CMHC- Durable Wood Frame Construction for All Climates

7/25/2019 CMHC- Durable Wood Frame Construction for All Climates

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D

URABLEWOOD-FRAME

C

ONSTRUCTIONFORALLCLIMATES

www.cmhc.ca

67102


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CMHCHOME TO CANADIANS

Canada Mortgage and Housing Corporation (CMHC) has

been Canadas national housing agency for more than 65 years.

Together with other housing stakeholders, we help ensure

that the Canadian housing system remains one of the best

in the world. We are committed to helping Canadians access

a wide choice of quality, environmentally sustainable and

affordable housing solutions that will continue to create

vibrant and healthy communities and cities across the country.

For more information, visit our website at www.cmhc.ca

You can also reach us by phone at 1-800-668-2642 or

by fax at 1-800-245-9274.

Outside Canada cal l 613-748-2003 or fax to 613-748-2016.

Canada Mortgage and Housing Corporation supports the

Government of Canada policy on access to information

for people with disabilities. If you wish to obtain this

publication in alternative formats, call 1-800-668-2642.


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DURABLE WOODFRAME

CONSTRUCTION FOR

ALL CLIMATES

CMHC offers a wide range of housing-related information. For

details, call 1 800 668-2642 or visit our home page at www.cmhc.ca

Cette publication est aussi disponible en franais sous le titre :Constructions ossature de bois durables pour toutes les zones

climatiques (67103)


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The information contained in this publication represents current research resultsavailable to CMHC, and has been reviewed by a wide spectrum of experts in thehousing industry. Readers are advised to evaluate the information, materials andtechniques cautiously for themselves and to consult appropriate professionalresources to determine whether information, materials and techniques are suitablein their case. The drawings and text are intended as general practice guides only.Project and site-specific factors of climate, cost, esthetics and so on must be takeninto consideration. Any photographs in this book are for illustration purposesonly and may not necessarily represent currently accepted standards.

Library and Archives Canada Cataloguing in Publication

Mattock, ChrisDurable wood-frame construction for all climates / Chris Mattock, Barry Craig.

Issued also in French under title: Constructions ossature de bois durablespour toutes les zones climatiques.Available also on the Internet.Includes index.ISBN 978-1-100-16986-6Cat. no.: NH15-448/2011E

1. Wooden-frame houses--Canada--Design and construction.

2. Wooden-frame buildings--Canada--Design and construction.3. House construction--Canada. I. Craig, Barry II. Canada Mortgage and HousingCorporation III. Title.

TH1101 M38 2011 694.1 C2011-980122-1

2011 Canada Mortgage and Housing Corporation

All rights reserved. No portion of this book may be reproduced, stored in aretrieval system or transmitted in any form or by any means, mechanical,electronic, photocopying, recording or otherwise without the prior written

permission of Canada Mortgage and Housing Corporation. Without limiting thegenerality of the foregoing, no portion of this book may be translated from Englishinto any other language without the prior written permission of Canada Mortgageand Housing Corporation.

Printed in Canada

Produced by CMHC


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Canada Mortgage and Housing Corporation i

Table of contents

TABLE OF CONTENTSPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Scope and Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Part 1: Introduction to the Building Envelope

Wood-Frame Construction . . . . . . . . . . . . . . . . . . . . . 7What is the Building Envelope? . . . . . . . . . . . . . . . . . . . . . . . 8

Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Crawl Space and Basement Foundations . . . . . . . . . . . . . . . . . 12

Slab-on-grade Foundations . . . . . . . . . . . . . . . . . . . . . . . . 16

Foundation Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Stick-Built Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Prefabricated Panel Framing. . . . . . . . . . . . . . . . . . . . . . . 21

Advanced Framing Techniques . . . . . . . . . . . . . . . . . . . . . 22

Roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Pitched Roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Roof Trusses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Site-framed Pitched Roofs. . . . . . . . . . . . . . . . . . . . . . . . 25

Low-Slope Roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Roof Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Ice Dams and Eave Protection . . . . . . . . . . . . . . . . . . . . . 32

Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Doors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Window Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Sealed Glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Low-emissivity Coatings . . . . . . . . . . . . . . . . . . . . . . . . 35

Gas Fills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Air Tightness, Water Resistance and Wind Load Resistance . . . . . . . 36

Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Flashings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Air Barrier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Skylights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Mechanical and Electrical Penetrations . . . . . . . . . . . . . . . . . . . . . . . . 40Foundation and Wall Penetrations . . . . . . . . . . . . . . . . . . . 40

Roof Penetrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41


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Chemical Incompatibility . . . . . . . . . . . . . . . . . . . . . . . . 130

Ventilation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Natural Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132Hybrid Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

Mechanical Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . 137

Exhaust Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Supply Only Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 140

Balanced Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

Building Science Summary . . . . . . . . . . . . . . . . . . . . . . . . 151

Cold Climate Buildings . . . . . . . . . . . . . . . . . . . . . . . . .152

Hot Climate Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . 156

Conclusion to Part 2 . . . . . . . . . . . . . . . . . . . . . . . . . . .160

Part 3: Climatic Considerations

Introduction to Part 3 . . . . . . . . . . . . . . . . . . . . . .163Categorizing Climates for Building Design and Construction . . . . . . .164

Climatic Zones for Building Design and Construction . . . . . . . . . .165

Climatic Influences . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

Heating and Cooling

Degree Days . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .166

Wind. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167

Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167

Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167

Solar Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

Local Climatic Effects . . . . . . . . . . . . . . . . . . . . . . . . . .168

Seasonal Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

Environmental Considerations for Moisture Control . . . . . . . . . . . .177Rain Control Design . . . . . . . . . . . . . . . . . . . . . . . . . .177

Wind-Driven Rain Moisture Control Performance of Cladding Systems . .178

Building Orientation and Roof Overhangs . . . . . . . . . . . . . . .180

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

Air Leakage Control Design . . . . . . . . . . . . . . . . . . . . . . .181

Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .182

Choice of Air Barrier System . . . . . . . . . . . . . . . . . . . . . .184

Moisture Balance and Vapour Diffusion Control Design . . . . . . . . .185

Materials for Vapour Diffusion Control. . . . . . . . . . . . . . . . .187

Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . .189

Zonal Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

Flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .191North American Marine Locations. . . . . . . . . . . . . . . . . . . .191

Conclusion to Part 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . 191


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Part 4: Construction Assemblies

Introduction to part 4 . . . . . . . . . . . . . . . . . . . . . .195Construction Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . 197

Very Cold (Vc) Climate: Assembly Details no. 1 . . . . . . . . . . . . .197

Cold Dry (Cd) Climate: Assembly Details no. 2 . . . . . . . . . . . . .201

Cold Humid (Ch) Climate: Assembly Details no. 3 . . . . . . . . . . .205

Warm or Hot Dry (Whd) Climate: Assembly Details no. 4 . . . . . . .209

Warm or Hot Humid (Whh) Climate: Assembly Details no. 5 . . . . . .213

Multi-Climate: Assembly Details no. 6. . . . . . . . . . . . . . . . . .217

Conclusion to Part 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . 221


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LIST OF FIGURESBuilding envelope components . . . . . . . . . . . . . . . . . . . . . . 8

Foundation types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Basement foundation with interior insulated wood-frame wall . . . . . 14

Basement foundation with exterior insulation . . . . . . . . . . . . . 15

Crawl space foundation . . . . . . . . . . . . . . . . . . . . . . . . . 16

Slab-on-grade foundation with perimeter insulation . . . . . . . . . . 17

Erection of walls in platform framing . . . . . . . . . . . . . . . . . . 20

Modified balloon framing . . . . . . . . . . . . . . . . . . . . . . . . 22

Roof Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Types of prefabricated roof trusses . . . . . . . . . . . . . . . . . . . . 25

Site-framed pitched roofs . . . . . . . . . . . . . . . . . . . . . . . . 26

Dormer roof framing . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Ventilated flat eave detail, Cold Roof design. . . . . . . . . . . . . . 28

Unventilated pitched roof eave detail, Hot Roof design . . . . . . . . 29

Ventilated pitched roof eave detail, Cold Roof design . . . . . . . . . 30

Soffit roof ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Ridge vent (A) and gable vent (B) . . . . . . . . . . . . . . . . . . . . 31

Ice dams and eave protection . . . . . . . . . . . . . . . . . . . . . . 33

Window types (viewed from exterior) . . . . . . . . . . . . . . . . . . 34Low-E glass coating for hot climates . . . . . . . . . . . . . . . . . . 36

Window sub sill flashing integrated with WRB . . . . . . . . . . . . . 37

Window head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Window sill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Stepped flashing for skylight . . . . . . . . . . . . . . . . . . . . . . 41

Prefabricated vent pipe flashing on roof . . . . . . . . . . . . . . . . . 42

Prefabricated vent pipe flashing at ceiling . . . . . . . . . . . . . . . . 43

Water entry by gravity . . . . . . . . . . . . . . . . . . . . . . . . . 49

Controlling water movement due to gravity. . . . . . . . . . . . . . . 49


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List of figures

Capillary suction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Water movement by capillary suction and some methods of control . . 51

Water entry by kinetic energy . . . . . . . . . . . . . . . . . . . . . 52

Preventing water entry by kinetic energy . . . . . . . . . . . . . . . . 52

Water entry by wind pressure . . . . . . . . . . . . . . . . . . . . . . 53

Preventing wind-driven rain entry with

pressure-moderated rainscreen . . . . . . . . . . . . . . . . . . . . . 54

Air leakage can move large amounts of water vapour due to

pressure differences across the building envelope . . . . . . . . . . . . . . . . 54

Vapour diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Moisture Penetration Control Mechanisms . . . . . . . . . . . . . . . 56

Examples of deflection . . . . . . . . . . . . . . . . . . . . . . . . . 58

Examples of deflection . . . . . . . . . . . . . . . . . . . . . . . . . 59

Examples of drainage . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Moisture-laden indoor air leaking into insulated cavities can lead

to condensation during cold months . . . . . . . . . . . . . . . . . . 61

Moisture-laden outdoor air leaking into an air-conditioned

building can lead to condensation formation during hot months . . . 62

Examples of drying in different wood-frame wall assemblies . . . . . . 64

Vapour diffusion and condensation in heating and cooling climates. . . 65

Exposure to wind-driven rain . . . . . . . . . . . . . . . . . . . . . . 67

Effects of overhangs on wall performance . . . . . . . . . . . . . . . . 68

Influence of overhangs and pitched roofs on wind and rain flow . . . . 69Rain deflection measures when roof overhangs are not possible . . . . . 70

Rainwater accumulation on cladding . . . . . . . . . . . . . . . . . . 71

Examples of Storage or Mass Wall Systems . . . . . . . . . . . . . . . 72

Example of Face-sealed Wall System. . . . . . . . . . . . . . . . . . . 73

Example of a Concealed-barrier Wall System . . . . . . . . . . . . . . 74

Example of Rainscreen Wall system . . . . . . . . . . . . . . . . . . . 75

Example of Pressure-Moderated Rainscreen Wall System . . . . . . . . 76

Example of PERSIST Wall System. . . . . . . . . . . . . . . . . . . 77

Moisture Removal Mechanisms in Walls . . . . . . . . . . . . . . . . 78

Wall Ventilation Techniques. . . . . . . . . . . . . . . . . . . . . . . 80Driving forces of air leakage . . . . . . . . . . . . . . . . . . . . . . . 82

Air leakage through ceiling penetrations . . . . . . . . . . . . . . . . 84


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Typical air leakage locations in wood-frame construction. Air will

leak either in or out of the building envelope, depending on air

pressure differentials. . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Possible air barrier locations in the building envelope . . . . . . . . . . 89

Combined polyethylene air and vapour barrier . . . . . . . . . . . . . 90

Vapour barrier tab at ends of partitions and air barrier over stud

wall top plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Airtight Drywall Approach (ADA) air barrier . . . . . . . . . . . . . . 92

Example of ADA wall penetration details . . . . . . . . . . . . . . . . 93

Preparing for a fan-door depressurization air tightness test . . . . . . . 94

Air tightness testing using fan-door depressurization . . . . . . . . . . 95

Air leakage in sheet metal ductwork is very high unless air-sealed

with a liquid sealer or durable aluminum foil tape. . . . . . . . . . . . 97Vapour diffusion from interior to exterior in a cold climate can result

in condensation formation and damage to the wall assembly . . . . . . 99

Placement of a vapour barrier on the inside of a wall cavity in a

cold climate can retard vapour diffusion into the insulated cavity

preventing condensation . . . . . . . . . . . . . . . . . . . . . . . . 100

Locating a vapour barrier on the inside of an insulated wall cavity in

an air-conditioned building in a hot-humid climate can also lead to

condensation formation and damage to the wall assembly. . . . . . . .101

Insulating a wall cavity on the outside of the sheathing with impermeable

insulation will resist water vapour diffusion in both winter and summer

and allow for drying of the structure to the interior and drying of thecladding to the exterior. Although more costly than an insulation filled

cavity, this option will work in many climates. . . . . . . . . . . . . .102

Placing low vapour permeance, rigid insulation on the exterior side

of a wall retards vapour diffusion into insulated wall cavities in

hot-humid conditions. . . . . . . . . . . . . . . . . . . . . . . . . .105

A vapour barrier can be located partway through a wall if the

temperature of the vapour barrier always stays above the dew point

temperature of the air with which it is in contact. . . . . . . . . . . .106

Placement of the vapour barrier with equal insulation value on each

side will prevent condensation formation during winter and summer in

climates with mild winters and hot-humid summers. . . . . . . . . . .106

Exterior Insulation and Finish System (EIFS) wall incorporatingdrainage channels, pressure-equalized two-stage joints and a water

resistant barrier membrane or coating. . . . . . . . . . . . . . . . . .107


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List of figures

Typical temperature gradient through a wall assembly for a

heating climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Psychrometric Chart . . . . . . . . . . . . . . . . . . . . . . . . . .114

Heat transfer by conduction. . . . . . . . . . . . . . . . . . . . . . . 115

Heat transfer by convection . . . . . . . . . . . . . . . . . . . . . . 116

Heat transfer by radiation . . . . . . . . . . . . . . . . . . . . . . . . 117

Heat transfer by air leakage . . . . . . . . . . . . . . . . . . . . . . .118

Pollution sources in the home include, mould, cigarette smoke, volatile

organic compounds (VOCs), such as formaldehyde, emitted from

particle board and other finishes, moisture generated by cooking,

bathing and washing, products of combustion from kerosene or gas

heaters, cleaning agents and soil gases. . . . . . . . . . . . . . . . . . 131

Natural ventilation driven by stack effect within a room . . . . . . . .133Wind-driven ventilation using single-sided ventilation . . . . . . . . .133

Ratio of building depth to ceiling height for wind-driven ventilation . .134

Building forms that aid buoyancy or stack effect driven ventilation . . .135

Building forms that aid buoyancy or stack effect driven ventilation . . .136

Modifying wind flow with landscaping . . . . . . . . . . . . . . . . . 136

Basic exhaust only ventilation system . . . . . . . . . . . . . . . . . .139

Central exhaust only ventilation system. . . . . . . . . . . . . . . . .140

Supply only ventilation system . . . . . . . . . . . . . . . . . . . . . 141

Basic balanced ventilation system . . . . . . . . . . . . . . . . . . . . 142

Recirculating central ventilation system . . . . . . . . . . . . . . . . .144Heat Recovery Ventilator (HRV) . . . . . . . . . . . . . . . . . . . . 145

Central Heat Recovery Ventilator (HRV) system with separate

exhaust and supply ducting . . . . . . . . . . . . . . . . . . . . . . .146

Central Heat Recovery Ventilator (HRV) system connected to furnace

for supply air distribution . . . . . . . . . . . . . . . . . . . . . . . . 147

Energy Recovery Ventilator (ERV) . . . . . . . . . . . . . . . . . . .148

Heat and moisture recovery in a heat wheel Energy Recovery

Ventilator (ERV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Requirements for a central distributed mechanical ventilation system . .151

Building-as-a-system for heating climates Step 1: Uninsulated

building envelope.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 152


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Building-as-a-system for heating climates Step 2: Insulation reduces

heat loss but can lead to condensation due to air leakage and cold temperatures

in the exterior sheathing and outer parts of the building envelope. . . . 152

Building-as-a-system for heating climates Step 3: A continuous air

barrier reduces condensation in insulated cavities by preventing air

leakage, but may cause indoor air quality problems resulting from

decreased fresh air delivery when uncontrolled air leakage has

been reduced. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

Building-as-a-system for heating climates Step 4: Indoor air quality

is improved by providing mechanical ventilation and by using low-toxicity

interior finishes. Heat losses increase as a result of additional ventilation. . 154

Building-as-a-system for heating climates Step 5: Heat recovery

ventilator (HRV) reduces heat losses resulting from ventilation by

up to 70 per cent. . . . . . . . . . . . . . . . . . . . . . . . . . . . 155Building-as-a-system for cooling climates Step 1:

Uninsulated building envelope. . . . . . . . . . . . . . . . . . . . . . 156

Building-as-a-system for cooling climates Step 2: Thermal

insulation reduces heat gains but can lead to condensation resulting

from air leakage and vapour diffusion. . . . . . . . . . . . . . . . . .157

Building-as-a-system for cooling climates Step 3: A continuous

air barrier throughout the building envelope minimizes entry of

moisture-laden air from the outside into insulated cavities. Vapour

diffusion is controlled by a vapour-resistant and water-resistant barrier

or by a low-permeance exterior sheathing.. . . . . . . . . . . . . . . .158

Building-as-a-system for cooling climates Step 4: Low toxicity

materials improve indoor air quality, but mechanical ventilation

increases cooling load. . . . . . . . . . . . . . . . . . . . . . . . . . . 159

Building-as-a-system for cooling climates Step 5:

Energy recovery ventilator (ERV) reduces heat and humidity

gain due to ventilation by up to 80 per cent. . . . . . . . . . . . . . .160

North American Climatic Zones* . . . . . . . . . . . . . . . . . . . .171

South American Climatic Zones* . . . . . . . . . . . . . . . . . . . . 172

European Climatic Zones* . . . . . . . . . . . . . . . . . . . . . . . 173

Asian Climatic Zones* . . . . . . . . . . . . . . . . . . . . . . . . .174

African Climatic Zones* . . . . . . . . . . . . . . . . . . . . . . . .175

Australasian Climatic Zones* . . . . . . . . . . . . . . . . . . . . . .176Plan details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .199

Section details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200


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Plan details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .203

Section details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

Plan details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .207

Section details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

Plan details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .211

Section details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Plan details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .215

Section details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

Plan details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .219

Section details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220


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Canada Mortgage and Housing Corporation 1

This document supplements CanadianWood-frame House Construction, CanadaMortgage and Housing Corporationsbest-selling book on wood-frameresidential construction in theCanadian climate.

Wood-frame construction, also calledtimber frame construction, has been asuccessful residential building methodfor centuries in many countries around

the world. This is proof of the durabilityof wood-frame construction in all climates.

There have been many changes inresidential design and construction:

n Economic necessity and a betterunderstanding of buildingperformance have been the maindrivers of change.

n New materials and methods ofconstruction have offered designersand builders new opportunities.

n

Governments and homeownershave encouraged and rewardedgreater energy efficiency andenvironmental responsibility.

With these changes came mistakes,

particularly when designers and builders

ignored the consequences of poor

building envelope design. From

mistakes, changes in construction and

research in building performance, we

now have a better understanding

of moisture control in buildings,which has led to improved building

envelope design.

The desire of homeowners for a more

controlled indoor environment, through

the use of heating systems and air

conditioning has resulted in the

building-as-a-system approach to thedesign and construction of housing.

It is no longer possible to ignore the

impact of the design of the buildingenvelope on the environmental

acceptability and comfort of the

indoor environment.

Durable Wood-frame Construction forAll Climates is a bridge between theestablished practices of the past andtodays conditions and imperatives.CMHC has learned from experiencethat when a sound and flexibleapproach is taken, properly designedand constructed wood frame buildings

are durable and can be successfullybuilt in all climates.

Durable Wood-frame Construction forAll Climates sets out a design andconstruction approach that enables

durable and sustainable wood-frame

construction to be built around the world.

PREFACE


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Introduction

INTRODUCTIONThis document has been developed toprovide designers and builders with theinformation they require to deal withthe durability aspects of wood-framebuildings in any climate, ranging fromvery cold to hot and humid. There isover a century of experience in Canadawith wood-frame construction, and overthis period it has been proven that ifwood-frame buildings are built andmaintained appropriately for the local

climate they will be durable and last avery long time. With Canadian companiesbeginning to supply wood-frameconstruction materials and designexpertise around the world, it isimportant to recognize the influence oflocal climates and cultural differences thatcan affect the durability of wood-framebuildings. Some of the cultural differencesthat can affect durability include the useof room-by-room heating or coolingrather than whole-house heating orcooling and the temperatures and relativehumidity at which homes are operated.Not only must the building be builtappropriately for the local climate butit must also be operated and maintainedcorrectly to ensure long-term durability.This may require the Canadian supplierto inform the local designers, buildersand homebuyers of the operationand maintenance requirements of aCanadian wood-frame house.

This document is divided into four parts.Part 1 provides an introduction to the

building envelope. Part 2 identifies thebuilding science principles thatcontribute to a buildings durability. Part 3introduces definitions for various

climates encountered around the world,the approaches for assessing variousconstruction methods and materials andhow well suited they are to particularclimates. Part 4 applies the informationdescribed in Parts 1,2 and 3 and givesexamples of specific construction detailsthat are well suited to climates rangingfrom very cold to hot and humidconditions. Appendices A and Bprovide additional supporting and

reference information.

Durability of a building envelope canonly be achieved if moisture movementin the building envelope is controlled ormanaged. While the majority of thisdocument will be devoted to addressingthe control of moisture in walls, moisturecontrol in all the other components ofthe building envelope, including thefoundation, the roof and the openings,must also be achieved. The design ofthe roof and attic space, the interface

between walls and windows and doorsand the prevention of moisture entryfrom ground water, are also veryimportant considerations.

Another equally important requirementis to provide a healthy indoorenvironment for the occupants of thehouse. Whether living spaces are healthyor not, depends on how they are designed,constructed and maintained as wellas how their heating, cooling andventilation systems are designed and

operated. The quality of an indoorenvironment is a shared responsibilitybetween the designer, the builder andthe occupants of the building.


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44Canada Mortgage and Housing Corporation4

Introduction

Finally, in designing and constructing adurable wood-frame building we must

consider the correct priorities for thecontrol of moisture. The larger the sourceof moisture, the more attention should begiven to it. The more critical an elementis to the success of the building envelopeperformance, the more it must beconsidered to assure that it is designedand constructed properly. The moresusceptible a material is to moisture,the more care must be taken to avoidaccumulation of excess moisture in it.The purpose of Part 2 is to review theprimary sources of moisture, the forcesthat drive moisture and rules that mustbe observed to achieve effective moisturemanagement. Failure to properly addresseach of these aspects can lead to partialor, in some circumstances, completefailure of the building envelope.

SCOPE AND CONTEXT

The intent of this guide is to provide anintroduction to how wood-frame housesshould be built to provide healthy andcomfortable living conditions and to

provide durable, long-lasting buildingenvelopes that will resist the naturalforces in a variety of climates. TheDurable Wood-Frame Constructionfor All Climates guide will focusprimarily on the design and constructionof wood-frame wall assemblies. Varioustypes of wood-frame wall constructionin several climatic zones will bedescribed and illustrated in detail.

The proper connection of wall assembliesto the foundation and roof assemblies,

and to the various openings in them, isvery important, so that continuity ofthe environmental barriers in the buildingenvelope components is ensured, fromfoundation to wall to roof and to allopenings in them.

However, because of the great variety ofdesigns, methods and materials ofconstruction, coming from local buildingpractices and cultural preferences, thisguide will describe only the basicrequirements for the foundation,

roof and opening components of thebuilding envelope, and illustrate onlythe most common types used inwood-frame construction.

The building science principles describedfor the design and construction of wallassemblies, also apply to foundations,roofs and openings. These componentsof the building envelope must beconstructed to perform most of therequired functions of walls, andadditional functions imposed by local

climate, customs, materials, buildingpractices and codes. This guide willdescribe the construction methods andmaterials used for durable wood-framewall assemblies and will not describe theother building envelope components offoundations, roofs and openings in anydetail. The Canadian Wood-frameHouse Construction guide publishedby Canada Mortgage and HousingCorporation (CMHC) and theBuilders Manual published by theCanadian Home Builders Association

(CHBA) should be consulted for moredetailed information on wood-frameconstruction methods and materials.


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Part 1: Introduction

to the Building Envelope


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Part 1 presents an overview ofwood-frame construction and themajor components that make up thebuilding envelope.

Wood-frame construction primarilyutilizes dimensional lumber, but can alsomake use of engineered wood products

or prefabricated structural insulatedpanels to construct wall, floor androof assemblies. These wood-frameassemblies are strong, durable, suitablefor all climates, and relatively fast andinexpensive. Wood-frame constructionhas a long history of durability andexcellent performance in North America,northern Europe, Asia and Oceania.Contemporary wood-frame constructionin Canada has evolved through manyyears of practice and improvements madeas a result of extensive research at the

National Research Council of Canada,Canada Mortgage and HousingCorporation and other organizations.

Wood-frame bui ldings must becarefully designed and constructed toprovide lasting protection against theelements in the exterior environment,and provide occupant health, comfortand safety in the interior environment.

Wood-frame construction has thefollowing advantages when comparedwith masonry construction:

n Wood-frame construction is fast, easyto build, easy to make alterations to.

n Wood is a renewable resource thatcan be sustained through effectiveharvesting, conservation andprotection of our forests.

n Wood-frame construction can beeasily and highly insulated toconserve energy and minimize

heating and cooling costs.n Wood-frame construction can

withstand wind, rain, snow andearthquake loads. It utilizesflexible framing components withstrong connections that can adjustto severe loads without damage.

n Wood-frame construction can beadapted to suit all climatic conditions.

n Wood-frame construction can easilymeet or exceed the fire safety and

acoustical privacy standardsrequired by building codes.

n Wood-frame construction techniquescan be easily learned by builders andrequires the use of only basic tools.It utilizes high quality materialsand tested and proven detailssaving time and money during theconstruction process.

n Wood-frame construction iseconomical and provides excellentperformance and long-term durability.

n Wood-frame construction islight-weight and requires smallerfoundations and structural supports.

WOODFRAMECONSTRUCTION


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Building envelope components

WHAT IS THE

BUILDING ENVELOPE?The building envelope is the skin of abuilding and acts as an environmentalbarrier, which is also calledenvironmental separator, between theexterior environment and the interiorenvironment of a building. It containsstructural members, such as floor joists,wall studs and roof trusses, whichsupport the exterior cladding and roofsurface of the building. It is comprisedof the following four basic components

(refer to Figure 1.1):

1. Foundation, including footings,foundation walls and floor slabs.

2. Walls, including all the exterior walls.

3. Roof, including ceilings, attics,eaves and the roof surface.

4. Openings, including windows,doors, skylights and mechanicaland electrical penetrations in theother components.

The building envelope contains the

following environmental barriers:1. Thermal barriers(or heat barriers)

control heat transfer, keep heat insidethe building in cold climates, andkeep heat outside in hot climates.They can be located anywhere inthe assembly, between the exteriorcladding and interior finish.Materials include batt, semi-rigid,board or loose fill insulation.

2. Moisture barriers(or weatherbarriers, or water-resistant barriers

(WRB)) prevent liquid moisture fromentering the building assemblies. Theyshould be located between theexterior cladding and the structure,usually over the roof and wallsheathing. Materials includehouse- wrap, building paper, flexibleand rigid membranes.

Roof

Walls

Openings

Foundation

Roof

Walls

Openings

oundation

Openings

Fondation

1.1


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3. Air barriers (or air barrier systems)

prevent air from moving throughthe building assemblies. They maybe located anywhere in the wallsand roof, but must completelyenclose the conditioned interiorenvironment. Materials includesemi-permeable house-wrap,gypsum board, plywood, OSB,air barrier tape and sealants.

4. Vapour barriers (or vapourretarders) control water vapourdiffusion. Vapour barriers are

required in cold climates;semi-permeable vapour retardersare required in hot climates. Theymust be located on the warm sideof the thermal barrier for bothcold and hot climates. Materialsinclude polyethylene film orflexible membranes, plywood,OSB, and gypsum board paintedwith low permeance paint.)

5. Radiant barriers(or radiationbarriers) control solar radiation

and heat in the form of radiation.They are placed in roofs, walls andopenings to reflect heat to the insideof the building in cold climates,and to the outside in hot climates.Materials for roofs and wallsinclude aluminum foil-backedgypsum board, aluminum foil-facedinsulation, foil-faced polyethylene,aluminum foil, and radiant barrierpaints. Materials for windows andskylights include low-emissivity(low-E) coatings, placed on the

outside of the inner pane of glassin cold climates, and on the insideof the outer pane in hot climates.

6. Sound barriers(or noise barriers)

control the amount of sound ornoise passing through the buildingassemblies and openings, in alldirections. They reduce airbornesound transmission (measured assound transmission class (STC),and structure-borne soundtransmission (measured as impactinsulation class (IIC)). They arelocated in the roof, walls, floorsand openings. Materials includegypsum board, plywood, OSB,insulation, concrete, glass and steel.

7. Fire barriers(including fire blocks,fire stops, fire separations and firewalls) control the movement of fire,heat, smoke, and other products ofcombustion through the buildingassemblies. They compartmentalizebuilding components and habitablespaces to prevent the spread of firethrough and between them. Fireseparations and firewalls have fireresistance ratings measured in minutesor hours. Materials include gypsum

board, insulation, wood, metal,concrete, masonry and sealant.

Each building envelope component canbe adapted to respond to the demandsof the local climate, the availability ofmaterials, local building techniques andregulations, and the preferences of thebuilding occupants. These componentsare constructed in a particular sequence,beginning with the foundation, then thefloors, then the walls, followed by theroof and ending with the openings in

the components. The building envelopecomponents must work together as asystem to perform the functions of abuilding envelope.


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Foundation types

assemblies will be described to emphasize

the importance of the continuity of theenvironmental barriers between thesebuilding envelope components.

A brief discussion of the four majorcomponents of the building envelope the foundation, walls, roof andopenings will explain how theseassemblies must be constructed to managethe physical forces that act upon thebuilding envelope in different climates,so that the result is a comfortableand healthy interior environment forthe occupants.

FOUNDATIONS

Three types of foundations may be usedwith wood frame construction: basementfoundations, crawl space foundationsand slab-on-grade foundations (refer toFigure 1.2). Basement foundations weredeveloped in cold climates inresponse to the need for a footinglocated far enough below grade that

the soil beneath the footing would not

freeze and heave in the winter. For thisreason, basement foundations areusually only found in cold climates.Slab-on-grade or crawl space foundationsare used in mixed and hot climates.

Moisture control is important in all typesof foundations to ensure durability ofthe building structure and the health andcomfort of the occupants. The followingmeasures should be taken to minimizeground water entry and condensationformation in foundation assemblies:

n

The area around foundations shouldbe well drained by providing drainageat the bottom of the footings aroundthe perimeter of the foundation.

n There should be a capillary breakbetween the foundation componentsand the surrounding soil. This cantake the form of a water impermeablemembrane, such as polyethylene,beneath the floor slab and betweenthe foundation walls and footings, and

Basement

Crawl space

Slab-on-grade1.2


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Basement foundation with interior insulated

wood-frame wall

Air barrier made continuous by air barrier tab

under wall plate, around rim joist and under

sill plate

Siding, sheating and framing at least

200 mm to 600 mm (8 to 24) above grade

Slope soil (5% min.) away

from foundation wall

Air gap membrane or free-draining fill to

redirect ground water to drainage tile

Dampproofing to grade level to prevent

capillary suction from drawing moisture

into the wall

38 x 89 (2 x 4) wood framing

with batt insulation

Perimeter drainage tile with

filter fabric sock sloped to storm

sewer, dry well or daylight and

covered with crushed stone

.15 mm (6 mil) polyethylene capillary break

placed between footing and foundation wall

Polyethylene AVB or airtight drywall barrierwith vapour retarder primer.15 mm (6 mil)

polythylene

capillary break

placed under

concrete floor

slab

building paper

moisture barrier

to grade

1.3


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Basement foundation with exterior insulation

Air barrier made continuous by air barrier

tab under wall plate, over rim joist and under

sill plate

50 mm (2 in.) extruded polystyrene or

expanded polystyrene insulation or

high density glass fibre or mineral fibre

board insulation

Dampproofing to grade level to prevent

capillary suction from drawing moisture

into the wall

Air gap membrane or free-draining fill

to direct ground water to drainage tile



Prepainted sheet metal, preservative treated

plywood, concrete board or cement parging

above grade insulation board protection


filter fabric sock sloped to

storm sewer, dry well or

daylight and covered with

crushed stone



Concrete foundation wall acts

as air barrier

Interior finishes

can be added

after concrete

foundation wall

has thoroughly

dried

.15 mm (6 mil)

polyethylene

capillary break

placed under

concrete floor

slab

50 mm (2 in.) extruded polysthyrene or

expanded polystyrene insulation or

hight density glass fibre or mineral fibre

board insulation

1.4


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A crawl space is heated, cooled andventilated in the same manner as therest of the house. In hot-humidclimates the entire floor slab mayrequire insulation to prevent condensationfrom forming on the slab.

Slab-on-grade Foundations

Slab-on-grade foundations are monolithicreinforced concrete slabs with thickenededges to support the wood-frame walls

above, or floating slabs surrounded byshallow concrete foundation walls orgrade beams. The slab-on-grade is placed

over a 0.15 mm (6 mil) polyethylenevapour barrier to prevent moisture fromentering, over at least 150 mm (6 in.) offreely-draining granular material. Unlikebasements and crawl spaces, there is nouseable space beneath the ground floorof the building for mechanical/electricalequipment, storage or living space.Slabs-on-grade are usually placed at orslightly above grade and must beprotected from the effects of freezing in

cold climates. Rigid insulation isinstalled on the outside edges of theslab and extends to the depth of the

Crawl space foundation

Underside of footing below

level of frost penetration


filter fabric sock sloped to

storm sewer, dry well or

daylight and covered with

crushed stone



Air gap membrane or free-draining fill to

redirect ground water to drainage tile



Siding, sheathing and framing at least

200 mm to 600 mm (8 to 24) above grade

Air barrier made continuous by air barrier tab

under wall plate, around rim joist and under

sill plate

.15 mm (6 mil)

polyethylene

capillary break

placed under

concrete floor

slab

building paper

moisture barrierto grade

1.5


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slab thickening or grade beam around

the perimeter of the slab. This reducesheat transmission loss from the slabedge and retains heat from the buildingin the ground below the footings, whichkeeps the soil from freezing. In coldclimates where greater protection fromfreezing is required, insulation is placedaround the perimeter of the foundationat the bottom of the footing andextended horizontally outwards belowgrade to protect the footings from frost(refer to Figure 1.6).

The design of a slab-on-grade foundationis determined by the type and bearingcapacity of the soil. The amount ofinsulation for frost protection isdetermined by the climatic conditions,soil type and foundation depth.

Foundation Insulation

One of the major issues to be consideredin foundation design and constructionis the location and type of thermalinsulation used to control heat loss andheat gain in foundations. Insulation ismost often placed on the inside of thefoundation walls in crawl space andbasement foundations because it is lesscostly than to insulate the exterior ofthe foundation walls. However,exterior foundation insulation is anacceptable alternative.

Interior insulation placement has thefollowing advantages:

n It is easy to build, particularly afterthe house has been completed.

n The insulation is protectedfrom insects.

Slab-on-grade foundation with perimeter insulation

High-density extrudedpolystyrene insulation

Polyethylene dampproofing

under slab

Reinforced concrete slab

edge designed for soil

load-bearing capacity

Fasten wall to slab with

expanding anchor

200 mm (8 in.)

Compacted granular

steel reinforcement -

consult structural engineer

1.6


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n If framing is used to support

the insulation, the framing alsoprovides a space for mechanical andelectrical services and support forthe interior finishes.

Interior insulation has the followingdisadvantages:

n The foundation wall assembly maybe unable to dry to the exteriordue to moisture in the soil. Theonly area for moisture to escapefrom the foundation assembly tothe outside is in the above grade

portion of the foundation wall.n In cold climates, the foundation

assembly is unable to dry to theinterior due to the presence of avapour barrier located on theinside of the foundation wall.

n Where construction moisture istrapped in the foundation assemblyor where moisture from condensationdue to diffusion or air leakage, orfrom ground water penetrationhas occurred, interior wood-frame

insulated walls may experiencemould growth and rot.

n The benefits of including the thermalmass of the concrete foundationwalls inside the thermal barriercannot be realized.

Exterior insulation has the followingadvantages:

n It raises the interior temperatureof the foundation wall,thereby preventing condensation

from forming.

n It allows drying of the foundation

assembly, particularly theconcrete, to the interior whenindoor relative humidity is low.

n It protects the dampproofing orwaterproofing membrane fromphysical damage during backfilling

n It protects the foundation walls fromfreeze-thaw cycles in cold climates,

n The thermal mass of the concretefoundation wall is inside the thermalbarrier, thereby protecting the wall

from thermal cycling and actingas a heat sink which moderatesinterior temperature swings.

Exterior insulation has the followingdisadvantages:

n The insulation can provide apathway for insect entry into thewood wall framing above, if notdesigned to resist insect infestation.

n It requires the use of moreexpensive insulation materials,

such as extruded polystyrene.n Exterior insulation is more

difficult and expensive to install,unless done before backfillingagainst the foundation walls.

n It can be damaged by backfilling.

n It is more difficult to finish theexposed face of exterior insulationthan an exposed concrete wall.


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WALLS

Framing

Two common methods of framing usedin wood-frame house construction arestick-built and prefabricated construction.Stick-built framing involves on-siteassembly of individual wall studs, floorjoists, roof rafters or trusses and walland roof sheathing. Prefabricated walland roof panels and modular buildingsare the most common types ofprefabricated construction.

Prefabricated panels are assembled in afactory and delivered in flat sectionsapproximately 2.7 m (9 feet) high andup to 6.1 m (20 feet) long, where theyare erected on site and connected to formwalls and roofs. Modular (or Volumetric)buildings are also assembled in a factoryand delivered as complete buildings orin large floor, wall and roof sectionswhich include mechanical and electricalsystems, windows, doors and interior andexterior finishes. Modular buildings maybe built with wood framing techniques,but often employ other proprietaryconstruction methods and materials whichmay differ from those commonly usedin wood frame construction. Therefore,modular building techniques are beyondthe scope of this document; however,stick-built and prefabricated panelframing will be discussed in this section.

Wall framing includes the vertical andhorizontal members of exterior wallsand interior partitions. These structuralmembers referred to as studs, wall plates

and lintels, serve as a nailing base forall exterior sheathing and finish materialsand support the upper floors, ceiling androof structures. All framing lumber

should be grade-stamped and contain not

more than 19 percent moisture, and mayrequire treatment for use in some countries.

Stick-Built Framing

Exterior wall studs are the verticalmembers to which the wall sheathing,exterior cladding and interior finishesare attached. Wood studs are made from50 x 100 mm (2 x 4 in.) or 50 x 150 mm.rough-cut lumber, and measure 38 x 89mm (1-1/2 x 3-1/2 in.) or 38 x 140 mm(1-1/2 x 5-1/2 in.) when finished. Inmetric measurement they are referred

to as 38 x 89 mm and 38 x 140 mm,and in imperial measurement they arereferred to as 2 x 4 in. and 2 x 6 in.,respectively. The studs are supported ona bottom plate or foundation sill plateand in turn support the double top plates.Studs are usually spaced at 400 mm(16 in.) on centre. This spacing may bereduced to 300 mm (12 in.) or increasedto 600 mm (24 in) on centre, dependingon the loads to be supported by the walland the limitations of the cladding andfinish materials. Wider studs of 190 or240 mm (8 or 10 in.) may be used toprovide increased space for more wallinsulation, but this increases the cost ofthe studs substantially.

Rigid or semi-rigid insulation can beapplied to the exterior of the studs toincrease the thermal resistance of the wallassembly without increasing the size ofthe studs. The studs are attached tohorizontal top and bottom wall platesof 38 mm (2 in.) thick dimensionallumber that are the same width as the

studs. Lintels are the horizontal membersplaced over window, door and otheropenings to carry the loads from abovethe opening to the adjacent wall studs.


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Unless required by building codes and

regulations, scaffolding is unnecessarywhen sheathing is applied to the wallframing prior to erection. Rigid typesof sheathing, such as plywood, orientedstrandboard and waferboard, willprovide adequate structural bracing toresist racking and keep the wall square.Other less rigid types of sheathing, suchas rigid glass-fibre, asphalt impregnatedfibreboard, polystyrene or polyurethanefoam board, do not provide the requireddiagonal bracing, and the wall must bereinforced with diagonal wood or metal

bracing let into the wall studs. Completewall sections are then tilted up from thefloor structure and fitted into place. Thewall sections are held in place bytemporary braces and the bottom platesare nailed through the subfloor to the floorframing members below (refer toFigure 1.7).

Once the assembled wall sections arealigned vertically, or plumbed, theyare nailed together at the corners andintersections. A second top plate, with

joints offset at least one stud space awayfrom the joints in the first plate below,is then added. The second top plate lapsthe first plate at the corners and interiorpartition intersections and, whennailed in place, provides an additionaltie in to the framed exterior walls.

Prefabricated Panel Framing

The details of prefabricated wall panelsare different for each manufacturer, butthe handling, assembly and performanceof most products are similar. Early typesof prefabricated panel framing weresimilar to stick-built framing, exceptthat the panels were made in a factorybefore they were assembled on the

building site. Prefabricated panel framing

has been refined over a 50 year history totake advantage of new materials, newmethods of construction, improvementsin technology, and the benefits ofprefabrication to high tolerances in acontrolled environment. Many panelizedwall systems contain glass fibre or mineralfibre insulation between wood studs andare clad with exterior plywood or orientedstrand board (OSB) sheathing andgypsum board interior finishes. Otherpanelized systems called StructuralInsulated Panel Systems, or SIPS, are

made with rigid polystyrene orpolyurethane insulation glued to andsandwiched between the interior andexterior plywood or OSB sheathing andcontain no studs to create thermalbridges. By reducing the amount ofwood and increasing the amount andthermal resistance value (RSI or R-value)of the insulation used, these panelizedwalls have much higher insulation valuesthan stick-built framed walls of thesame thickness.

Prefabricated wall panels are usuallydelivered to the construction sitecomplete with exterior cladding andsheathing, structure, insulation, moisture,air and vapour barriers, and can alsoinclude openings, such as windows anddoors. The panels can be erected quicklyby a small crew with only minimalknowledge of wood-frame construction.Interior finishes are installed after thebuilding envelope has been closed in tokeep out the weather. Many interiorfinishes are susceptible to moisture and

transportation damage and,consequently, are installed after thewall and roof panels have been erectedand doors and windows installed.


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Prefabricated panel framing is used in-

creasingly in areas that are served byship, rail and truck transportation andat sites within a 1,000 km radius of aprefabrication factory. Studies bythe Athena Institute have shown thatbuildings constructed from prefabricatedpanels and transported by shipacross the North Atlantic Ocean, produceless greenhouse gas emissions thansimilar buildings constructed on sitewith local materials.

Advanced Framing Techniques

Advanced Framing Techniques, alsoknown as Reduced Materials Intensity orOptimum Value Engineering, isstick-built framing that has beenmodified to reduce material use andcosts. Advanced framing techniques canalso improve the thermal efficiency ofthe building envelope by reducing thequantity of wood studs and plates usedand increasing the amount of insulation,thereby minimizing the negative effectof heat transmission through the solidwood members. Because wood studs

conduct heat at a higher rate than

insulation, this effect, known as thermalbridging, reduces the overall thermalresistance of the building assembly. Inadvanced framing techniques, the studsare placed at the maximum 600 mm(24 in.) allowable spacing permitted bybuilding codes, with the floor joists androof trusses or rafters located directlyabove the studs. This ensures the roofand floor loads fall directly onto the studs,making a second top plate unnecessary.

In advanced framing techniques, lintelsare supported with single, rather thandouble jack studs and exterior corners areframed with two studs instead of three.Because the structural connections are lessrobust than in conventional framing,galvanized steel nailer plates and hangersare used to connect thestuds and lintels,and all joints in the single top plates.Advanced framing techniques reducethermal bridging by eliminating up to20% to 25% of the framing members,thus reducing materials intensity andleaving more space for insulation in the

wall and ceiling assemblies.

Modified balloon framing

Polyethylene VB tab behind

ledger, sealed to

polyethylene VB on walls

above and below

Lower floor wall framing

Upper floor wall framing

Joist

Joist ledger attached

to wall framing

Joist hanger

attached to ledger

1.8


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deflect rain, snow and ice, and therebydecrease the chance of liquid moisturegetting into the walls. An overhang of600 mm (24 in.) or more is recommendedfor damp and wet climates. The vertical

edge of the eave, or fascia, is coveredby wood or metal, and the horizontalunderside of the eave, or soffit, iscovered by wood, perforated metal or acementitious material. Of the manytypes of pitched roofs, the gable roof isthe most common and the simplest toconstruct, especially with the use oflightweight roof trusses. Otherconfigurations, such as the hip roof, aremore complex, but can also be easilyframed with roof trusses (refer to

Figure 1.9).

Roof Trusses

Pre-engineered and pre-assembled rooftrusses are triangulated wood structures

made with dimensional lumber chordand web members connected with metalplate gang nailers. They are designed bya structural engineer or with computersoftware and assembled in a factory

environment, which allows their qualityand precision to be strictly controlled.

Roof trusses are widely used in housesbecause they use less material thansite-assembled roof structures and speedup the process of enclosing the roof.Pre-assembled roof trusses are transportedby truck from the factory to theconstruction site, and installed by theframers. They provide support for theroof sheathing, a surface for theattachment of the ceiling finish material

and a space for ceiling insulation. Sinceroof trusses are commonly spaced at600 mm (24 in.) on centre, 19 x 75 mm(1 x 3 in.) wood strapping at 400 mm(16 in.) on centre must be applied to

Roof Types

3. Gable

6. Mansard5. Gambrel4. Hip

1. Flat 2. Monopitch

1.9


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supporting the dormer wall studs andthe valley rafters (refer to Figure 1.12).The roof sheathing may be installedbefore the dormer is framed, and thencut flush with the framing members

around the dormer opening. A bottomplate added on top of the sheathingsupports the side wall studs enclosingthe dormer and also serves as a nailingbase for the dormer wall sheathing. Thewalls and roof of the dormer are completedin the same way as those in the rest ofthe house. To reduce erection time andimprove the quality of the framing,dormers can be pre-fabricated in afactory and installed over pre-arrangedopenings in the roof. Except for verysmall attic spaces, uninhabited attics

require an access hatch for periodicinspection and maintenance of the attic.

Low-Slope Roofs

Low-slope and so-called flat roofs areusually less durable than pitched roofsin climates with heavy rain or snow

conditions because they dont deflectmoisture from precipitation as efficientlyas pitched roofs. In low-slope roofconstruction, the roof rafters can alsosupport the ceiling, and are called roofjoists. The size and spacing of the roofjoists required by the National BuildingCode of Canada is based on the roof andceiling loads. Roof joists for low-sloperoofs may be laid level or on a slope, withroof sheathing and a roof membrane.When the joists are laid level to maintaina flat ceiling in the living space below, a

slope of not less than 1:50 (1/4 inch perfoot) should be provided by addingsecondary tapered framing members ontop of the roof joists. This will ensure

roof sheathing applied prior

to construction of dormer

Note:size of window to allow for

proper flashing, detailing and

finishing of roofing

double header

side stud

valley rafter

joist hangers

double rafter

jack rafter

1.12

Dormer roof framing


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A common method of providingventilation in roof spaces is to install

louvered and screened openings in thesoffits at the eaves and at or near the topof a pitched roof (refer to Figures 1.16and 1.17). Air movement through the

vents is caused by wind pressuredifferentials and natural convection in

the attic space due to stack effect. Soffitvents are most effective when combinedwith pot vents, ridge vents or gableend vents located high on the roof.

Ventilated pitched roof eave detail, Cold Roof design

Minimum 50 mm (2 in.) air space

above insulation

Weather resistive barrier

(WRB) over entire wall with

joints shingle-lapped

Vented soffit

Metal fascia and eavestrough

Roofing extends beyond metal

drip edge to deflect water

and ice

1.15

Soffit roof ventilation

airflow

pre-finished fascia

perforated soffit

baffle

1.16


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Vents should be distributed uniformlyon opposite sides of all roofs, with atleast 25 percent of the openings locatedat the top and at least 25 percent of theopenings located at the bottom of theroof space. Figure 1.17 shows a ridgevent and a gable vent for pitched roofs.

Vents and ceiling insulation should beinstalled so that airflow through thevents and the roof space is not restricted.Vents must not allow the entry of rain,

snow or insects. Blocking or wind bafflesshould be installed between the rooftrusses or rafters where the wall meetsthe eave, to prevent wind washing anddisplacement of the attic insulation.Corrosion-resistant metal or plasticmaterial should be used for the vents and

for the screening in ventilator openings.These venting techniques are notrecommended for severely cold climateswhere high winds can force fine snow

Ridge vent (A) and gable vent (B)

wind flow

airflow

airflow

1.17


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air seals; their operating portions protect

the window assemblies from moisturepenetration; and, when provided, theinsect screens are placed to the inside ofthe opening sash where they areprotected from the weather. Figure 1.19illustrates the most common typesof windows.

Most windows in houses built before1980 have wood frames and sashes.Wood is hygroscopic, meaning that itabsorbs moisture, and reacts to changesin temperature and humidity by swellingor shrinking; therefore wood windowsrequire frequent maintenance and repairsif they are to continue to function aseffective environmental separators.Aluminum, polyvinyl chloride (PVC)and fiberglass window frames have nowbecome more popular than wood for newhouses because they are more resistantto weather and require less maintenance.The use of PVC may be discouraged bysome, due to the greenhouse gases thatare produced during production. Forthe look of wood windows on the inside

and the weather resistance of PVC oraluminum on the outside, woodwindows are available that have a naturalwood finish on the interior and are cladon the exterior with a thin layer ofPVC or aluminum.

Sealed Glazing

Windows for cold climates arecomprised of a minimum of two sheetsof glass separated by aluminum, stainlesssteel, silicone or nylon spacers and sealedin a hermetic, air-tight glazing unit.Although double-glazed windowsconsisting of two sheets of glass arecommon, triple-glazed windows arebecoming increasingly popular where

heating costs are high. Insulated spacers

at the edges of the glass, made with lowconductance materials, such as solidsilicone, greatly improve the thermalresistance of the glazing unit. The interiorglass surface in a typical InsulatedGlazing Unit (IGU) remains clean andfree of condensation so long as thehermetic seal is not broken. The air spacewithin sealed glazing units provides asmall thermal resistance value, causingthe interior glass surface to be warmerthan the exterior surface in cold weatherand cooler than the exterior surface in

hot weather. Windows for Very Coldclimates should have a minimum ofthree sheets of glass enclosing twosealed air spaces, to provide sufficientthermal resistance in the glazing unit.The width of the air space betweeneach sheet of glass in an insulatedglazing unit should be between 13 and19 mm (1/2 and 3/4 in.), which is wideenough to reduce heat loss due toconduction, but too narrow to permitair convection within the air spacethat causes convective heat loss. If the

interior glass surface is kept abovethe dew point temperature of theroom air by providing sufficientthermal resistance in the glazing unitand interior heating in the room,condensation will not form on theinside surface of the glazing unit.

Low-emissivity Coatings

In Cold climates, a factory-appliedlow-emissivity, or low-E coating onthe exterior surface of the interior sheet

of glass in a double glazed unit reflectsthe heat back into the interior of thehouse, thus reducing radiation heat lossthrough the glass and increasing theeffective thermal properties of the


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window unit. In Hot climates,reflective low-E coatings should beapplied to the interior surface of theexterior sheet of glass to keep the heatfrom the suns radiation out of the house.Smart low-E coatings have beendeveloped that allow low-angle wintersun to enter the house, while reflecting

high-angle summer sun to the outside.Figure 1.20 illustrates how a low-Ecoating prevents part of the solarradiation from entering the house.

Gas Fills

The air within a typical insulatedglazing unit can be replaced with aninert gas, such as argon or krypton,which has a higher insulation valuethan air. Gas-filled glazing units alsohave lower conductive and convectiveheat loss than conventional air-filled

windows, and provide higher overallthermal resistance values.

Air Tightness, Water Resistanceand Wind Load Resistance

Windows used in Canada are regulatedby the CSA A-440 (2000) standard forair tightness, water resistance and windload resistance and by CSA A-440.2(2000) for energy performance ofwindows. Different window types, such

as storm, operable and fixed windows,must meet minimum air tightnessrequirements, measured in cubic metresper hour per metre of crack length ofair leakage. Windows must also be watertight at different exterior air pressuredifferentials, measured in Pascals (Pa)of pressure. Windows must also provideminimum resistance to deflection ofsash and frame, and blowout of glazingunits due to wind pressure, measuredin Pascals (Pa) of pressure. Tables 1-3 inthe CSA A-440 (2000) standard

contain the minimum requirementsfor air leakage, water tightness andwind load resistance for windows.

36% - 70%

LOW-E

30% - 64%

1.20

Low-E glass coating for hot climates


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Installation

Windows should be installed in the wallassembly in much the same way as doors.Although installation may not beregulated by codes and standards,window units that are inadequatelysealed to the air barrier system or thatare improperly flashed may result inexcessive air leakage and severe waterdamage to the building envelope. Windowunits should be securely attached to therough frame opening and have the spacebetween the frame and the roughopening filled with air-tight expanding

polyurethane foam. They must also becarefully sealed to the wall air barriermaterial to ensure continuity of the airbarrier system. All window units requirea metal or flexible membrane flashingover the head and under the sill, whichis shingle-lapped under the waterresistant barrier (WRB), also called thewater-resistant barrier. The sill andhead flashings must deflect water eitherdirectly to the exterior, or into a drainagecavity that protects the inner wall

surface from moisture damage and

eventually deflects all water to the outside.To further protect the buildingenvelope from moisture penetration atthe perimeter of a window unit, thehead flashing should extend beyond therough frame opening on both sides, anda sub sill flashing should be installed atthe bottom of the rough frame opening.These are described in more detail inthe next section. Figure 1.21 shows aflexible sub sill flashing, executedbefore the window unit is installed.

FlashingsWith exposure to climatic conditionsand prolonged usage, all doors andwindows will eventually leak.Pre-finished metal or flexible membraneflashings should be used above andbelow all openings in the buildingenvelope to deflect water to the exterior.The opening in which the door orwindow is installed should be designedand constructed to shed all moisture tothe exterior to prevent damage to the

1.21

Window sub sill flashing integrated with WRB

wood stud wall framing

sub-sill blocking slopes to exterior

weather resistive barrier (WRB)

peel-and-stick sub-sill flashing min. 150 mm(6 in.) up sides of rough opening

OSB sheathing


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the roof framing and sealed to the air

barrier system at the walls of theskylight opening and at the ceiling.When the air barrier in a wood-framebuilding consists of a polyethylenesheet attached to the inside surface ofthe roof framing, the polyethylenemust be extended up and sealed to theinside face of the skylight frame. Whenthe air barrier is a membrane on theexterior of the roof sheathing, it shouldbe sealed to the outside of the skylightframe and covered with a pre-finishedmetal flashing to protect it from

exposure to ultraviolet radiation, water,snow, ice and mechanical damage.Some skylight manufacturers supplystep flashings specifically designed fortheir units and for a variety of roof slopesand finishes, which enables the installerto effectively integrate the skylightunit into the roof waterproofingsystem. Figure 1.24 illustrates a steppedbase flashing for a skylight unit on apitched roof.

Mechanical and

Electrical PenetrationsMechanical and electrical penetrationscreate openings in the building envelopethrough which water may enter, air mayleak and heat may escape. The mostcommon penetrations are exhaust ductsfor bathrooms and kitchens, intakeand exhaust piping for furnaces,boilers, ventilators, electrical outlets,light fixtures, wiring and conduit,dryer vents, chimney flues, and plumbingstacks, drains, pipes and vents and gas

piping. Flues, stacks and vents oftenoccur in roof assemblies and the otherpenetrations mentioned often occur infoundation and wall assemblies.

Foundation and Wall Penetrations

Penetrations through the buildingenvelope should be avoided by locatingpipes, ducts and wires in interior wallswhenever possible. However, somemechanical and electrical elements willpass through the building envelope andmust be properly sealed to the airbarrier system, protected with flashingsto deflect water to the outside andsealed to prevent moisture penetration.Strips of self-adhesive, air and moistureimpermeable, flexible membranematerial should be adhered to the

penetrating pipes, conduits and ductsand carefully sealed to the adjacent wallassembly to prevent air leakage,moisture penetration and heat loss.The water resistant barrier (WRB)on the adjacent wall surface shouldbe shingle-lapped over the flexiblemembrane at the top of the penetration,and under the membrane below thepenetration. The space between thepipe, duct or conduit and thesurrounding wall should also be filled

with insulation to prevent heat loss.Where a pipe, drain, duct or conduitpenetrates a foundation wall orconcrete floor slab, the space aroundit should be filled with non-shrinkgrout and a continuous flexible,waterproof sealant bead applied to thejoint between the penetration andthe concrete. Wherever possible,mechanical and electrical penetrationsshould be made perpendicular to theair barrier, so that the opening can beproperly sealed.


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Part 2: Environmental Control Strategies

MOISTURE MOVEMENT

When moisture moves through thebuilding envelope, significant damagecan be done to the various constructionassemblies. Therefore, the movementof moisture must be carefully controlled.

For moisture penetration to occur, threeconditions must exist simultaneously;there must be a source of moisture,there must be an opening throughwhich the moisture can penetrate, andthere must be a driving force to movethe moisture through the opening.

Without all three of these conditionsbeing present at the same time, moisturepenetration cannot occur. For example,if moisture is present and there is anopening, but there is no force to drivethe moisture, moisture penetrationcannot occur. Or, if there is moisturepresent and there is a driving force,but there is no opening, moisturepenetration cannot occur.

Moisture can penetrate the smallest ofopenings, such as the pores in porous

materials. Therefore, the moisturebarrier must be continuous and carefullysealed to eliminate any openingsthrough which moisture can enter thebuilding envelope.

Sources of Moisture

One of the keys to preventing moisturedamage to buildings is to understandwhere moisture comes from, howmoisture moves and how condensationis formed. Moisture is found in nature

in three forms: as a solid (i.e. ice), as aliquid (i.e. water) and as a gas (i.e. watervapour). The sources of moisture thatcan affect the performance and durabilityof building envelopes are as follows:

1. Precipitation: Moisture in the

form of rain, sleet, snow or hail isfound in nature and provides thelargest source of moisture affectingthe building envelope componentsof walls and roofs.

2. Ground Water: The water contentof the soil can vary from wet todry and can provide moisture thataffects the foundation componentof building envelopes.

3.Water Vapour:Water vapourcontained in both the indoor and

outdoor environments provides asource of moisture that can transformfrom a gas to a liquid through theprocess of condensation.

4. Occupants:The occupants ofbuildings produce large quantitiesof moisture through daily activitiessuch as breathing, cooking,washing and cleaning.

5. Construction Materials:Allconstruction materials containmoisture, for example concrete andwood contain large quantities ofmoisture. The moisture can be storedseasonally in existing materials orcan come from new materials. Inboth cases the moisture can bediffused into the building envelope.

Moisture Movement Forces

The sources of moisture outlined abovecan be forced through an opening inthe building envelope in a number of

ways. The following driving forces arethe mechanisms that cause the movementof moisture through the building envelope:


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Part 2: Environmental Control Strategies

The effect of wind pressure as adriving force on water through thebuilding envelope cladding can beneutralized by the use of pressureequalized two stage joints or a pressureequalized rainscreen. Since the pressureacross the cladding or rainscreen is notalways fully equalized, this system

is sometimes referred to as pressuremoderated (refer to Figure 2.8). Thistype of construction requires the use ofa rigidly supported impermeable airbarrier that resists wind pressure andallows the air space behind the claddingor the pressure equalization chambertoquickly pressurize or depressurize inreaction to the wind pressure applied tothe rainscreen.

5. Air Leakage:

All air contains some water vapour and

as air moves through a building envelopeassembly, driven by an air pressuredifferential, the air brings water vapouralong with it (refer to Figure 2.9).

When this air meets a cold surface,condensation can occur. Air leakagecan occur in both directions throughthe building envelope, as air movesfrom an area of higher pressure to anarea of lower pressure. The transport ofwater vapour by air leakage can becontrolled by installing a continuous

air barrier to prevent the uncontrolledmovement of air through the buildingenvelope. Air barriers will be covered indetail later in this section.

6. Vapour Diffusion:

This is the movement of water vapourfrom an area of high water vapourpressure to an area of low watervapour pressure. Water vapour willdiffuse through air and solid materials(refer to Figure 2.10). Some materialssuch as gypsum board and concrete

block allow water vapour to diffuseeasily through them and are said to behighly water vapour permeable. Othermaterials such as glass, aluminum foil

Water entry by wind pressure

Air leakage

through wall

assembly

Water driven

through large

openings by

air pressure

differential

between the

outside and

inside across

the envelope

Water on

surface ofcladding 2.7


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7/25/2019 CMHC- Durable Wood F

CMHC- Durable Wood Frame Construction for All Climates

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