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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Just like building physics, performance based building design was hardly an issue before the energy crises of the 1970ies. 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.
In continuation of Vol. 1 this second volume discusses light-weight construction with wooden and metal elements, roofing systems, facades, and ends with finishes and the overall risk analysis. Most chapters build on a same scheme: overview, overall performance evaluation, design and construction.
The work 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.
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Seitenzahl: 349
Veröffentlichungsjahr: 2012
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Contents
Preface
0 Introduction
0.1 Subject of the book
0.2 Units and symbols
0.3 References and literature
1 Timber-framed construction
1.1 In general
1.2 Performance evaluation
1.3 Design and execution
1.4 References and literature
2 Sheet-metal outer wall systems
2.1 In general
2.2 Performance evaluation
2.3 Design and execution
2.4 References and literature
3 New developments
3.1 Transparent insulation
3.2 Multiple skin and photovoltaic outer walls
3.3 References and literature
4 Roofs: requirements
4.1 In general
4.2 Performance evaluation
4.3 References and literature
5 Low-sloped roofs
5.1 Typologies
5.2 Roofing membranes
5.3 Compact low-sloped roofs
5.4 Protected membrane roofs
5.5 References and literature
6 Pitched roofs
6.1 Classification
6.2 Roof covers in detail
6.3 Basic assemblies
6.4 Performance evaluation
6.5 Design and execution
6.6 References and literature
7 Sheet-metal roofs
7.1 In general
7.2 Metal roof cover
7.3 Performance evaluation
7.4 Design and execution
7.5 References and literature
8 Windows, outer doors and glass façades
8.1 In general
8.2 Glass
8.3 Windows and doors
8.4 Glass façades
8.5 References and literature
9 Balconies, shafts, chimneys and stairs
9.1 In general
9.2 Balconies
9.3 Shafts
9.4 Chimneys
9.5 Stairs
9.6 References and literature
10 Partitions; wall, floor and ceiling finishes; inside carpentry
10.1 Overview
10.2 Partition walls
10.3 Building services
10.4 Wall finishes
10.5 Floor finishes
10.6 Ceiling finishes
10.7 Inside carpentry
10.8 References and literature
11 Risk analysis
11.1 In general
11.2 Risk definition
11.3 Performing a risk analysis
11.4 Example of risk analysis: cavity walls
11.5 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
Cover: Low Energy Brick Building (Students’ Residence), KULeuven, Belgium
Photo: 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.
© 2013 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: 978-3-433-03023-3
ePDF ISBN: 978-3-433-60248-5
ePub ISBN: 978-3-433-60249-2
mobi ISBN: 978-3-433-60250-8
oBook ISBN: 978-3-433-60251-5
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 1970s. With the need to upgrade energy efficiency, the interest in overall building performance grew. The volume on applied building physics discussed a performance rationale and performance requirements at the building and building enclosure level, with emphasis on heat, air, moisture checks. As in the third volume, volume four continues this rationale for structural aspects, acoustics, fire safety, maintenance and buildability. And as with volume three, it is the result of 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 and literature from around the world has been used, with a list of references and literature at the end of each chapter.
The book can be used by undergraduates and graduates in architectural and building engineering and also building engineers who want to refresh their knowledge may also benefit. The level of discussion assumes a sound knowledge of building physics, along with a background in structural engineering, building materials and building construction.
Acknowledgements
A book of this magnitude reflects the work of many, not only the author. Therefore, first of all, we want to thank our thousands of students. They gave us the opportunity to test the content and helped in upgrading by the corrections they proposed and the experience they offered in learning what parts should be better explained.
This is a text that has been written standing on the shoulders of those who came before us. Although we started our career as a structural engineer, our predecessor, Professor Antoine de Grave planted the seeds from which our interest in building physics, building services and performance based building design slowly grew. 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 is.
During the four decades at the Laboratory of Building Physics, several researchers and Ph. D.-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, professor at the University of Ghent (UG); Rongjin Zheng, associate professor at Zhejiang University, China, and Bert Blocken, professor at the Technical University Eindhoven (TU/e), 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 each time again. The many ideas I exchanged with Kumar Kumaran, Paul Fazio, Bill Brown, William B. Rose, Joe Lstiburek and Anton Ten Wolde in Canada and the USA were also of great help. A number of reviewers took time to examine the book. Although we do not know their names, we also thank them here.
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, June 2012
Hugo S. L. C. Hens
0
Introduction
0.1 Subject of the book
This is the second part of the third volume in a series of books 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
Performance Based Building Design 2 continues the application of the performance based engineering rationale, discussed in ‘Applied Building Physics: Boundary Conditions, Building Performance and Material Properties’ to the design and construction of building assemblies. In order to do that, the text considers the performance requirements presumed or imposed, their prediction during the design stage and the technology needed for realization.
Performance Based Building Design 1 ended with massive outer walls. Performance Based Building Design 2 begins with lightweight building and outer wall systems: timber-framed and metal-based. Then low-sloped, pitched, and metal roofs follow to finish the enclosurerelated subjects with glazed surfaces and windows. Attention then turns to balconies, chimneys, shafts, staircases, inside partitions, and finishes. The volume closes with a chapter on risk analysis. Of course, for principals acceptable risk is an important issue. As in Performance Based Building Design 1, the impact of performance requirements on design and execution is highlighted. For decades, the Laboratory of Building Physics at the KULeuven not only tested highly insulated massive façade assemblies, but also lightweight façade assemblies and roofs. The results are used in the discussions.
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 of importance 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.
Symbol
Meaning
Indices
A
Air
c
Capillary, convection
e
Outside, outdoors
h
Hygroscopic
i
Inside, indoors
cr
Critical
CO
2
, SO
2
Chemical symbol for gases
m
Moisture, maximal
r
Radiant, radiation
sat
Saturation
s
Surface, area, suction
rs
Resulting
v
Water vapour
w
Water
ϕ
Relative humidity
Notation
[ ], bold
Matrix, array, value of a complex number
Dash
Vector
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, Louvain, p. 135.
1
Timber-framed construction
1.1 In general
In the Low Countries on the North Sea, timber was the common construction material for rural and municipal dwellings until the 13th – 14th century. Brick construction was an aristocrat’s privilege. Many devastating town fires, the sociological fact that bricks stood for wealth and growing wood shortages slowly turned brick building into the new standard.
Timber construction still is the reference in many countries worldwide, like the US, Canada, Norway, Sweden, Finland, Russia, Japan and other countries rich in forests and often with a cold climate. There, the framed type has an important advantage compared to massive construction: it is easy to insulate, which is why even in northwest Europe timber-frame construction has regained popularity, now for passive houses. However, the disadvantages also deserve mentioning: hardly any thermal inertia, air tightness critical and less moisture tolerant than brick construction.
In timber framing, load- and non-bearing outer and partitions walls consist of a framework of timber studs and crossbeams, called plates. The outer wall frames are externally finished with structural sheathing. Where the studs bear all vertical loads and the outer wall ones have also to withstand the wind component, normal to the façade, the sheathing provides overall stiffness against horizontal loading. It also prevents buckling of the studs parallel to their lowest inertia radius. From the three common framing approaches – platform, balloon, post and beam – the platform type, composed of storey-high stud walls and timber floors is the most popular (Figure 1.1).
Construction looks as follows: once the foundations and foundation walls are ready, the ground floor is laid, in humid climates preferably a concrete deck, though in dry climates also timber joists with plywood or OSB (oriented strand board) deck apply, the crosscut end sides being closed with header plates. In such case, ripped half-width standard timber beams form the floor joists with struts at half-span excluding lateral buckling. Then one fixes the bottom plates, after which the studs are nailed and coupled with top plates. To stabilize the frame corners, doubling these is an option. After, a plywood, OSB or stiff insulation board (XPS) sheathing is nailed to the outer wall frames. The joists of the second floor, which are fixed at the top plates then follow. Header plates again close the crosscut end sides and plywood or OSB forms the running surface. The same cycle restarts for the second storey: bottom plate, studs, top plates, sheathing, floor joists, running surface, etc.
A timber framework or rafters, axis to axis at the same distance as the studs, shape the loadbearing roof structure with an external sheathing once more providing stiffness. Timber framing ends with wrapping up the outer walls with waterproof, wind tight building paper, stapled from bottom to top on the sheathing with the higher strips overlapping the lower ones. Platform framing lends itself to modular construction and prefabrication.
From inside to outside the outer wall assembly looks like (Figure 1.2): inside lining (gypsum board); (service cavity); air (always) and vapour (when necessary) retarder; bays between studs filled with insulation (mineral wool or cellulose); plywood, OSB or stiff insulation board sheathing; building paper; outside finish (timber siding, brick veneer, EIFS, etc).
Aside from timber framing, also metal framed construction exists, with metal studs and plates replacing the timber ones.
Figure 1.1. Platform type (1: joists, 2: header plate, 3: running surface, 4: top plates, 5: sheathing, 6: studs, 7: bottom plates).
Figure 1.2. Timber-framed outer wall, reference assembly (1: inside lining, 2: service cavity, 3: air and vapour retarder, 4: thermal insulation; 5: sheathing, 6: building paper, 7: outside finish).
1.2 Performance evaluation
1.2.1 Structural integrity
Timber-framed buildings are so lightweight that anchoring in the foundation walls is necessary to prevent displacement under extreme wind load (Figure 1.3).
Figure 1.3. Timber-framed construction, anchoring in the foundation walls.
Wind loading and buckling of the outer and partition wall studs demands proper attention. The sheathing or inside finishes block it in the lowest moment of inertia direction. The direction normal to the walls needs a control. Table 1.1 gives the buckling factors vertical loads have to be multiplied by, as a function of the stud’s slenderness (i):
(1.1)
with L the effective stud span (in timber framed construction equal to the distance between bottom and top plates), I the moment of inertia around the neutral axis of the combination stud/sheathing (if shear-stiff coupled) and A total active cross section.
If this product gives stresses in the timber beyond acceptable, or, if for a given span the stud’s radius of inertia is too low, then two options are left: diminishing the centre-to-centre distance between studs or using deeper ones. The first is disadvantageous in terms of whole wall thermal transmittance whereas the second allows larger insulation thicknesses, thus, a lower whole wall thermal transmittance.
Table 1.2 summarizes the mechanical properties of softwood and plywood. For the stiffness against horizontal loads, the same rules as for massive construction hold: the floors as rigid horizontal decks, at least 3 sheathed or wind-braced walls whose centre planes do not cross in one point, the stiff walls preferentially distributed in a way the resulting wind load vector crosses their stiffness centre.
Table 1.1. Buckling factors (slenderness vertically in steps of 10, horizontally in steps of 1).
Table 1.2. Mechanical properties of softwood and plywood.
1.2.2 Building physics: heat, air, moisture
1.2.2.1 Air tightness
Air tightness of timber-framed envelopes is not taken for granted. The outside finish, the building paper, the sheathing, as well as the insulation, all are air-permeable. Contributing factors are, for the building paper, the overlaps between the strips, for the sheathing the joints between boards and for the thermal insulation the material itself and the gaps between insulation, studs and plates. It is the inside finish to guarantee air-tightness. Non-perforated gypsum board linings without cracks between boards have an air permeance of (Ka) ≈ 3.1 · 10–5 ΔPa–0.19. For an air pressure difference of 10 Pa, that value limits air leakage to 0.43 m3/(m2 · h). However, when sockets and others perforate the lining and cracks form between boards, this value may increase by a factor of 10, which is why inclusion of an additional air barrier deserves recommendation. In moderate and cold climates, one used a PE-foil, stapled against the timber frame, preferentially with a service cavity left between foil and inside lining. Recently, OSB with taped joints emerged as an alternative (Figure 1.4). But also with additional air barrier, perfect air-tightness is hard to realize. Even excellent workmanship did not result in tested air leakages below 3 dm3/(m2 · h) at 1 Pa air pressure difference. In hot and humid climates, it is up to the outside finish to guarantee air-tightness.
Figure 1.4. Taped OSB as air barrier.
Figure 1.5. Timber-framed construction: caring for a continuous air barrier in the envelope.
Also three-dimensionally, a timber-framed construction offers a network of leaks. Via the junctions with the envelope, outside air may permeate partition walls, while conversely inside air can flow to the outside through the sockets in the partitions. At each floor level, air may flow façade to façade between the joists, a phenomenon causing unexpectedly high heat losses, quick ceiling soiling, and mould where the outside air enters. All this demands an envelope with continuous air barrier. Therefore, the following recommendations prevail: (1) include PE-strips at each floor between header plate and header insulation, (2) fix PE-strips in all junctions between outer and partition walls, (3) tape the overlaps to the air barrier (Figure 1.5).
Fully filling the space between sheathing and air barrier prevents air looping along the thermal insulation. A hotbox test on a two meters high timber framed wall insulated with 8 cm thick XPS-boards demonstrated that partial fills are critical. These are too stiff to link up perfectly with studs, plates, sheathing, and inside lining, creating leaks across and air layers at both sides of the insulation that way. At a temperature difference of 18.7 °C there was no uniform heat loss of 4.5 W/m2 but large differences between the flow rates up and down the inside and outside surface were noted, see Table 1.3.
Height
m
Heat flow rate
W/m2
outside surface
inside surface
1.7
30.9
3.7
0.3
5.7
11.5
The reason is air looping, with cold air rising at the warm side of the insulation, warm air falling at the cold side, changeover from warm to cold on top of the insulation and changeover from cold to warm down the insulation. The data also suggest that thermal stack between hot and cold box activates outflow up, and inflow down the wall.
The building paper wrap should guarantee wind-tightness.
1.2.2.2 Thermal transmittance
The discussion relates to outer walls only. For roofs and floors, reference is made to the chapter on floors in Performance Based Building Design 1 and the chapters that follow on roofs. As always, the clear and whole wall thermal transmittances (U) differ, the last accounting for studs, top and bottom plates. In the case of an airtight outer wall, the series/parallel circuit of Figure 1.6 allows a fair guess of the whole wall thermal transmittance, as do also the following linear thermal transmittances (ψ):
With mineral wool or cellulose as thermal insulation and a brick veneer as outside finish, the thicknesses of Table 1.4 give whole wall thermal transmittances of 0.4, 0.2 and 0.1 W/(m2 · K) for 40 and 60 cm centred studs.
Figure 1.6. Timber framed wall as series/parallel circuit.
Table 1.4. Timber framed outer wall: insulation thicknesses (first number using ψ’s, second according to series/parallel circuit).
Figure 1.7. Top right engineered timber stud.(a), (b), (c) are the steel shape studs Table 1.5 is based on.
For the values 0.4 and 0.2 W/(m2 · K) both methods fit. Yet, for a value 0.1 W/(m2 · K), the gap is manifest, showing that such low value demands a three-dimensional calculation. 0.2 W/(m2 · K) gives wall thicknesses touching an acceptable 40 cm. Instead, 0.1 W/(m2 · K) needs economically questionable thicknesses. The use of engineered studs and plates (Figure 1.7) gives some relief.
With metal frames, thermal bridging effects are more pronounced, with the Figure 1.6 circuit giving no reliable results anymore. Only measurement or three-dimensional calculations do. Take the first wall in Table 1.5. Its studs consist of cold-formed U-steel shapes with wall thickness 1.2 mm. Compared to the clear wall thermal resistance, the whole wall value drops by 38.2%, while timber studs limit that drop to 8.8%.
XPS as sheathing material, plus a smaller contact area between sheathing and steel studs or the use of perforated or thermally cut steel shapes gives the best results. The last bring the whole wall thermal transmittance in line with timber-framed walls.
In addition, the impact of workmanship when insulating the bays has been studied experimentally. Figure 1.8 shows some typical imperfections, while Table 1.6 lists their measured effect on the whole wall thermal transmittance. Increase peaks when air looping develops as is the case with narrowly cut insulation, creating 50 mm wide leaks at both studs.
Table 1.5. Clear and whole wall thermal resistance of the steel framed walls of Figure 1.7.
Table 1.6. Whole wall thermal transmittance in case of workmanship inaccuracies.
Figure 1.8. Typical workmanship inaccuracies.
1.2.2.3 Transient response
On a daily basis, timber-framed outer walls have an admittance way below 3.9 W/(m2 · K) (for a surface film coefficient indoors of 7.8 W/(m2 · K)), while the dynamic thermal resistance hardly differs from the steady state thermal resistance and temperature damping does not even approach a value 15. Better thermal insulation hardly changes things, see Table 1.7.
Table 1.7. Temperature damping, dynamic thermal resistance, and admittance (1-day period).
Through that, limited glass area, effective solar shading, and well-designed nighttime ventilation gain importance in moderate climates. Of course, an alternative is to combine a timber-framed envelope with heavy weight inside partitions and floors. To underline the difference, Figure 1.9 gives the fabric related room damping as function of window area for a room with a volume of 4 × 4 × 2.7 m3, a 4 × 2.7 m2 timber-framed outer wall, clear wall thermal transmittance of 0.16 W/(m2 · K), timber framed partition walls and joisted floors and, for the same room but now with brick partitions and concrete floors. With massive inside partitions and floors, damping increases by a factor of 4.
Figure 1.9. Fabric related room damping, integral timber framed versus outer wall only, in combination with massive partition walls and floors.
1.2.2.4 Moisture tolerance
Due to water sensitivity of the softwood used, timber-framed construction is inherently less moisture tolerant than massive construction. Above a moisture ratio of 20% kg/kg the risk to see mould colonizing the timber increases sharply whereas above 30% kg/kg fungal attack and bacterial rot become likely. To avoid problems the following requirements should be fulfilled:
Requirement 1
A vapour permeable outside finish facilitates fast drying of building moisture. Tests in the moderate, humid climate of Newfoundland, Canada, on eight walls proved building paper with low diffusion resistance is quite effective. All walls had a PE air and vapour retarder at their inside. Wall 1 and 2 were insulated with 14 cm mineral wool. Their frame was OSB sheathed and covered with building paper. Insulation in walls 3 to 6 was 8 cm mineral wool. For 3 and 4 the sheathing consisted of dense, 38 mm thick mineral wool boards covered with a vapour permeable spun-bonded foil. 5 and 6 had a 38 mm thick XPS sheathing, covered with the same spun-bonded foil. Wall 7 was insulated with 14 cm wet sprayed cellulose and finished with an OSB sheathing. Wall 8 finally got 127 mm EPS as insulation, which was covered with a vapour permeable foil. All walls had humid studs and plates with a building moisture ratio from 26 to 30% kg/kg. Table 1.8 gives the measured moisture ratio after one-year exposure.
Table 1.8. Drying of timber framed walls (St John’s, New Foundland).
Walls facing south dried faster than north facing ones. After 1 year, the studs of walls 3 and 4, the one with vapour permeable finish, are driest, with a moisture ratio largely below 20% kg/kg. Wall 8 lags behind somewhat. Wall 1, 2, 5, and 6 perform worse. To the north, they still show moisture ratios quite above 20% kg/kg, while to the south they drop just below. The situation in wall 7 is frankly dramatic. There, the high moisture content of the wet sprayed cellulose humidified the studs. Remarkably, due to air looping around and in the insulation moving air from the warm to the cold side at the top, all walls studs dry fastest there. On its way to the bottom, the air cools down causing water vapour picked up at the top to condense down on the sheathing.
Requirement 2
Draping the building paper so the overlaps allow functioning as second drainage plane, avoids rain from wetting the sheathing and timber frame. In addition, overhanging edges mask the delicate façade to roof transition while a backsplash zone in waterproof material above grade is not a redundant luxury with a wood siding or stucco outside finish.
Requirement 3
Requirement 3 determines how to solve the details above grade. In a humid climate, foundation walls and ground floor decks are best executed in a stony material on which the timber-framed walls are mounted. Between grade and lowest bottom plate one must respect a difference in level of at least 20 cm. Also, a continuous damp proof layer should separate the lowest bottom plate from the foundation walls or floor deck. The same damp proof layer is needed everywhere studs contact stony materials that can turn wet.
Requirement 4
Without a continuous air retarder, air-tightness of timber-framed outer walls remains defective. Even when correctly mounted, an air permeance below 10–5 kg/(m2 · s · Pa) at 1 Pa air pressure difference is hardly realizable. As Figure 1.10 underlines, even at moderate air outflow, vapour resistance of the inside finish and building paper have a marginal impact on the amount of condensate deposited at and in the sheathing.
Not only do amounts of condensate vary with height, the worst situation occurs when the leaks at both sides of the insulation are far apart. Clearly, deducing vapour resistance requirements from a Glaser calculation does not work.
Figure 1.11 illustrates the effect of local leaks in the inside finish and the sheathing, coupled to air looping in and around the insulation.
Simulation with more complete models gave following guidance:
Clearly, the requirements in indoor climate class 2 and 3 are far from severe. Or, timber framed outer walls in that type of buildings do not demand excessive vapour tightness at their inside. Air-tightness is what matters.
Table 1.9. Timber framed outer walls, relation between the diffusion thickness of building paper and air/vapour retarder (Uccle moderate climate).
Requirement 5
Surely highly insulated timber framed outer walls finished with a brick veneer may suffer from solar driven vapour flow. An example are passive houses, where the outer walls consist of a timber framed inside leaf, lined inside with an air-tightened OSB sheathing and finished at the cavity side with a very vapour permeable wood fibre board (Figure 1.12). A 3 cm wide unvented cavity separates that inside leaf from a capillary active, 9 cm thick brick veneer, which at the rain side acts as rain buffer storing up to 14 litres per m2 and more. During warmer weather after a rainy period, part of that moisture diffuses across the inside leaf to the inside where it humidifies the OSB. As the veneer stays at 100% relative humidity year round, relative humidity in the OSB inside lining fluctuates annually as shown in Figure 1.12.
Superimposed is a daily relative humidity oscillation at the OSB’s cavity side with peaks over 90% in summer. In fact, temperature at the backside of a wet west over south-west to south looking brick veneer may pass 35 °C during warm summer days. Related vapour saturation pressure then reaches 5260 Pa, high enough to create a daily vapour flow to the inside, which further humidifies the OSB. Solar driven vapour flow activates the OSB’s formaldehyde release during the summer months.
Practitioners have no clue of the problems solar driven vapour flow may cause. Avoidance however is simple, as it suffices using building paper that has a slightly higher diffusion resistance than the air/vapour retarding foil or sheathing inside. As Figure 1.13 underlines, such solution fits within the relations of Table 1.9. A less safe alternative consist of ventilating the cavity between brick veneer and timber-framed leaf.
Figure 1.12. Passive house, solar driven vapour flow: above temperature at the veneers backside, below relative humidity in the inside OSB air retarder.
Figure 1.13. Timber-framed outer wall: relation between the diffusion thickness of the air/vapour retarder and the building paper.
1.2.2.5 Thermal bridges
Limited thermal bridging is a clear advantage of timber-framed construction. Only when very low whole wall thermal transmittances are imposed, does one need engineered studs and alternative solutions for header plates, frame corners, window reveals and lintels, see Figure 1.14. Metal framed construction is a different story. As Table 1.5 showed, correct stud and plate shaping and the use of thermally insulating sheathing then becomes very important.
Figure 1.14. Timber-framed outer wall: adapting header plate design to avoid thermal bridging.
1.2.3 Building physics: acoustics
Figure 1.15. Timber-framed party wall.
1.2.4 Durability
Timber is moisture sensitive and deforms anisotropic under hygric load. How to avoid unacceptable wetting is explained above. Platform framing should absorb hygric movements without damage: studs one floor high and sheathing jointed per floor. Each floor deck then acts as a kind of hinge, excluding high hygric movement induced bending moments in the studs.
Figure 1.16. Clapboarding.
Care should be taken with synthetic and timber outside finishes. Synthetics deform thermally, wood hygrically. In both cases the best practice is to use small elements, fastened in a way movement remains possible, as is the case with slated finishes, synthetic siding, aluminium siding, timber siding and timber clapboarding. There, the upper planks cover the nails of the lower ones (Figure 1.16). None of these finishes, however, assures air-tightness. Siding and clapboarding are even not rain-tight, which is why the building paper must be draped in a way it acts as drainage plane.
1.2.5 Fire safety
Timber and timber-framed construction is fairly combustible. Application therefore is only allowed for low-rise construction, up to three storeys, while the inside finish must be fire safe, as gypsum board is. Constructing party walls can anyhow be done in a way, overall fire resistance touches 90′ or more. It suffices to assemble them as sketched in Figure 1.15: two leafs, separated by fire proof wood wool cement boards, the bays between studs filled with mineral wool and both leafs lined with a double layer of gypsum board.
1.2.6 Maintenance
If correctly designed and built – airtight, moisture tolerant, no problematic thermal bridging, hygric movement absorbed without cracking – the maintenance intensity of timber framed hardly differs from massive construction. Of course, maintenance outdoors depends on the finish.
1.3 Design and execution
1.3.1 Above grade
Once the foundation walls are finished, thermally cutting the floor support minimizes thermal bridging to the substructure and ground. Cellular glass blocks are well suited for that (Figure 1.3). After casting the concrete ground floor, the surfaces where the timber framed walls come are levelled. That way the bottom plates are in continuous contact with the deck without needing wedges, a blameworthy practice. Under the bottom plates comes a waterproof layer, and then one anchors these plates with tension bolts into the deck. All joints between plates and waterproof layer are also sealed. An alternative is to use a thick enough polymer bitumen or bitumen pasta as waterproofing.
1.3.2 Frame
Walls are tied together by coupling the single top plates with steel connectors or by using a double top plate with the upper one staggered over the wall’s depth (Figure 1.17). For lintels with limited span two sides down mounted studs are used. Larger spans are solved using insulated headers composed of studs and plates with plywood or OSB shearing at both sides. Sometimes a timber ring girder is applied (Figure 1.17). Double studs, of which one acts as jack stud supporting the header, line window and door bays at their sides. An alternative is to fix the headers using steel header hangers.
Figure 1.17. Timber-framed construction: frame detailing.
1.3.3 Thermal insulation
Best suited is mineral wool or glass fibre. For so-called sustainability reasons, one also uses sprayed cellulose. Sprayed PUR as alternative guarantees better air-tightness. Anyhow, all bays demand a complete fill between sheathing and airtight layer, which is stapled or mounted after the insulation is put in place.
1.3.4 Air and vapour retarder
As said above, continuity is of prime importance here. The layer also does not tolerate perforation after mounting. Therefore, it is highly recommended to leave a 3 to 5 cm deep service cavity between air and vapour retarding layer and inside lining. Electricity guiding rods and pipes are installed after the air and vapour retarding foil is fixed and the supporting laths for the lining nailed. Then the service cavity is filled with stiff mineral wool boards to guarantee enough mechanical support for the foil not to rip off under wind pressure. Despite all this, it is still common practice to fix all guiding rods in the frame bays before inserting the insulation.
1.3.5 Building paper
We discussed the functions of the building paper above: second drainage plane and additional wind barrier. For more than a decade, spun-bonded foils with good air- and water-tightness but low diffusion thickness, only 0.01 to 0.02 m, have been on the market. A good choice except when a brick veneer or other highly water buffering outer finish is used. In such a case, a control on solar driven vapour flow is a necessity. Anyhow, any building paper foil wraps the sheathing in horizontal stripes, starting at the bottom and going up the envelope with the next strip overlapping the one below over 10 cm or more. Down the building paper, a tray should drain any run-off back to the outside. At door and window bays, the foil is wrapped around the headers and side studs.
1.3.6 Variants
The following variants are quite common: (1) replacing the OSB or plywood sheathing by thermally insulating XPS- or stiff glass fibre boards, (2) composing the construction of prefabricated, modular outside and partition wall timber-framed elements. Variant (1) largely minimizes thermal bridging but in many cases, additional cross bracing tied into the top and bottom plates must guarantee horizontal stiffness.
As said above, timber-framed gained popularity for passive house construction in countries with massive construction tradition. The reason is the ease to insulate with uneconomical thick insulation packages, while keeping thermal bridging minimal. A popular outer wall assembly looks like (from inside to outside):
The assembly must guarantee maximum air-tightness and exclude winter interstitial condensation. However, as already shown, the design completely overlooks the negative effects of solar driven vapour flow for walls with brick veneer.
1.4 References and literature
[1.1]
Febelhout, Tekst:
wat is houtskeletbouw, niet gedateerd
(in Dutch).
[1.2]
Latta, J. K. (1973).
Walls, windows and roofs for the Canadian climate.
Special Technical Publication No. 1 of NRC, Division of Building Research, 94 pp.
[1.3]
Architects Journal (1973). Technical Study, Timber.
[1.4]
Trethowen, H. (1976).
Condensation in cavities of building structures.
New Zealand Journal of Science, Vol. 19, 311–318.
[1.5]
STS 23 (1979). Houtbouw (in Dutch).
[1.6]
Laboratorium Bouwfysica (1980).
Invloed van lekken in het dampscherm op het condensatiegedrag van constructiedelen
. Rapport 80/1 (in Dutch).
[1.7]
Mayer, K., Künzel, H. (1980).
Test results concerning the ventilation of the air spaces behind siding using small scale elements
. Report Institut für Bauphysik.
[1.8]
NBI (1981).
Building Research Data Sheets, Timber Frame Walls
. Building Details A523.255.
[1.9]
Samuelson, I. (1981).
Moisture conditions in internally insulated underground walls.
Report CIB-W40, Copenhagen meeting, Swedish National Testing Institute, Borås.
[1.10]
NRC (1983).
Division of Building Research, Exterior walls, Understanding the Problems.
Proceedings of the Building Science Forum 1982, Proceedings No. 6.
[1.11]
STS 23 (1983). Houtbouw, addendum (in Dutch).
[1.12]
Laboratorium Bouwfysica (1983).
Bouwfysische analyse van een woningbouwsysteem.
Rapport 83/38 (in Dutch).
[1.13]
Johnson, K. A. (1985).
Interstitial condensation, Building Technical File.
Pilkington Research and Development, No. 8, January.
[1.14]
Harderup, L. E., Claesson, J., Hagentoft, C. E. (1985).
Prevention of moisture damage by ventilation of the foundation.
Report Department of Building Technology, Lund Institute of Technology.
[1.15]
Laboratorium Bouwfysicsa (1985).
Systeembouw-woning ‘Futurhome’, Isolatieproblemen.
Rapport 86/5 (in Dutch).
[1.16]
Christensen, G. (1985).
Summer condensation in post-insulated exterior walls.
Report CIB-W40, Holzkirchen meeting, Danish Building Research Institute.
[1.17]
Vansant, B. (1986).
Hygrothermisch gedrag van houtskeletbouwwanden met hoge thermische prestaties
. Eindwerk KU Leuven (in Dutch).
[1.18]
Scanada Consultants Ltd (1986).
Computer model of the drying of the exterior portion of wood framed walls
. Report.
[1.19]
Andersen, N. E. (1987).
Summer condensation in an unheated building.
Report CIB-W40, Borås meeting, Danish Building Research Institute, Borås.
[1.20]
Bankvall, C. (1987).
Thermal performance of building envelopes with high thermal resistance
. Report, Swedish National Testing Institute.
[1.21]
Hens, H. (1987).
Buitenwandoplossingen voor de residentiële bouw: houtskeletbouw.
Intern rapport Laboratorium Bouwfysica (in Dutch).
[1.22]
Oefeningen houtskeletbouw (1987–1988, 1988–1989, 1990–1991) (in Dutch).
[1.23]
Gockel, H. (1987).
Wood frame construction.
DBZ, 8.
[1.24]
McCusaig, L., Stapledon, R. (1987).
A study of the drying potential of various wood-frame systems used in atlantic Canada
. Internal report.
[1.25]
Johnson, K. A. (1988).
Improving the thermal performance of timber framed walls.
Building Technical File, Pilkington Research and Development, No. 21, pp. 31–38.
[1.26]
Nieminen, J. (1989).
Building Physical Behaviour of a Wooden Wall Structure
. CIB World Congress, Paris.
[1.27]
Burch, M. D., Thomas, W. (1991). An Analysis of Moisture Accumulation in a Wood Frame Wall Subjected to Winter Climate. NISTIR 4674.
[1.28]
CMCH (1991). Air Barrier Technology Update.
[1.29]
Proceedings of the ASHRAE/DOE/BTECC Conference on the Thermal Performance of the Exterior Envelopes of Buildings V (1992). ASHRAE, Clearwater Beach.
[1.30]
Burch, M. D., TenWolde, A. (1992).
Controlling Moisture in the Walls of Manufactured Housing
. NISTIR 4981.
[1.31]
Sandin, K. (1993).
Moisture conditions in cavity walls with wooden framework.
Report Department of Building Materials, Lund University.
[1.32]
Lstiburek, J., Carmody, J. (1993).
Moisture Control Handbook: Principles and Practices for Residential and Small Commercial Buildings
. Van Nostrand Reinhold, New York.
[1.33]
Hens, H. (1994).
Houtbouw, bouwfysisch en bouwkundig.
Informatiedag ‘Bouwen met Hout’, Febelhout (in Dutch).
[1.34]
Hens, H. (1994).
Lichte Bouwsystemen, een algemene bouwfysische beoordeling.
TI-KVIV studiedag ‘Lichte Bouwsystemen’ (in Dutch).
[1.35]
Information Holz (1994).
Holzbau Handbuch.
Reihe 1, Entwurf und Konstruktion, Teil 3, Folge 2 (in German).
[1.36]
Proceedings of the ASHRAE/DOE/BTECC Conference on the Thermal Performance of the Exterior Envelopes of Buildings VI (1995). ASHRAE, Clearwater Beach.
[1.37]
Information Holz (1995).
Holzbau Handbuch.
Reihe 1, Entwurf und Konstruktion, Teil 3, Folge 3 (in German).
[1.38]
Zarr, R., Burch, D., Fanney, H. (1995).
Heat and Moisture Transfer in Wood-Based Wall Construction: Measured versus Predicted
. NIST Building Science Series 173.
[1.39]
Uvslokk, S. (1995).
U-values of wall constructions, influence of workmanship.
Annex 24 report, Norwegian Building research Institute.
[1.40]
Künzel, H. (1995).
Regenschutz, Feuchteschutz und Wärmeschutz von Fachwerkwänden, Hinweise für Sanierung und Neubau
. WKSB, Heft 35, pp. 1–9 (in German).
[1.41]
MacGowan, A., Desjarlais, A. (1997).
A Investigation of Common Thermal Bridges in Walls.
ASHRAE Transactions, Vol. 103, Part 1.
[1.42]
Tuluca, A., Devashish, L., Jawad, H. Z. (1997).
Calculation Methods and Insulation Techniques for Steel Stud Walls in Low-Rise Multifamily Housing
. ASHRAE Transactions, Vol. 103, Part 1.
[1.43]
Building Science Corporation (1997).
Builder’s Guide: Mixed Climates.
[1.44]
Proceedings of the ASHRAE/DOE/BTECC Conference on the Thermal Performance of the Exterior Envelopes of Buildings VII (1998). ASHRAE, Clearwater Beach.
[1.45]
Building Science Corporation (1998).
Builder’s Guide: Hot-Dry & Mixed-Dry Climates
.
[1.46]
Energy Efficient Building Association (2000).
