Dw by the derivative of the water retention curve gives the liquid con-duction coefficient Dφ.

Figure 3: Moisture storage function of reed sandstone from Sand (Germany) deter-mined from sorption and pressure plate tests [10].

If present, liquid transport dominates vapour diffusion by some orders of magnitude. Therefore it has to be considered carefully when liquid water has an impact on the building component e.g. when wind-driven rain hits a cavity wall or a solid wall made of natural stone. In contrast to thermal conduction or vapour diffusion, liquid transport is highly non-linear because the diffusivity often shows an exponential increase with water content. As an example for typical liquid diffusivity functions of mineral building materials, the functions

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Dw= f(w) of a reed sandstone from the quarry in Sand (Germany) are plotted in Figure 4. The liquid diffusivities used to simulate water absorption and re-distribution vary by a factor of almost 100 between the equilibrium moisture content at 80% RH (starting point of the curves) and the free water saturation (w/wf = 1) of the material. Therefore liquid transport is very important when a porous material is wet while its influence decreases rapidly when the material dries out.

Figure 4: Liquid diffusivity of reed sandstone from Sand determined from transient water absorption and redistribution measurements by NMR scanning [10]. The liquid transport coefficients, also called liquid diffusivity (separate functions are used for

simulating water absorption and drying processes) depends exponentially (logarithmic scale of y-axis) on the water content of the material.

When all materials and their hygrothermal properties in the investigated assembly are specified, it is time to select the boundary conditions. In order to obtain a realistic simulation of the hygrothermal performance of building com-ponents exposed to natural weather the following climatic parameters must be provided:

— exterior air temperature;

— exterior relative humidity;

— short-wave radiation (global & diffuse solar radiation);

— long-wave radiation (thermal sky radiation);

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— precipitation;

— wind speed and direction;

— interior air temperature;

— interior relative humidity.

Due to the diurnal changes of the exterior climate conditions hourly mete-orological data sets are generally required. Depending on the purpose of the hygrothermal analysis, data from extreme years (see EN 15026 [7]) may be more suitable than average meteorological data (e.g. Test Reference Years) that are normally used for energy calculations. Since the interior climate shows less fluctuation daily or even monthly mean data may be sufficient for many appli-cations.

The impact of the exterior and interior climate conditions on the building envelope is also governed by the so-called surface transfer that describes the transport processes at the exterior and interior surfaces. Usually there is a stagnant film of air at the surface which acts as a resistance to heat and vapour flow. This film resistance depends inversely on the air flow velocity at the sur-face and is therefore usually higher at the interior side of the building (realistic transfer coefficients are specified in WTA 6-2 or EN 15026). The heat transfer coefficients often include the heat exchange by long-wave radiation which is o.k. for the interior surface but may lead to inaccurate results at the exterior side. Especially when exterior condensation (and subsequent façade deface-ment due to algae und fungi) is an issue, an explicit radiation balance includ-ing long-wave radiation to the sky is required.

The effect of the solar radiation on the exterior surface is determined by the short-wave absorptivity as well as the long-wave emissivity of the surface layer. Because metals have generally a lower emissivity (low-E) than non-metals (E ≈ 0.9) metallic surfaces become hotter during solar radiation than other surfaces with similar short-wave absorptivity. On the other hand, the low thermal emissivity of a metal foil can help to reduce the heat flow towards the interior of the building during intense solar radiation (radiant barrier).

The last input block in Figure 2 contains the initial conditions and calcula-tion specifications (duration, numerical accuracy, etc.). Since the temperature distribution over the building component adapts normally comparatively fast to the boundary conditions a uniform temperature close to the expected mean can be selected as start value. Moisture transport processes are, however, rather slow. Therefore a realistic initial distribution may be important for the outcome of the simulation. If construction moisture is present it must be speci-fied. For initially dry building materials it is good practice to start with their equilibrium moisture content at 80% relative humidity.

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The hygrothermal simulation results shown in the two output blocks in Figure 2 consist of heat and moisture fluxes and transient cross-sectional dis-tributions of temperature, relative humidity and water content. They may be presented as profiles for specific points in time or as temporal variations at a specific location within the building component. A common presentation of the results is the stepwise visualization of the transient distributions in a movie like fashion. In order to assess the hygrothermal performance of a particular building component, these results have to be interpreted which may require some expert knowledge.

3.2 Result evaluation and interpretation

At first it should be checked if the resulting variations of temperature, rela-tive humidity and water content in the different materials do not exceed the limits specified for these materials. Then the annual moisture balance should be analyzed. Building components containing some initial moisture should dry out. Continuous moisture accumulation is usually a sign for failure. However, these failure criteria may not be sufficient to ensure a good long-term perform-ance of the building component. Therefore special post-process models may be required for a satisfactory result interpretation. Figure 2 tentatively (blocks in grey) shows the need for models that use the hygrothermal results as input data and compare them to different performance criteria such as energy consump-tion, risk of corrosion, mould growth or rot.

4 Application example: exposed natural stone wall

In order to develop better means to preserve monumental heritage build-ings such as castles and cathedrals made of natural stone the German ministry for research and technology launched a project that focused on the moisture behaviour and related degradation processes of natural stones. Within the frame of this project the hygrothermal properties of several species of natural stone were determined as basis for numerical simulations. Parallel to the inves-tigations in the laboratory, a field test was carried out exposing stone samples at one side to natural weather and at the other side to a well-defined interior climate. The detailed recordings of the boundary conditions on both sides of the stone samples also served as input data for the hygrothermal simulations. During the field test the natural stone samples which formed part of a wall where periodically removed and weighed to determine their water content. In addition the samples were scanned from time to time by a nuclear magnetic resonance (NMR) apparatus in order to obtain an instant moisture distribution profile over the cross-section of the wall.

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Figure 5: Top: Temporal variations of the integral water content of the exposed natu-ral stone samples determined by weighing in comparison with the simulation results. Bottom: Moisture distributions in the samples determined by NMR scanning at two

points in time compared to the simulated moisture profiles for the same dates.

A comparison of the experimental results with the results of hygrothermal simulations by the WUFI® model is presented in Figure 5. The top graph shows the total water content variations of three samples of the same natural stone species (reed sandstone from Sand)) compared to the calculated curve. The steep increases in water content at several occasions during the observa-tion period are due to driving rain events which interrupted by bright spells re-sult in a slow moisture accumulation of the originally dry samples. In the bot-tom graph the measured and calculated moisture distributions over the cross-section of the natural stone wall (the samples are part of the wall) are com-

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pared for two distinct points in time. The presented temporal variations as well as the moisture distributions show an excellent agreement between hygrother-mal simulation and experiment. This proves that the hygrothermal model de-scribes the moisture behaviour of exposed natural stone walls accurately. It should be noted, however, that these results are based on a detailed analysis of the hygrothermal properties in the laboratory and on-site registration of the in-terior and exterior climate conditions including hourly measurements of the driving rain load by rain gauges integrated in the test façade. Additionally, natural stones represent an ideal material concerning homogeneity and vari-ability which means their properties show no alterations with location and time.

The comparison with the experiment inspires confidence in the simulation results. Since walls of heritage buildings are usually thicker than the wall in the field test, the simulations were repeated with the same material properties and with a typical meteorological dataset for a wall of 60 cm thickness. The aim of this study was to get an average moisture and temperature distribution in such a wall as a basis for further analysis of degradation processes. There-fore the simulations were run with the same annual climate conditions until a dynamic equilibrium was achieved. Since problems often occur at walls ex-posed to wind-driven rain a western orientation has been selected.

The results are plotted in Figure 6 as annual variation range and mean dis-tribution of temperature and water content. Due to the dark beige colour of the sandstone the maximum surface temperature in summer may exceed 40 °C. The lowest temperature is close to the minimum air temperature in winter. To-wards the interior of the building the temperature variations are gradually dampened and the annual mean distribution (solid line in top graph) forms a straight line between the average exterior and interior conditions which can be expected because heat transfer is described by a linear differential equation.

However the picture changes when the moisture distributions in the bottom graph of Figure 6 are analyzed. First of all, the annual variations in moisture content due to natural weather do not affect the interior side of the wall; in fact the climatic influence does not go beyond the exterior third of the wall. Even more surprising is the shape of the mean distribution which does not form a straight line. Similar to the thermal processes, the fluctuations in water content due to direct impact of hygrothermal load cycles (wind-driven rain / solar ra-diation) peak at the exterior surface. However, the average water content at the surface lies below 3 vol.-% which is almost the same as at the interior surface. Some Millimetres beneath the exterior surface the average water content rises steeply and reaches a maximum of approx. 7 vol.-% at a depth of about 5 Cen-timetres. This phenomenon is caused by the strong – non-linear – dependence of liquid transport on water content which has already been discussed by refer-ring to the liquid diffusivity functions in Figure 4.

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Figure 6: Annual variation (colored zones) and mean distribution (solid lines) of tem-perature and moisture content in an external wall made of reed sandstone. These re-sults are obtained by running the simulations with the same meteorological dataset of

one year until a dynamic equilibrium is reached.

The practical consequences of this moisture behaviour for porous materials exposed to wind-driven rain are manifold. Material degradation is often caused by dilatation processes in the micro-structure due to the expansion of freezing water or crystallizing salts. Especially in the case of frost the risk of damage increases with water content. This would explain why the material layer be-neath the surface is often more degraded than the exposed surface layer, which may lead to spalling or delaminating of the exterior crusts. There are also posi-tive aspects. Rain water hitting the surface is absorbed and stored in a layer some distance beneath the surface which is not accessible to micro-organisms. Stored rain water in concrete façades prevents carbon dioxide diffusion and therefore protects the layer with the reinforcing steel from carbonation and consequently the steel from corrosion.

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5 Rehabilitation guidelines and moisture control

Some heritage constructions are more vulnerable to moisture loads than modern buildings. Figure 7 depicts a typical example for a moisture sensitive building with a half-timbered structure. The beauty of the timber-framed fa-çades of such buildings contributes to the attraction of old towns which ex-plains why their visible structure should not be covered cladding or an exterior insulation system (e.g. ETICS). If such a façade is exposed to wind driven rain some water will penetrate into the joints between the studs and the filling (mostly consisting of clay or fired brick masonry) as demonstrated in Figure 8, left. In order to avoid moisture damages the driving rain load has to be limited (traditionally timber-framed houses have not been erected in regions with high precipitation loads) and the drying potential to both sides of the wall must be sufficient as indicated on the right hand side of Figure 8. This is usually the case because most of these buildings are several hundred years old and those with an insufficient drying potential do not exist any more. However, if the drying potential is altered by rehabilitation measures problems may arise.

Figure 7: Half-timbered heritage house in Fritzlahr (Germany).

Most half-timbered houses in Germany have a wall thickness of 12 – 16 cm which means that their thermal performance is quite poor. In the long run these houses can only be preserved if their indoor comfort and energy consumption lives up to modern expectations. Therefore, an additional insulation is neces-sary which may only be applied at the interior of the wall for esthetical rea-

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sons. If a vapour retarder is installed over the interior insulation the wall’s dry-ing potential to the interior will be severely reduced and without a vapour re-tarder there is too much condensation on the wooden studs during the heating period. In order to account for both effects, the German standard for moisture control of constructions DIN 4108-3 [11] prescribes a very narrow diffusion resistance range (1.0 m ≤ sd ≤ 2.0 m) for the interior insulation including va-pour retarder. This compromise between prevention of interstitial condensation and optimization of the drying potential is based on hygrothermal simulation results and several experimental studies.

However, even with such a compromise the application of an interior insu-lation poses still a risk because the drying potential of the wall without insula-tion is much higher for two reasons. Firstly, there is almost no diffusion resis-tance towards the interior and secondly, the wall is warmer which also acceler-ates the drying process. There is nothing that can be done against the cooler wall once the interior insulation is in place because it is the prime purpose of the insulation to keep the heat in the house. There is, however, the possibility to enhance the drying potential towards the interior by applying a vapour re-tarder with variable diffusion resistance [12] or by using capillary active insu-lation materials such as Calcium-Silicate boards which wick water away from the surface of the original wall by capillary action.

Figure 8: Rain water penetration and drying potential by vapor diffusion to the exte-rior as well as to the interior of a timber frame wall.

6 Conclusions and future perspective

In recent years, hygrothermal simulation models have become useful tools for moisture control of buildings. This includes the moisture analysis of walls

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and roofs of heritage constructions that must be dealt with special care when restoration or rehabilitation measures are envisaged. Hygrothermal simulations have been successfully employed to predict the moisture behaviour of building materials and envelope systems under different indoor and outdoor climate conditions. There are a number of ways moisture can enter the building enclo-sure and interstitial condensation due to vapour diffusion which has been dealt with extensively in the past is hardly the most important one. Actually, the most important and unexpected damage cases were caused by rain water pene-tration sometimes coupled with solar radiation (solar vapour drive).

Another important source of moisture in old masonry buildings is rising damp. There are many counter measures such as sawing and sealing, bore hole injection and different (mostly ineffective) electrical methods on the market to tackle this problem. In some cases it is enough to exchange the old render or plaster with a new more vapour permeable render or plaster. If salts are re-sponsible for the rising damp effect, the application of a salt extracting render (renovation mortar systems) specified in [13] may be the best solution. The problem of rising damp can also be modelled if a two-dimensional hygrother-mal simulation tool is available, e.g. [14]. However, due to the higher com-plexity of 2D calculations and greater uncertainties in the boundary conditions the results may be less accurate. But they can still indicate the potential prob-lem and help to select appropriate solutions.

Hygrothermal simulations offer the opportunity to find the most effective

solution between two opposing tasks: to prevent or at least limit the moisture entry into a building component and to let moisture, that has entered, dry out as fast as possible. Design optimizations that result in more moisture tolerant constructions are already encouraged by the European standard EN 15026 [7] and even more importantly by the new ASHRAE Standard 160 [15]. Last but not least there is also a working group at ASTM that will issue a new Standard on hygrothermal simulation models in the near future.

By combining hygrothermal building envelope simulation with whole building simulation it is also possible to predict the interior climate based on natural weather and building operation. This may be especially important for heritage constructions, old furniture and artefacts because they need more sta-ble indoor conditions. The variation of the relative humidity is strongly influ-enced by the buffering effects of the building envelope materials as well as the transient outdoor conditions. For the development of suitable ventilation and heating strategies these effects have to be taken into account. Several whole building models for the simulation of the heat and moisture transfer effects which influence the indoor climate have been developed and validated recently within the frame of the international project IEA-Annex 41 “MoistEng”. Ramos [16] for example studied the effect of moisture buffering materials.

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7 References

[1] ASHRAE (American Society for Heating, Refrigerating and Air-conditioning Engineers): Handbook of Fundamentals 2009 Chapter 23.

[2] Freitas, V. P. Transferência de humidade en paredes de edeficios, Disser-tation Universidade do Porto, 1992.

[3] Mendes, N., et al. UMIDUS : A PC Program For The Prediction Of Heat And Moisture Transfer In Porous Building Materials. Proceedings of Building Simulation ’99, Volume 1: pp. 277-283.

[4] Trechsel, H.R. 2001: Moisture Analysis and Condensation Control in Building Envelopes. American Society for Testing and Materials (ASTM) Manual 40.

[5] WTA Merkblatt / Guideline 6-2 D/E: Simulation wärme- und feuchte-technischer Prozesse / Simulation of Heat and Moisture Transfer. May 2002 / Oct. 2004.

[6] WTA-Merkblatt 6-3 D: Rechnerische Prognose des Schimmelpilzwachstumsrisikos. Dec. 2005.

[7] EN 15026: Hygrothermal performance of building components and build-ing elements – Assessment of moisture transfer by numerical simulation. April 2007.

[8] Künzel H.M.: Simultaneous Heat and Moisture Transport in Building Components. - One- and two-dimensional calculation using simple pa-rameters. IRB Verlag 1995, http://publica.fraunhofer.de/eprints/urn:nbn:de:0011-px-566563.pdf.

[9] prEn 15026: Hygrothermal performance of building components and building elements – Assessment of moisture transfer by numerical simula-tion. August 2004.

[10] Krus, M.: Moisture Transport and Storage Coefficients of Porous Mineral Building Materials - Theoretical Principals and New Test Methods, 1996.

[11] DIN 4108-3: Protection against moisture subject to climate conditions. July 2001.

[12] Künzel, H.M.: Adapted vapour control for durable building enclosures. Proceedings 10DBMC, Lyon April 2005, TT2-86.

[13] WTA-Merkblatt 2-9/04: Sanierputzsysteme (renovation mortar systems). [14] Torres, M. I. M. Modelização da transferência da humidade – Humidades

ascensionais, Dissertation Faculdade de Ciências e Tecnologia da Univer-sidade de Coimbra, 2006.

[15] BSR/ASHRAE Standard 160P: Criteria for Moisture Control Design Analysis in Buildings. Public review draft Sept. 2008.

[16] Ramos, N. M. M. A Importancia da Inercia Higroscopica no Comporta-mento Higrotermico dos Edificios, Diss. Universidade do Porto, 2007