Performance Based Building Design 1 E-Book

Hugo S. L. Hens

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 336

Veröffentlichungsjahr: 2012

Beschreibung

Just like building physics, performance based building design was hardly an issue before the energy crises of the 1970s. With the need to upgrade energy efficiency, the interest in overall building performance grew. The term "performance" encompasses all building-related physical properties and qualities that are predictable during the design stage and controllable during and after construction. The term "predictable" demands calculation tools and physical models that allow evaluating a design, whereas "controllable" presumes the existence of measuring methods available on site. The basis for a system of performance arrays are the functional demands, the needs for accessibility, safety, well-being, durability, energy efficiency and sustainability and the requirements imposed by the usage of a building.

As the first of two volumes, this book applies the performance rationale, advanced in applied building physics, to the design and construction of buildings. After an overview of materials for thermal insulation, water proofing, air tightening and vapour tightening and a discussion on joints, building construction is analysed, starting with the excavations. Then foundations, below and on grade constructions, typical load bearing systems and floors pass the review to end with massive outer walls insulated at the inside and the outside and cavity walls. Most chapters build on a same scheme: overview, overall performance evaluation, design and construction.
The book is absolutely recommended to undergraduates and graduates in architectural and building engineering, though also building engineers, who want to refresh their knowledge, may benefit. The level of discussion assumes the reader has a sound knowledge of building physics, along with a background in structural engineering, building materials and building construction. Where and when needed, input and literature from over the world was used, reason why each chapter ends listing references and literature.

Leseprobe

Contents

Preface

0 Introduction

0.1 Subject of the book

0.2 Units and symbols

0.3 References and literature

1 Performances

1.1 In general

1.2 Definitions and basic characteristics

1.3 Advantages

1.4 Performance arrays

1.5 Design based on performance metrics

1.6 Impact on the building process

1.7 References and literature

2 Materials

2.1 In general

2.2 Array of material properties

2.3 Thermal insulation materials

2.4 Water, vapour and air flow control layers

2.5 Joints

2.6 References and literature

3 Excavations and building pit

3.1 In general

3.2 Realisation

4 Foundations

4.1 In general

4.2 Performance evaluation

4.3 Foundation systems

4.4 Specific problems

4.5 References and literature

5 Building parts on and below grade

5.1 In general

5.2 Performance evaluation

5.3 Design and execution

5.4 References and literature

6 Structural options

6.1 In general

6.2 Performance evaluation

6.3 Structural system design

6.4 References and literature

7 Floors

7.1 In general

7.2 Performance evaluation

7.3 Design and execution

7.4 References and literature

8 Outer wall requirements

8.1 In general

8.2 Performance evaluation

8.3 References and literature

9 Massive outer walls

9.1 Traditional masonry walls

9.2 Massive light-weight walls

9.3 Massive walls with inside insulation

9.4 Massive walls with outside insulation

9.5 References and literature

10 Cavity walls

10.1 In general

10.2 Performance evaluation

10.3 Design and execution

10.4 References and literature

11 Panelized massive outer walls

11.1 In general

11.2 Performance evaluation

11.3 Design and execution

11.4 References and literature

Professor Hugo S.L. C. Hens

University of Leuven (KULeuven)

Department of Civil Engineering

Building Physics

Kasteelpark Arenberg 40

3001 Leuven

Belgium

Coverphoto: © Hugo Hens

Library of Congress Card No.:

applied for

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de.

© 2012 Wilhelm Ernst & Sohn

Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Rotherstr. 21, 10245 Berlin, Germany

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Coverdesign: Sophie Bleifuß, Berlin, Germany

Typesetting: Manuela Treindl, Fürth, Germany

Printing and Binding: betz-druck GmbH, Darmstadt, Germany

Print ISBN: 978-3-433-03022-6

ePDF ISBN: 978-3-433-60196-9

ePub ISBN: 978-3-433-60197-6

mobi ISBN: 978-3-433-60198-3

oBook ISBN: 978-3-433-60195-2

To my wife, children and grandchildren

In remembrance of Professor A. de Grave

Who introduced building physics as a new discipline

at the University of Leuven (KULeuven), Belgium, in 1952

Preface

Overview

Just like building physics, performance based building design was hardly an issue before the energy crises of the nineteen seventies. Together with the need for more energy efficiency, the interest in overall building performance grew. The tome on applied building physics already discussed a performance rationale, and contained an in depth analysis of the heat, air, moisture performance requirements at the building and building enclosure level. This third tome builds on that rationale although also structural aspects, acoustics, fire safety, maintenance and buildability are considered now. The text reflects thirty eight years of teaching architectural, building and civil engineers, coupled to more than forty years of experience in research and consultancy. Where and when needed, input from over the world was used, reason why each chapter ends with a list of references and literature.

The book should be usable by undergraduates and graduates in architectural and building engineering, though also building engineers, who want to refresh their knowledge, may benefit. The level of discussion assumes the reader has a sound knowledge of building physics, along with a background in structural engineering, building materials and building construction.

Acknowledgments

A book of this magnitude reflects the work of many, not only of the author. Therefore, first of all, we like to thank the thousands of students we had. They gave us the opportunity to test the content and helped in upgrading it.

The text should not been written the way it is, if not standing on the shoulders of those, who preceded. Although we started our carrier as a structural engineer, our predecessor, Professor Antoine de Grave, planted the seeds that slowly fed our interest in building physics, building services and performance based building design. The late Bob Vos of TNO, the Netherlands, and Helmut Künzel of the Fraunhofer Institut für Bauphysik, Germany, showed the importance of experimental work and field testing for understanding building performance, while Lars Erik Nevander of Lund University, Sweden, taught that application does not always ask extended modeling, mainly because reality in building construction is much more complex than any simulation can reflect.

During the four decennia at the Laboratory of Building Physics, several researchers and PhD-students got involved. I am very grateful to Gerrit Vermeir, Staf Roels Dirk Saelens and Hans Janssen who became colleagues at the university; to Jan Carmeliet, now professor at the ETH-Zürich; Piet Standaert, a principal at Physibel Engineering; Jan Lecompte, at Bekaert NV; Filip Descamps, a principal at Daidalos Engineering and part-time professor at the Free University Brussels (VUB); Arnold Janssens, associate professor at the University of Ghent (UG); Rongjin Zheng, associate professor at Zhejiang University, China, Bert Blocken, professor at the Technical University Eindhoven (TU/e) and Griet Verbeeck, professor at KHL, who all contributed by their work. The experiences gained by working as a structural engineer and building site supervisor at the start of my career, as building assessor over the years, as researcher and operating agent of four Annexes of the IEA, Executive Committee on Energy Conservation in Buildings and Community Systems forced me to rethink the engineering based performance approach every time again. The many ideas I exchanged and got in Canada and the USA from Kumar Kumaran, Paul Fazio, Bill Brown, William B. Rose, Joe Lstiburek and

Anton Ten Wolde were also of great help. A number of reviewers took time to examine the book. Although we do not know their names, we like to thank them.

Finally, I thank my family, my wife Lieve, who managed living together with a busy engineering professor, my three children who had to live with that busy father and my many grandchildren who do not know their grandfather is still busy.

Leuven, February 2012

Hugo S. L. C. Hens

0

Introduction

0.1 Subject of the book

This is the third book in a series on building physics, applied building physics and performance based building design:

Building Physics: Heat, Air and Moisture

Applied Building Physics: Boundary Conditions, Building Performance and Material Properties

Performance Based Building Design 1

Performance Based Building Design 2

Both volumes apply the performance based engineering rationale, discussed in ‘Applied Building Physics: Boundary Conditions, Building Performance and Material Properties’ to the design and construction of building elements and assemblies. In order to do that, the text balances between the performance requirements presumed or imposed, their prediction during the design stage and the technology needed to realize the quality demanded.

Performance requirements discussed in ‘Applied Building Physics: Boundary Conditions, Building Performance and Material Properties’, stress the need for an excellent thermal insulation in cold and cool climates and the importance of a correct air, vapour and water management. It is therefore logical that Chapter 2 starts with a detailed overview of insulation materials, waterproof layers, vapour retarders, airflow retarders and joint caulking, after Chapter 1 recaptured the performance array at the building assembly level. In the chapters that follow the building assemblies that together shape a building are analyzed: foundations, basements and floors on grade, the load bearing structure, floors and massive facade systems. Each time the impact of the performance requirements on design and construction is highlighted. For decades, the Laboratory of Building Physics at the K. U. Leuven also did extended testing on highly insulated massive facade assemblies. The results are used and commented.

0.2 Units and symbols

The book uses the SI-system (internationally mandatory since 1977). Base units are the meter (m), the kilogram (kg), the second (s), the Kelvin (K), the ampere (A) and the candela. Derived units, which are important, are:

For the symbols, the ISO-standards (International Standardization Organization) are followed. If a quantity is not included in these standards, the CIB-W40 recommendations (International Council for Building Research, Studies and Documentation, Working Group ‘Heat and Moisture Transfer in Buildings’) and the list edited by Annex 24 of the IEA, ECBCS (International Energy Agency, Executive Committee on Energy Conservation in Buildings and Community Systems) are applied.

Table 0.1. List with symbols and quantities.

Table 0.2. List with suffixes and notations.

0.3 References and literature

[0.1] CIB-W40 (1975). Quantities, Symbols and Units for the description of heat and moisture transfer in Buildings: Conversion factors. IBBC-TNP, Report No. BI-75-59/03.8.12, Rijswijk.

[0.2] ISO-BIN (1985). Standards series X02-101 – X023-113.

[0.3] Kumaran, K. (1996). Task 3: Material Properties. Final Report IEA EXCO ECBCS Annex 24. ACCO, Leuven, pp. 135.

1

Performances

1.1 In general

This chapter starts by providing some definitions and the performance arrays. It then gives an analysis of the interaction between a rigorous application of performance metrics and building, followed by the possible impact of performance formulation on the construction process.

1.2 Definitions and basic characteristics

The term ‘performance’ encompasses all building-related physical properties and qualities that are predictable during the design stage and controllable during and after construction. Typical for performances is their hierarchical structure with the built environment as highest level (level 0) followed by the building (level 1), the building assemblies (level 2) and finally layers and materials (level 3). Relation between the four levels is typically top-down. ‘Predictable’ demands calculation tools and physical models that allow evaluating a design, whereas ‘controllable’ presumes the existence of measuring methods available on site. In some countries, the selection of building performance requirements had legal status. That coupled with a well-balanced enforcement policy guarantees application. One could speak of must and may requirements. Must is legally required, whereas may is left to the principal.

1.3 Advantages

The main advantage of a performance-based rationale is the objectification of expected and delivered building quality. For too long a time, designers juggled with ‘the art of construction’ without defining what kind of art was involved. With a rigorous application of performance metrics, the principal knows the physical qualities he may expect. In forensic cases, performance requirements provide a correct reference, which is not the case with the art of construction. A performance approach may also stimulate system based manufacturing. And finally, performance metrics could steer the building sector in a more research based direction.

1.4 Performance arrays

The basis for a system of performance arrays are the functional demands, the needs for accessibility, safety, well-being, durability, energy efficiency and sustainability and the requirements imposed by the usage of a building. For the arrays, see Table 1.1 and 1.2.

Table 1.1. Performance array at the building level (level 1).

1 In countries like The Netherlands, Germany and Austria fire safety belongs to building physics. In other countries, it doesn’t.

Table 1.2. Performance array at the building assembly level (level 2).

1 In countries like The Netherlands, Germany and Austria fire safety belongs to building physics. In other countries, it doesn’t.

1.5 Design based on performance metrics

1.5.1 The design process

‘Designing’ is multiply undefined. At the start, information is only indefinitely known. Each design activity may produce multiple answers, some better than others, which however cannot be classified as wrong. That indefiniteness demands a cyclic approach, starting with global choices based on sparse sets of known data, for buildings listed as project requirements and design intents. The choices depend on the knowledge, experience and creativity of the designer. The outcomes are one or more sketch designs, which then are evaluated based on the sets of imposed or demanded level 0 and 1 performance requirements. One of the sketch designs is finally optimized and the rest not meeting the performances are discarded. The result is a pre-design with form and spatiality fixed but the building fabric still open for adaptation.

With the pre-design, the set of agreed-on data increases. During the stages that follow, refinement alternates with calculations that have a double intent: finding ‘correct’ answers and adjusting the fabric to comply with the performance requirements imposed. That last phase ends with the final design, encompassing the specifications and the construction drawings needed to realize the building.

1.5.2 Integrating a performance analysis

Designing evolves from the whole to the parts and from vaguely to precisely known data and parameters. These are generated by the design itself, allowing performance analysis to become more refined as the design advances.

During the sketch design phase only level 1 performance requirements such as structural integrity, energy efficiency, comfort and costs receive attention. As most data are only vaguely known, only simple models facilitating global parametric analysis can be used. This isn’t unimportant as decisions taken during sketch design fix many qualities of the final design.

At the pre-design stage along with level 1, the level 2 performance requirements also have to be considered as these govern translation of form and spatiality into building construction. As more parameters and data are established, evaluation can be more refined. The load bearing system gets its final form, the enclosure is designed and the first finishing choices are made. Options are considered and adjusted from a structural, building physical, safety, durability, maintainability, cost and sustainability point of view.

Detailing starts with the final design. Designing becomes analyzing, calculating, comparing, correcting and deciding about materials, layer thicknesses, beam, column and wall dimensions, reinforcement bars and so on. The performance metrics now fully operate as a quality reference. Proposed structural solutions and details must comply with all level 2 and 3 requirements, if needed with feedback to level 1. That way, performances get translated into solutions. Performances in fact do not allow construction. For that, each design idea has to be transformed into materials, dimensions, assemblies, junctions, fits, building sequences and buildability, with risk, reliability and redundancy as important aspects.

Performance requirements also should become part of the specifications, so contractors may propose alternatives on condition they perform equally or better for the same or lower price.

1.6 Impact on the building process

For decades, the triad <principal/architect/contractor> dominated the building process. The principal formulated a demand based on a list of requirements and intents. He engaged an architectural firm, which produced the design, all construction drawings with consultant’s help (structural engineers, mechanical engineers and others), and the specifications on which contractors had to bid. The lowest bidder got the contract and constructed the building under supervision of the architect.

That triad suffers from drawbacks. The architect is saddled with duties for which he or she is hardly qualified. Producing construction drawings is typically a building engineering activity. Of course, knowledge about soil mechanics, foundation techniques, structural mechanics, building physics, building materials, building technology, and building services was procured but always after the pre-design was finished, that means after all influential decisions had been made. The split between design and construction further prevented buildability from being translated into sound construction drawings, which today, still, hardly differ sometimes from the pre-design ones. Details and buildability are left to the contractor, who may lack the education, motivation and resources for that. The consequences can be imagined. No industrial activity experiences as many damage cases as the building sector.

A performance rationale allows turning the triangle into a demand/bidder model. The demand comes from the principal. He produces a document containing the project requirements and intents. That document is much broader than a list of physical performances. Site planning, functional requirements at building and room level, form, architectural expression and spatiality are all part of it. Based on that document, an integrated building team, which includes the architect, all consulting engineers and sometimes the contractor is selected based on the sketch design it proposes. The assigned team has to produce the pre- and final drawings, included structure, building services, all energy efficiency aspects and, if demanded, an evaluation according to LEED, BREEAM or any other rating systems. If the contractor is part of the team, the assigned team also has to construct and decommission the building. Otherwise, a contractor is chosen based on a price to quality evaluation.

1.7 References and literature

[1.1] VROM (1991). Teksteditie van het besluit 680 (Text edition of the decree 680). Bouwbesluit, Den Haag (in Dutch).

[1.2] Rijksgebouwendienst (1995). Werken met prestatiecontracten bij vastgoedontwikkeling, Handboek (Using performance based contracts for real estate development, handbook). VROM publicatie 8839/138, 88 p. (in Dutch).

[1.3] Stichting Bouwresearch (1995). Het prestatiebeginsel, begrippen en contracten (The performance concept, notions and contracts). Rapport 348, 26 p. (in Dutch).

[1.4] Australian Building Codes Board News (1995). Performance BCA, 14 p.

[1.5] CERF (1996). Assessing Global Research Needs. CERF Report #96-5016 A.

[1.6] Lstiburek, J., Bomberg, M. (1996). The Performance Linkage Approach to the Environmental Control of Buildings. Part 1, Journal of Thermal Insulation and Building envelopes, Vol. 19, Jan. 1996, pp. 224–278.

[1.7] Lstiburek, J., Bomberg, M. (1996). The Performance Linkage Approach to the Environmental Control of Buildings. Part 2, Journal of Thermal Insulation and Building envelopes, Vol. 19, April 1996, pp. 386–402.

[1.8] Hens, H. (1996). The performance concept, a way to innovation in construction. Proceedings of the 3rd CIB-ASTM-ISO-RILEM Conference ‘Applications of the Performance Concept in Building’, Tel Aviv, December 9–12, p. 5-1 to 5-12.

[1.9] Hendriks, L., Hens, H. (2000). Building Envelopes in a Hollistic Perspective. Final report IEA-Annex 32, IBEPA, Task A, ACCO, Leuven, 101 p. + add.

[1.10] ANSI/ASHRAE/USGBC/IES (2009). Standard 189.1 for the design of high-performance green buildings except low-rise residential buildings.

[1.11] ANSI/ASHRAE/USGBC/IES (2010). 189.1 User’s manual.

[1.12] Hens, H. (2010). Applied building physics, boundary conditions, performances, material properties. Wilhelm Ernst und Sohn (a John Wiley Company), Berlin.

2

Materials

2.1 In general

The second chapter first reviews materials used for thermal insulation. It then considers vapour barriers, also called vapour control layers, and air barriers, more generally known as air control layers. The last part examines joints between building components.

2.2 Array of material properties

Each knowledge field evaluates materials according to their properties. The storage and transport of heat, moisture and air in and across materials is also quantified that way, with density ρ and porosity Ψ – the weight per unit volume of material and the volume taken in by the pores in a unit volume of material – as basic characteristics. Even the consequence of heat, air and moisture presence is described using properties, with some combinations of properties mirroring unique physical features, see Table 2.1.

Table 2.1. Array of thermal, hygric and air-related material properties.

2.3 Thermal insulation materials

2.3.1 Introduction

Thermal insulation materials were developed in order to minimize heat transport. That requires reducing thermal conductivity (λ) to the utmost, an objective that could not be reached without knowing how heat is transferred across a porous material.

2.3.2 Apparent thermal conductivity

2.3.2.1 In general

The property ‘thermal conductivity’ stands for the ratio between the vector ‘heat flow rate’ somewhere in a material and the vector ‘temperature gradient’ there. In isotropic materials, that ratio is a scalar whereas in anisotropic materials it is a tensor with a value along the x-, y- and z-axis: λx, λy and λz. For those materials, Fourier’s second law becomes:

(2.1)

But this definition does not apply for a highly porous insulation material. Their apparent thermal conductivity is described as the heat passing a 1 m3 large cube with adiabatic lateral surfaces per unit time for 1 K temperature difference between top and bottom face. That condition is met per m2 in an infinitely vast, 1 meter thick layer with 1 °C difference between both isothermal faces. Measurement of the apparent thermal conductivity is based on that description. A material sample of thickness d meter is mounted between a warm and a cold plate. Once steady state is reached, the temperature difference (Δθ) over and heat flow (Φ) across the central part of the sample is logged. When the test apparatus is wrapped adiabatically and the central area A is small compared to the sample area, heat flow develops perpendicularly to thickness and the apparent thermal conductivity becomes:

(2.2)

2.3.2.2 Impact of the transport modes

Heat flow across a dry porous material combines four transport modes (Figure 2.1): (1) conduction along the matrix, (2) conduction in the pore gas, (3) convection in that gas and (4) radiation in all pores between the pore walls. If humid, two additional modes intervene: (5) conduction in the adsorbed water and (6) latent heat transfer.

Apparent thermal conductivity as measured is not a fixed material property but a characteristic whose value depends on factors directly linked to these transport modes.

(1) + (2) Conduction along the matrix and in the pore gas (λc)

If only these two intervened, the equivalent thermal conductivity should be:

(2.3)

Figure 2.1. Heat transfer in a porous material.

where Ψ is total porosity, λM thermal conductivity of the matrix and λG thermal conductivity of the pore gas. According to that formula, apparent thermal conductivity lowers with increasing porosity. Porosity is now given by:

(2.4)

with ρs specific density of the matrix material. The same matrix material with higher porosity, yet with lower overall density, will thus see its apparent thermal conductivity drop, a fact proven experimentally (Figure 2.2). Also, a matrix material with lower thermal conductivity or a pore gas that insulates better than stagnant air gives relief. A very low apparent thermal conductivity is reached with vacuum pores (λG ≈ 0).

But, when another gas than air fills the pores, diffusion of oxygen and nitrogen into the pores, diffusion of pore gas to the surroundings and adsorption of pore gas in the matrix material, slowly lifts the apparent thermal conductivity to a final equilibrium. The lift looks like this in the first weeks:

with coefficient C1 inversely proportional to the diffusion resistance factor of the insulation material and proportional to the temperature with exponent n (Tn, n < 1, T in K). Later, lifting slows down to:

where coefficient C2 depends on diffusion resistance factor and temperature as C1 does. A high diffusion resistance factor means a slow lift. Or, if one wants to store another gas than air in the pores, the matrix should be as vapour retarding as possible. An alternative is to face the insulation boards with a vapour-tight lining.

Thus, an insulation material must be low density; the pores should be filled with an insulating gas that is better than stagnant air and have a matrix that conducts heat inefficiently.

(3) Convection in the pores

Convection only develops in larger pores. Its impact is quantified by multiplying the thermal conductivity of the pore gas in Equation (2.3) with the Nusselt number (X ≥ 1):

(2.5)

Convection increases heat flow. Or, pores in an insulation material should be so small that the Nusselt number is 1. A good insulation material thus is not only very porous; the pore volume must consist of very small pores.

(4) Radiation in the pores

In every pore, except if perfectly reflecting, pore walls at different temperatures exchange radiant heat. The result is a radiant term (λR) complementing the apparent thermal conductivity of formula [2.5]:

(2.6)

with:

(2.7)

In these formulas, n represents the number of pore walls along the insulation thickness d.ρ and τ are long wave reflectivity, respectively long wave transmissivity of the pore walls, while e1 and e2 stand for the long wave emissivity, side insulation, of the linings at both sides. FRC is a correction factor, accounting for the interaction between radiation, convection and conduction. In it, Δθ is the temperature difference across the thickness d of the insulation material, whereas Δθ1 and Δθn are the temperature differences between the facing at both sides and the first pore wall encountered.

The radiant term has a ‘temperature to the 3th power’ impact on the apparent thermal conductivity. As the thermal conductivity of matrix and pore gas is also temperature sensitive, the overall dependence is:

(2.8)

with 0 < n < 1. For −20 ≤ θ ≤ 50 °C, [2.8] is closely matched by:

The lighter the insulation material, i.e. the thinner the pore walls or the larger the pores, the higher the coefficient aR and the more temperature dependant the apparent thermal conductivity is. As there are less large pores across the layer thickness than small pores, whereas thin pore walls have higher long wave transmissivity than thicker pore walls, in both cases, radiation gains importance.

A radiant side effect is an apparent thermal conductivity increase with insulation thickness. In fact, as the number of pore walls n can be replaced by the ratio between layer thickness d and mean pore width dP (d/dP), layer thickness is explicitly present in the numerator and hidden in the denominator of Equation (2.6). That way, the term shifts from zero for layer thickness zero to an asymptote λR∞ for infinite thickness:

(2.9)

This asymptote increases with larger pores (dP↑) and the pore walls transmitting more radiation, i.e. when an insulation material has a lower density. That way, radiant exchanges obstruct apparent thermal conductivity from a continuous drop with density. In fact, once below a limit density, a further drop is only possible by enlarging the pores or thinning the pore walls. In both cases, radiation gains importance, turning the apparent thermal conductivity into a sum of a monotonously decreasing conductive and increasing radiant part. That way, an optimum density exists for which at a given layer thickness the equivalent thermal conductivity is minimal. As a formula:

(2.10)

for mineral wool at 20 °C (see Figure 2.3):

Figure 2.3. Mineral wool: apparent thermal conductivity versus density.

(5), (6) Conduction in the adsorbed water layers, latent heat exchanges

Moist materials not only see extra heat conduction in the adsorbed water layers and condensed water islands, they also suffer from latent heat flow by evaporation/diffusion/condensation/backflow in the pores. That extra conduction results in a linear relationship between apparent thermal conductivity and moisture ratio for porous building materials and a parabolic relationship between apparent thermal conductivity and volumetric moisture ratio for insulation materials:

(2.11)

where λd in both equations is the apparent thermal conductivity for the dry material. Latent heat flow however adds a term:

(2.12)

where lb is the latent heat of evaporation. Apparent thermal conductivity thus becomes:

(2.13)

Thanks to evaporation/diffusion/condensation/backflow in the pores, temperature affects apparent thermal conductivity in humid insulation materials more than by radiation only. Influence also quickly increases with a decreasing vapour resistance factor (μ). The largest impact in fact is seen in vapour permeable materials such as mineral wool, where evaporation of moisture at the warm side causes a jump in apparent thermal conductivity (Figure 2.4).

Figure 2.4. Mineral wool: apparent thermal conductivity versus volumetric moisture ratio. The unbroken line is moisture at the warm side; the dotted line represents moisture at the cold side.

2.3.3 Other properties

2.3.3.1 Mechanical

Due to their very high porosity, insulation materials have limited strength and stiffness. Under mechanical load, they behave like mattresses rather than elastic-plastic. Low stiffness in turn incurs creep, relaxation and sometimes remarkable form instabilities. Therefore insulation materials hardly may perform load bearing functions, although there are exceptions like good compression strength when used in floors and foundations whereas in sandwich panels the insulation layer must withstand shear.

2.3.3.2 Physical

Moisture

Most insulation materials are non-hygroscopic. All in fact have macro-pores and limited specific pore surface. Consequently, they hardly adsorb water vapour whereas capillary condensation only happens at relative humidity near 100%. However, excluding capillary action in fibrous insulation materials requires water-repellent treatment. Only closed-pore insulation materials guarantee imperviousness for water heads, while limiting vapour diffusion across the pores and interstitial condensation in the pores demands a high vapour resistance factor, again requiring closed pores. That favours foams as opposed to fibrous materials, which are vapour permeable, pervious for water heads and non-capillary only when treated with a hydrophobic resin. Whether insulation materials lose strength and stiffness, degrade biologically and rot when moist, depends on the matrix material.

Air

Good air-tightness demands closed pores. Foams are no problem, but not with fibrous materials, which are extremely air permeable.

2.3.3.3 Fire

Also here the matrix material qualifies. Insulations based on organic and synthetic materials typically belong to the classes ‘flammable’ whereas the non-organic ones are mostly inflammable.

2.3.3.4 Sensitivity to temperature, IR and UV

Again, the matrix material plays the main role. Synthetics behave worse whereas organic and non-organic materials hardly give problems.

2.3.4 Materials

Insulating building materials, insulation materials and new developments are discussed using following scheme.

Short description

Properties

Density

Heat

Moisture

Air

Strength and stiffness

Behaviour

In general

Under mechanical load

Sensitivity to temperature, IR and UV

Under moisture load

Exposure to fire

Others

Usage

In addition, some attention is given to radiant barriers.

2.3.4.1 Insulating building materials

Brick masonry

Increasing the thermal resistance of brick masonry demands (1) limiting the meters run of horizontal and head joints per m2 of wall, (2) developing lower density bricks, (3) using insulating mortars and (4) increasing wall thickness. Less meter run of joints means using fast bricks. Lower density combines lighter potsherd with optimal perforation patterns.

Figure 2.5. Perforation patterns in fast bricks: (2) and (3) better than (1).

Properties

Density

Lies between 750 and 880 kg/m

3

for lightweight fast bricks. Dense brickwork may weigh 2000 kg/m

3

.

Thermal

Specific heat capacity

Dry 840 J/(kg · K), independent of density

Thermal resistance

Up to 0.5 m

2

· K/W for a 14 cm thick lightweight fast brick wall, density 900 kg/m

3

. For comparison, a 14 cm thick normal fast brick wall does not pass 0.28 m

2

· K/W. A 30 cm thick light weight fast brick wall could reach 1.7 m

2

· K/W.

Hygric

Moisture content

Bricks are hardly hygroscopic. They have high capillary moisture content.

Diffusion thickness

Masonry has a lower diffusion thickness than the bricks due to badly filled joints and micro cracks between bricks and joints. A good estimate is

μ

eq

d

≈ 5

d

(m).

Capillary water absorption coefficient

From moderate (0.05 kg/(m

2

· s

0.5

)) to high (0.8 kg/(m

2

· s

0.5

)), depending on the brick’s porous structure.

Usage

Lightweight fast brickwork is well suited as inside leaf in cavity walls and for massive walls with rain right outside render. However, it does not replace a good thermal insulation. For that, the apparent thermal conductivity is too high. In addition, embodied energy is not negligible.

Concrete

The first step on the way to a better insulating concrete consists of replacing gravel by lighter particles: furnace slag, expanded clay, perlite or polystyrene pearls. Density and apparent thermal conductivity drop as does strength and stiffness, but shrinkage and creep increase. The lowest apparent thermal conductivity is attained by skipping gravel or any other addition and foaming the sand/mortar mixture through gas formation in an autoclave. The result is ‘aerated concrete’, manufactured in blocks with dimensions up to 59.5 × 29.5 × 29.5 cm and as facade or roof elements. Its foamed structure allows sawing and milling.

Properties

Density

While ‘normal’ concrete weights ≈ 2200 kg/m

3

, expanded clay concrete has a density between 650 and 1600 kg/m

3

, with 1600 kg/m

3

for structural application and 650 kg/m

3

for non-bearing uses. Polystyrene concrete weights 260–800 kg/m

3

, 260 kg/m

3

as post-fill. Aerated concrete ranges between 350 and 800 kg/m

3

.

Thermal

Specific heat capacity

Dry 840 J/(kg · K), independent of density

Apparent thermal conductivity

Tables attribute 1.6 to 2 W/(m · K) as ‘dry value’ to ‘normal’ concrete. Measurement gave 2.6 W/(m · K). Expanded clay concrete gives 0.024 exp (0.0027

ρ

) for 600 <

ρ

< 1200 kg/m

3

. With polystyrene concrete, the value is 0.041 exp (0.00232

ρ

) for 250 <

ρ

< 800 kg/m

3

. For aerated concrete it is: 0.12 + 0.000375

p

for 450 <

p

< 620 kg/m

3

.

Hygric

Moisture content

The cement gel turns concrete into a hygroscopic material.

Diffusion thickness

Drops with decreasing density and increasing moisture content.

Capillary water absorption coefficient

Concrete is not very capillary. First the second digit behind the decimal point differs from zero.

Strength and stiffness

Although density is quite low, constructing 4–5 floors high is still possible with heavier aerated concrete blocks.

Behaviour

Under moisture load

Hygric shrinkage increases with lower density. Reason is less particle resistance when going from gravel over expanded clay and polystyrene pearls to no particle resistance at all with aerated concrete! When building with that material, shrinkage demands proper detailing. Aerated concrete also has high building moisture content, up to 200–250 kg/m

3

and an initial thermal conductivity, which exceeds the air-dry value of 0.14–0.2 W/(m · K).

Exposure to fire

Due to the combination of non-combustibility, good insulation and low thermal expansion, aerated concrete has excellent fire resistance.

Usage

Aerated concrete is an alternative to thermal insulation. An air-dry aerated concrete wall, thickness 30 cm, has a clear wall thermal transmittance below 0.5 W/(m2 · K).

2.3.4.2 Insulation materials

A material is called ‘insulating’, when its dry apparent thermal conductivity does not pass 0.07 W/(m · K). Classification happens according to structure, behaviour, application or matrix material. In the case of matrix material, the scheme becomes:

Only the materials in standard letters are commented in detail.

We also discuss new developments such as transparent (TIM) and vacuum insulation (VIP).

Sea grass, wool, straw and flax are called ‘sustainable’ by bio-ecologists, only because they are ‘natural’. Their quality in terms of ‘durability’ however, is never referred to. All four are hygroscopic, moisture sensitive and combustible. Many people are allergic to wool. Upgrading durability and lowering combustibility demands addition of chemicals, among them borax salts. Whether these materials are still ‘natural’ with these additions is left unmentioned.

Cork

The basic material is the stripped bark of the cork oak. After grinding, the bark particles are autoclaved in steam at 350 °C. That expands them, kills moulds and bacteria, while part of the VOCs evaporates and the resin binds the particles into blocs. These are then cut to size. An alternative is to dry heat the cork particles, drench them into bitumen and press that mixture into boards.

Properties

Densit

Between 80 and 250 kg/m

3

. Quite high for an insulation material.

Thermal

Specific heat capacity

Dry ±1880 J/(kg · K), independent of density

Apparent thermal conductivity

For a density of 111 kg/m

3

, temperature between 0 and 0 °C and volumetric moisture ratio between 0 and 6% m

3

/m

3

:

Hygric

Moisture content

Due to its organic origin, cork is hygroscopic. It also shows some capillarity.

Vapour resistance factor

Between 5 and 20. The value drops with higher relative humidity (in reality with moisture content).

Air

Its open porous structure makes cork air permeable.

Strength and stiffness

Cork has low compressive strength. A 0.11 MPa large compression during one day results in 10% strain for 145 kg/m

3

dense boards.

Behaviour

Under mechanical load

Cork creeps. 1 day at 0.05 MPa compression increases strain from 1.5 to 5%. Allowable stress is therefore limited to 1/3 of that at 10% strain (

σ

10

).

Sensitivity to temperatures, IR and UV

Cork scores quite well. Thermal expansion coefficient is quite high (±40 · 10

–6

K

1

), but resistance against low and high temperatures is excellent and UV gives some discoloration only.

Moisture load

Like all organic materials, cork swells when wetted and shrinks when drying. If wet for a long enough period, it turns mouldy and may rot.

Exposure to fire

Cork burns.

Usage

Although cork was well suited to insulate low-sloped roofs and refrigerators, plastic foams took over that segment of the market. Never apply cork where high relative humidity is likely! Using it to upgrade airborne and contact sound insulation also makes no sense, as the material is too stiff. However heavy boards do well as vibration damper.

Cellulose

The basic materials are unused newspapers. To limit combustibility and mould sensitivity, the fibres are mixed with borax salts. The material applies as dry or wet sprayed loose fill. It is also available as dense boards.

Properties

Density

Ranges from 24 to 60 kg/m

3

. Depends among others on spraying pressure.

Thermal

Specific heat capacity

Dry ≈ 1880 J/(kg · K), independent of density

Apparent thermal conductivity

Air dry given by with

d

thickness in mm,

θ

temperature in °C and

ρ

density in kg/m

3

Hygric

Moisture content

Due to their organic origin, cellulose fibres are hygroscopic. The borax salts still increase sorption, see

Figure 2.6

. The fibres also show capillarity.

Vapour resistance factor

Is not higher than 1.9 for a density of 50 kg/m

3

. Drops with increasing moisture content. The low value is due to the fibrous structure.

Air

Fibrous structure makes the material air permeable, k

a

≈ 1.6 · 10

–3

s.

Strength and stiffness

Loading loose fill beyond the weight of the fibrous mass is excluded.

Figure 2.6. Cellulose fibre: sorption/desorption.

Behaviour

Under mechanical load

At low density, static and dynamic forces induce irreversible settling (

s, t

in years). Measured (a for annum, year):

Under moisture load

Cellulose shows quite some hygric swelling and shrinkage. Wet spraying results in drying shrinkage. At moisture contents above 20% kg/kg, the fibres clog and may start rotting.

Exposure to fire

Despite borax salt addition, cellulose fibres are combustible. With thicker insulations, such as in passive houses, collapsing roofs during a fire create hazardous situations for fire fighters.

Drawbacks

Cellulose dust may induce respiratory problems. During spraying, a mask should be worn. Also, the borax salts are not without problems. Simple exposure can cause respiratory and skin irritation. Ingestion of the salts may give gastrointestinal distress including nausea, persistent vomiting, abdominal pain, and diarrhoea. Effects on the vascular system and brain include headaches and lethargy, but are less frequent. In severe poisonings, a beefy red skin rash affecting palms, soles, buttocks and scrotum has been described. With severe poisoning, erythematous and exfoliative rash, unconsciousness, respiratory depression, and renal failure happen.

Usage

Cellulose fibres are an alternative to glass fibre and mineral wool. Typical applications are insulation of timber-framed walls, insulation of timber low-sloped roofs with insulation between purlins and insulation of attic floors. The dense boards may be used to insulate pitched roofs. However, cellulose fibres should never be used in air spaces exposed to very high relative humidity. This could be a problem when used in low-slope roofs and timber-framed walls with brick veneer. There solar driven vapour flow from wet veneers back to the inside during warm weather may humidify the fibres

Glass fibre and mineral wool

Glass fibre is produced using (recycled) glass, whereas mineral wool has diabase stone as a basic material. Glass and stone are melted, after which a spinning head stretches the melt into fibres with diameter < 10 μm. These fall through a spray of phenol or silicon binder on a conveyor belt, on which the facings for blankets and bats lie. Conveyor belt and fibre blankets, bats or boards then pass a heated press where the binder hardens and the insulation gets its final density and thickness. Then the blankets, bats and boards are cut to size. The spectrum of finished products ranges from loose fill over blankets and bats to soft, semi-dense and very dense boards.