Project Description

climatic change consequences.  geometrical details , 4 Ds: Deflection, Drainage, Drying, Durability,  flooding,  rising damp,  freeze-thaw cycling, wind-driven rain,  precipitation,  Moisture, temperature, convection, UV exposure ,

Carsten Rode, Associate Professor, Ph.D., Department of Civil Engineering, Technical University of Denmark

Birgitte Dela Stang and Morten Hjorslev Hansen, Senior Researchers, Ph.D., Danish Building Research Institute

Purpose, hypotheses and relevance

Moisture and temperature levels and variations in time and space play a crucial role in degradation processes of building materials, such as silicate materials, metals and polymeric materials where also UV-radiation is a very important factor. An exterior wall can consist of more than 10 different material layers. Furthermore, a wall element is often inhomogeneous in the plane because of counteracting structural and insulating properties. The moisture and temperature conditions inside such a wall are highly dependent on the material combinations and the climate conditions on both sides of the wall. The main parameters are listed in the keyword list above. 

    The background and motivation for the project is that most damages that happen to building components occur in places with a complex geometry that cannot be handled correctly by today’s design tools. This could for instance be where different materials meet in joints and where conditions most often have a multidimensional nature. Also apparently regular construction elements have multidimensional parts and features whose hygrothermal conditions should be considered better in the design of buildings, e.g. near beams and columns in common building elements. These loci often represent thermal bridges in the constructions, and they involve the assembly of different components and materials, so there is an increased risk of unintentional airflow or accumulation of moisture. The combination of these factors too often lead to the degradation of materials.

The purpose of the project is to develop a computational model for multidimensional transient Heat, Air and Moisture (HAM) flow in building constructions. The model will provide a suitable toolbox for fast and sound computations of moisture and temperature conditions in building components.

    This project intends to produce a tool that enables the analysis of conditions leading to degradation of building components. Critical temperature and moisture conditions and UV-exposure are partly known and partly collected from field tests and controlled experiments in a laboratory environment.

    It is the intention that with such a combination of models and systematic collection of empirical knowledge, it will be possible to predict better those degradation processes of building products, which are realised in practice.

Multidimensional Heat, Air and Moisture Transport in building components can be modelled. With such models and knowledge about degradation mechanisms, durability and performance of build-in building components can be estimated.

A computational model for multidimensional transient Heat, Air and Moisture flow in building constructions that is verified against measured data will provide better understanding of the mechanisms of transient flow in building materials. The theories behind such models are well known and used in numerous fields of research. Still, complete understanding of the significant parts and parameters of these complex combinations of different models is missing. The model will be a powerful tool to improve this knowledge.

    Deterioration and service life prediction of building materials is generally based on empirical knowledge. One of the reasons for this is the lack of knowledge of the alternating moisture and temperature conditions in the materials. The history of these conditions is crucial for prediction of growth of fungus or deterioration by rot in biological materials. Moisture, temperature and UV-radiation are central to degradation processes of polymers. Access to computations of moisture and temperature conditions in building components will improve the basis for better predictions of deterioration and service life of building components. As such it will improve the link between laboratory studies of deterioration and observations of deterioration in practice.

    A computational model for transient and multidimensional Heat, Air and Moisture flow in building constructions that is verified will serve as a “virtual moisture lab”. I.e. accelerated tests run in climate simulators in the laboratory can be validated against simulations run in the computational model. Simulations may even replace many expensive tests run today.

    The influence of climate changes on existing and future buildings, e.g. from severe rainfalls and floodings will add significantly to the relevance of the proposed project – not least for future applications.

Combining computed moisture and temperature levels with knowledge of deterioration mechanisms will allow for improved predictions of service life that is an essential parameter in calculations of life    cycle costing and in life cycle analysis.

    Based on previous experiences from a one-dimensional model for combined heat and moisture transfer, it is evident how an original research tool can really improve the technical skills in the building industry after it has been prepared for utilisation in practice by actors in the building industry.

Societal relevance

The building industry is one of the largest industries in Denmark. In 2004 a total of 120 billion DKK was invested - 30% of these investments were placed in major repair. It is evident that increased service life of such large investments will play an important economic role.

The project will set up, test and verify a coherent suite of hygrothermal and degradation models to predict and improve the durability of building assemblies

    The project is carried out as a collaborative Ph.D.-project between the Danish Building Research Institute and the Technical University of Denmark, and it will involve research competence of the Danish Building Research Institute, as well as supervision from the Technical University of Denmark.

The plans for development of a comprehensive HAM model are ambitious, but it will be realized using experiences from models in 1D earned by the applicants over the past more than 15 years.

The 2D model should comprise the following issues:

     –Moisture flow in materials by vapour diffusion, capillary suction and convective air flow.

     –Materials properties vary with moisture content, so the model will be strongly non-linear.

     –The model should comprise hysteresis and temperature dependency in the description of the characteristics for moisture retention.

     –Heat transfer in materials by conduction and by latent heat transfer.

     –Heat and mass transfer in cracks by convection.

     –Surface heat transfer phenomena by convection and radiation, incl., long wave radiation to the sky and other ambient conditions.

     –Moisture sources at exterior surfaces such as from wind driven rain or soil moisture.

     –UV-A and UV-B irradiation on exterior surfaces.

     –Provision of new data and established knowledge from research in durability and degradation mechanisms. Especially looking to the potential for replacing old, time consuming and expensive experimental investigation techniques.

     –Full scale tests in climate simulator of multi-layer wall elements to generate validation data. The test will result in time series of temperatures, static air pressure, relative humidity and air velocity in cross-sections of the wall elements in addition to time series of the boundary conditions.

     –The majority of material properties (retention curves, water vapour permeability, thermal conductivity etc.) used in the model validation will be extracted from available data. However is it anticipated that a minor part of the material properties have to be measured in the project.

A fairly recent overview of HAM analysis methods is given in ASTM (2001).

    Transient, one-dimensional models for combined Heat, Air and Moisture Transport in building component have been reasonably well established for about two decades now. Many such models were developed and benchmarked as part of the International Energy Agency’s research project “Annex 24” on "Heat, Air and Moisture Transport in Insulated Envelope Parts”. The model MATCH developed by the main applicant was among the benchmarked tools. MATCH was developed around 1990 (Rode Pedersen, 1990), and after its publication as a research model, it was as the first model of its kind prepared such that it could be used by third party users in design of components of the exterior Envelope of Buildings. The model today is still among the few which incorporate a model for sorption hysteresis and for temperature dependency of the sorption properties (Rode & Clorius, 2004).

    Some 2D transient HAM models have been developed already by researchers from for instance VTT, TU Dresden, Oak Ridge National Lab., and Florida Solar Energy Center during the past 15 years. Generally however, such tools are not in the public domain, and may only have been partly documented and validated. The models are scarcely maintained, and can very often be operated only by the person(s) who developed them. The models lack some of the features mentioned above about advanced retention properties.

    Current development in HAM-models is focusing on the whole building, e.g. such as the modelling that takes place within the International Energy Agency’s research project on “Whole Building Heat, Air and Moisture Response” (IEA 2004). The main applicant is a co-leader of the modelling subtask of this project, which has participation from 39 institutions from 4 continents in the world. Results from the STVF-project on Hygrothermal Performance of Whole Buildings (led by the main applicant) serve as valuable input for the IEA project.

    Durability of various building materials has been the subject of intensive research over the past three decades as the triennial conferences on this subject shows (CSTB, 2005). A large number of deterioration models exist describing mechanisms such as polymer degradation, corrosion of steel, carbonation of concrete, formation of microcracks due to for instance freeze-thaw action or shrinkage-swelling, growth of mould fungi, biological degradation due to wood decaying fungi and rot, etc. Few of these works have used HAM as a tool for the prediction of durability (service life). The relatively few examples have concentrated on HM transfer in monoliths of porous media. (Carmeliet, 1995) used 2D-heat and moisture transfer to predict durability of concrete repair mortars. (Johannesson, 1998, 2000) used a multi-component heat and mass transfer model to predict durability (freeze/thaw resistance and corrosion of reinforcing steel) of concrete.

DTU has for more than 15 years earned experience in the most advanced modelling of transient heat and moisture flow that can be made in one dimension (Rode Pedersen, 1990; Rode & Clorius, 2004).

    DTU has extensive experience in investigating the moisture transport properties of building materials, and has recently carried out a number of studies of the properties for moisture retention and transport under transient conditions (Hansen, 1986; Peuhkuri, 2003)

    The expertise of the Danish Building Research Institute (SBi) is in this project is concentrated on accelerated ageing (Nicolajsen & Hansen, 1999; Hansen & Nicolajsen, 1999), laboratory testing of moisture accumulation in building components using a climate simulator (Stang et al. 2002) and full scale in situ testing of moisture accumulation in a moisture test house (Hansen et al., 2002; Brandt & Hansen, 1999).

Publication of PhD-thesis and 3-5 scientific papers in international peer reviewed journals and at conferences of relevance.

Significance of results for future research/research training

    An important element of the proposed project is to convey these experiences also to a new generation of researchers. The model developed in the projected will serve as framework for future research training in the area.

^1,2,3:milestones

References

ASTM (2001) Manual 40 Moisture Analysis and Condensation Control in Building Envelopes. Editor: Heinz R. Treschsel. American Society of Testing and Materials.

Brandt, E. and Hansen, M.H. (1999) Measuring moisture content in wood with built in probes - 20+ years experience. Proc. 8th Int. Conference Durability of Building Materials and Components, Vancouver May 30-Jun 3, 1999. Vol. 1 (pp. 669-679).

CSTB (2005) Proc. 10 Int. Conf. Durability of Building Materials and Components, 17-20 Apr 2005, Lyon. Proceedings CD. CSTB. Paris.

Carmeliet, J. (1995) Computational modelling of hygrothermal damage in cementitious materials. Proc. VIII Int. Congress Polymers in Concrete, Oostende 3-5 Jul 1995.

Hansen, K.K. (1986) Sorption Isotherms – A Catalogue. Technical report 162/86. Building Materials Laboratory, Technical University of Denmark.

Hansen, M. H., Nicolajsen, A. and Stang, B. D. (2002) On the influence of cavity ventilation on moisture content in timber frame walls, 6th Symp. Building Physics in the Nordic Countries, Trondheim Jun 17-19 2002.

Hansen, M.H. and Nicolajsen, A. (1999) Protection of wood by design, Proc. 8th International Conference on Durability of Building Materials and Components, Vancouver, May 28 - Jun 3 1999, pp. 723-733.

Hens, H. & Carmeliet, J. (2002) Performance Prediction for Masonry Walls with EIFS Using Calculation Procedures and Laboratory Testing. Journal of Thermal Envelope and Building Science, Vol. 25, No. 3, pp. 167-187.

IEA (2004) International Energy Agency, Energy Conservation in Buildings and Community Systems programme, Annex 41, “Whole Building Heat, Air and Moisture Response” http://www.kuleuven.be/bwf/projects/annex41/.

Johannesson, B. (1998) Modelling of transport processes involved in service life prediction of concrete. Lic. thesis. Lund University of Technology. TVBM-3083.

Johannesson, B. (2000) Transport and sorption phenomena in concrete and other porous media. Ph.D. thesis. Lund University of Technology. TVBM-1019.

Nicolajsen, A. and Hansen, M.H. (1999) Laboratory tests on wood claddings. Proc. 5th Symp. Building Physics in the Nordic Countries, Gothenburg, Aug 24-26, 1999. Vol. 2 (pp. 661-668).

Rode Pedersen, C. (1990) Combined Heat and Moisture Transfer in Building Constructions. Ph.D. thesis. Report 214. Thermal Insulation Laboratory, Technical University of Denmark.

Performance of Exterior Envelopes of Whole Buildings IX, International Conference. Clearwater Beach, FL, USA. Dec 5-10, 2004.Modeling of Moisture Transport in Wood with Hysteresis and Temperature Dependent Sorption Characteristics.Rode, C. and Clorius, C.O. (2004)

Peuhkuri, R. (2003) Moisture Dynamics in Building Envelopes. Ph.D. thesis. Report R-071. Department of Civil Engineering, Technical University of Denmark.

Stang, B. D., Nicolajsen, A. and Hansen, M.H. (2002) Moisture in combined concrete timber-frame walls without vapour barrier. Proc. 6th Symp. Building Physics in the Nordic Countries, Trondheim, Jun 17-19.