Faculty of Engineering - UNIMAS Institutional Repository of Roof Ventilation on Cooling... ·...

EFFECT OF ROOF VENTILATION ON COOLING PERFORMANCE

OF A BUILDING

Gillan Lio Siang Kwang

Master of Engineering

(Civil)

2012

Faculty of Engineering

ii

ACKNOWLEDGEMENT

I would like to extend my greatest thanks to my mother, father and sister who are the

pillar of my strength and existence. I would like to thank God whom has made me into who I

am now and give me hope and blessing each and every day.

Greatest appreciation goes to my supervisor Assoc. Prof. Dr Azhaili Baharun for his

guidance and for the precious time spent in making my thesis a success. I would also like to

thank my co-supervisor Dr Siti Halipah Ibrahim for her guidance and for providing me the

information I needed to do my thesis. In addition to that, I would like to thank all whom have

contributed, be it directly or indirectly for my thesis.

Thank you very much.

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ABSTRACT

The purpose of this study is to show the effectiveness of roof ventilation on cooling

performance of a building. The function and design of a roof is important because it is

constantly exposed to the sun’s radiation throughout the day. Improper removal of heat on the

roof will cause heat transfer to the inner building resulting in thermal discomfort to the

occupants. Roof ventilation is one of the best ways to help in removal of heat. There are

different types of roof ventilation used which are wind turbine ventilation, soffit ventilation,

ridge ventilation and gable ventilation. EnergyPlus software was used as simulation to

analyze the effectiveness of different types of roof ventilation. Before using the EnergyPlus

for simulation, validation work was done to show the accuracy of the software. The validation

work was done by comparing actual data collected on site to simulated data using EnergyPlus

software. The data compared is the temperature. The deviation showed a range of 2% to 15%.

After the validation work, the simulation of different types of roof ventilation was done. The

simulation was done by comparing different types of roof ventilation with an unvented

building to show whether there are improvement in terms of temperature. The result showed

that wind turbine ventilation has the least improvement while soffit ventilation gave the best

improvement by lowering the temperature of the building. Overall the result showed that roof

ventilation can help in improving the cooling performance of a building.

iv

ABSTRAK

Tujuan kajian ini adalah untuk mengkaji keberkesanan pengudaraan bumbung kepada prestasi

penyejukan bangunan. Fungsi dan reka bentuk bumbung adalah penting kerana ia sentiasa

terdedah kepada sinaran matahari sepanjang hari. Penyingkiran haba yang kurang memuaskan

di atas bumbung akan menyebabkan pemindahan haba ke bangunan dalaman yang

mengakibatkan ketidakselesaan haba kepada penghuni. Pengudaraan bumbung adalah salah

satu cara yang baik untuk membantu dalam penyingkiran haba. Terdapat pelbagai jenis

pengudaraan bumbung yang digunakan seperti turbin angin, pengudaraan soffit, pengudaraan

rabung dan pengudaraan gable. EnergyPlus telah digunakan sebagai simulasi untuk

menganalisis keberkesanan pelbagai jenis pengudaraan bumbung. Sebelum menggunakan

EnergyPlus untuk simulasi, kerja pengesahan telah dilakukan untuk menunjukkan ketepatan

perisian tersebut. Kerja pengesahan telah dilakukan dengan membandingkan data sebenar

yang dikumpul pada tapak dengan data simulasi daripada EnergyPlus. Parameter data yang

dibandingkan adalah suhu. Sisihan berbeza dari 2% hingga 15%. Selepas kerja pengesahan,

simulasi jenis pengudaraan bumbung telah dilakukan. Simulasi telah dilakukan dengan

membandingkan pelbagai jenis pengudaraan bumbung dengan sebuah bangunan biasa untuk

menunjukkan sama ada terdapat penurunan dari segi suhu. Hasilnya menunjukkan bahawa

angin pengudaraan turbin mempunyai penurunan suhu terkurang manakala pengudaraan soffit

memberikan penurunan suhu terbaik. Keseluruhan keputusan menunjukkan bahawa

pengudaraan bumbung boleh membantu dalam peningkatkan prestasi penyejukan bangunan.

v

TABLE OF CONTENTS

AKNOWLEDGEMENT ii

ABSTRACT iii

ABSTRAK iv

LIST OF TABLES viii

LIST OF FIGURES ix

LIST OF SYMBOLS xi

CHAPTER TITLE PAGE

1 INTRODUCTION 1

1.1 General Introduction 1

1.2 Problem Statement 2

1.3 Objective 3

1.4 Scope of Study 4

1.5 Expected Findings 4

1.6 Thesis Outline 4

2 LITERATURE REVIEW 6

2.1 Introduction 6

2.2 Type of Roof Commonly Used in Malaysia 6

2.2.1 Pitched Roof 7

2.2.2 Flat Roof 9

2.3 Roof Truss 10

2.3.1 Materials for Roof Truss 10

2.3.2 Type of Roof Truss 11

2.4 Types of Roof Ventilation 12

2.5 Roof Materials Commonly Used in Malaysia 15

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2.6 Ceiling Material Commonly Used in Malaysia 16

2.7 Heat Transfer 17

2.7.1 Conduction 18

2.7.2 Convection 19

2.7.3 Thermal Radiation 20

2.8 Thermal Comfort 22

2.9 EnergyPlus Software 24

3 METHODOLOGY 27


3.2 Process of Study 28

3.3 Overview of the Thesis 29

3.4 Site Data Selecion 29

3.4.1 Site Data Collection 32

3.4.2 Infrared Thermo-Hygrometer 32

3.4.3 Measured Point in the Low Cost House 33

3.5 Software Used in the Study 34

3.5.1 Google Sketchup 34

3.5.2 EnergyPlus 35

3.6 Simulation Using EnergyPlus 35

3.6.1 Limitation 37

3.7 Simulation Work 37

4 RESULTS AND ANALYSIS 38

4.1 Introduction 38

4.2 Validation Work 38

4.3 Simulation of Different Type of Roof Ventilation 41

4.3.1 Comparison between House with Wind Turbine

Ventilation and Unvented House 43

4.3.2 Comparison between House with Gable


4.3.3 Comparison between House with Soffit

vii


4.3.4 Comparison between House with Ridge


5 CONCLUSION AND RECOMMENDATION 56


5.2 Conclusion 56

5.3 Recommendation 57

REFERENCES 58

APPENDIX 62

Appendix A Actual Data versus Simulated Data 62

Appendix B Vented Data versus Unvented Data 65

Appendix C Architectural Drawings of Low Cost House 73

viii

LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 Material Properties Applied in the Model 36

ix

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Low Rise Building Envelope Heat Gain 2

1.2 Cause and Effect Diagram 3

2.1 Configurations and Terminology of a Pitched Roof 6

2.2 Common Types of Pitched Roofs 8

2.3 Construction Method of Roof Fall 9

2.4 Type of Roof Truss 11

2.5 Types of Roof Ventilation 13

2.6 Thermal Effect of Roof Material 16

2.7 Thermal Radiation From Roof To Inner Building 18

2.8 ASHREA Summer and Winter Comfort Zones 23

3.1 Flow Chart of the Project 28

3.2 Front view of the Selected Low Cost House 29

3.3 Overview Location of the Low Cost House

(Google Map 2012) 30

3.4 Selected Low Cost House (Google Map 2012) 30

3.5 Lane Entrance to the Selected Low Cost House 31

3.6 Living Room of the Selected Low Cost House 31

3.7 Infrared Thermo-Hygrometer 33

3.8 Measured Points in the House 34

3.9 View of the House Model in Simulation 36

4.1 Comparison between Actual Data and Simulation Result

for Temperature at Wall (Side) 40


for Temperature at Roof (Front) 40


for Temperature at Wall (Side) 41

x

4.4 House without Roof Ventilation 42

4.5 Taken in the Simulation for Comparison (Front View) 42

4.6 Modeled Wind Turbine 44

4.7 Comparison between Unvented House and Wind Turbine

(Left Wall) 44

4.8 Comparison between Unventilated House and Wind Turbine

(Right Wall) 45

4.9 Comparison between Unvented House and Wind Turbine

(Front Roof) 45

4.10 Modeled Gable Vent 47

4.11 Comparison between Unvented House and Gable Vent

(Left Wall) 47


(Right Wall) 48


(Front Roof) 48

4.14 Modelled Soffit Vent 50

4.15 Comparison between Unvented House and Soffit Vent

(Left Wall) 50


(Right Wall) 51


(Front Roof) 51

4.18 Modelled Ridge Vent 53

4.19 The Openings in the Ridge Vent 53

4.20 Comparison between Unvented House and Ridge Vent

(Left Wall) 54


(Right Wall) 54


(Front Roof) 55

xi

LIST OF SYMBOLS

q - Thermal

hc - Convection Coefficient

t1 & t2 - Surface temperature and bulk fluid Temperature

A - Area of heat transfer

k - Thermal conductivity

Eb - Black body emissive power

σ - Stefan- Boltzmann constant

T - Absolute Temperature

F - View factor

ε - Emissivity

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

The thermal performance of a building envelope is important to ensure a comfortable

indoor climate and building’s energy efficiency. There are many aspects in a building that can

contribute to low energy. One of those aspects is the roof. A roof is located at the uppermost

part of a building. The roof helps to protect and shield the building from the effects of

weather. Due to the tropical climate in Malaysia, roofs are primarily used to shelter against

the sun’s radiation and rain. The building roof covers 50-70% of the total heat entry of inner

environment as shown in Figure 1.1 (Vijaykumar et al., 2007).

During hot season, the function and design wise of the roof is important to provide a

comfortable living environment. As the sun’s radiation radiate throughout the day on the roof,

heat will be produced on the outer and inner side of the roof. Roof ventilation is considered

as passive cooling which is one of the main contributions to thermal performance and low

energy efficiency. If improper ventilation is provided to the roof, heat will be trapped within

the roof. If this happens, it will eventually reach the inner building causing an uncomfortable

living environment. Hence, by knowing the effects of roof ventilation on cooling performance

of a building, a more in depth discovery can be made.

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Figure 1.1 Low Rise Building Envelope Heat Gain (Source: Vijaykumar et al., 2007).

1.2 Problem Statement

In building construction, the significant of roof ventilation in performance cooling is often

ignored. Tropical climate zones such as in Malaysia rely heavily on roof to provide not just

shading but good low energy and thermal cooling. The alternative solution is to use air

conditioning for cooling. However, majority of people cannot afford it due to financial

constraints. Furthermore, there are other major problems of using air conditioning or other

mechanical ventilation which can be illustrated in Figure 1.2. Hence, it is vital to provide a

good analysis on the effect of roof ventilation so that further information can be provided to

explore the effectiveness of the roof system used in Malaysia.

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Figure 1.2 Cause and Effect Diagram (Source: Al Yacouby et al., 2011)

1.3 Objective

The objectives are shown below.

i. To gather information about the type of roof structures, roof ventilations and

roof materials used in residential house through literature review.

ii. To collect experimental data on heat transfer through roofing on a selected site

iii. To validate EnergyPlus software with data collected from site

iv. To find the effectiveness of roof ventilation system for minimizing roof heat

transfer into room through simulation using EnergyPlus software

4

1.4 Scope of Study

EnergyPlus software will be used to simulate the cooling effect of roof ventilations.

Before using EnergyPlus software for simulation, validation work must be done to show the

reliability and accuracy of final result produced by the software. The collection of the data

will be focused on the low cost house in Kuching. Equipment from the lab will be used to

compare the result with the software. The only parameter used for the validation and

simulation work is the temperature.

1.5 Expected Findings

From the study, the accuracy of the Energy Plus software can be validated when

compared with actual data on site. The determination of the different type of roof ventilation

can be obtained from the software simulation.

1.6 Thesis Outline

Chapter 1 will cover the general introduction, problem state, objective and scope of

the study. This chapter will explain about the thesis which concerns the cooling effect of roof

ventilation and what are the main focuses of achieving it.

Chapter 2 will describe the type of roof structures, roof ventilations and roof materials

used in residential houses. This chapter will also discuss about heat transfers which explain

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how the heat from outside condition can affect the inside environment of the house. Besides

that, thermal comfort is also discussed to show the thermal comfort range found in Malaysia.

The description and discussion on EnergyPlus software is also included.

Chapter 3 will cover the procedure undertaken to achieve the objective of the study. A

flowchart will show the overall process to conduct the study. Validation and simulation work

will be explained in detail in this chapter.

Chapter 4 will cover the results obtained from the validation and simulation work. The

results will be analyzed and explained in detail in this chapter.

Chapter 5 will cover the conclusion of the overall study that had been made.

Recommendation will also be presented to give further improvement of the study.

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CHAPTER 2

LITERATURE REVIEW

2.1 General

To better understand the roof ventilation, the roof structure itself must be understood.

Aside from that, the heat transfer of for the roof needs to be discussed in order to have more

information on how heat is transferred from the roof to the living area.

2.2 Type of Roof Commonly Used In Malaysia

There are basically two types of roofs that are popular in Malaysia which are flat roofs

and pitched roofs. The choice of type of roofs used is mostly based on the demand, cost,

climates and aesthetics. Flat roofs are not actually flat but having a certain angle that is less

than the pitched roofs. Flat roof has an angle of up to 10˚ where as pitched roof is more than

10˚ (Fleming, 2005). Both of the roofs will be discussed further on the following sub-

chapters.

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2.2.1 Pitched Roof

Pitched roof describes the angle of the roof that compares the horizontal run and the

vertical rise. To better understand pitched roofs, some configurations and terminologies

should be known before discussing further.

Figure 2.1 Configurations and Terminology of a Pitched Roof (Source: Bryan, 2005)

From Figure 2.1, eaves are on edges where their main function is to collect water for

disposal. They contain gutters which are connected to the downpipes that bring water to

drains. Verges are the junction of a roof and a gable end wall. They are at edges where water

is not expected to be collected. Ridge, hip and valley are all formed when the pitch surfaces

meet. The valley effectively collects water while the ridge and the hip shed water away down

each of the roof surfaces. It must be ensured that the water is later carried by the eaves. If the

tiles themselves cannot be “swept” around the valley to maintain the waterproofing of the

covering, a gutter has to be formed to collect and direct water to the gutters and downpipes at

the eaves (Bryan, 2005). Abutments are the meeting between roof surfaces and walls. The

joint between them has to be sealed so that the water runs to the eaves and not into the roof

Verge

Hip Valley

Eaves

Verge

Eaves

Gable

Abutmen

t

Ridge

8

space. Pitch roofs can be divided into several types. The types of pitched roofs are explained

in Figure 2.2.

Hip Roof

This roof has four sloping sides running

upward towards the center of the building

to create a ridge. Rafters at the corners are

diagonal from the bottom edge to meet the

ridge.

Dutch Hip Roof

It is basically a hip roof with a small gable

at either end. The gables can be used for

ventilation

Shed or Lean-to Roof

The pitch of this roof is one direction only.

It is mostly used where temporary

construction is needed and where sheds or

additions to buildings are erected. One

wall is higher than those on the opposite

side.

Gable Roof

The roof has two roof slopes that meet at

the center which is the ridge forming a

gable. It is the most common roof as it is

simple, economical and maybe used on

any type of structure.

9

Figure 2.2 Common Types of Pitched Roofs

2.2.2 Flat Roof

When compared to pitched roof, flat roof is less dominant due the level of rainfall and

strength of the wind that are found in Malaysia. While the flat roof possibly offers a lower

initial cost, this is only achieved with the less expensive continuous roof coverings such as

three-layer felt. Flat roof has a joisted structure, similar to a floor but has to be provided with

a fall to ensure effective removal of rainwater into the gutters. This is achieved by either

sloping the joists or placing the tapered battens known as firring pieces on top of the joists.

Figure 2.3 will show the construction methods of roof fall.

Sloping Roof Joist

By obtaining the slope or fall, sloping of the

roof joists has to be done. This type of roof is

normally used when there is no level ceiling

required.

Valley Roof

The roof is formed by two intersecting hip

or gable roofs. The two roofs will meet at

the valley.

10

Tapered Firring Pieces

Attaching tapered pieces of timber to the roof

joist can help construct the slope or fall of the

roof. This type of construction will produce a

level ceiling.

Deepened Joist (Parallel Fittings)

The firings are nailed at right angles to the

fall of the roof.

Figure 2.3 Construction Method of Roof Fall

2.3 Roof Truss

2.3.1 Materials for Roof Truss

Most of the materials used as roof truss in Malaysia are timber or steel. Compared to

steel, timber is found to be more popular as it is cheaper and readily available. Especially

when dealing with low cost and medium cost housing construction, timber would most likely

be a more preferred choice. But for high cost housing construction, either steel or timber

would be used depending on the requirements. There are actually some advantages and

disadvantages of using timber and steel as roof truss. As mentioned, timber is readily

available and cheap. Hence it is economical and also lightweight. However, it prompts to

structural decay. Water absorbed by wood framing can cause a reduction in its compression,

tensile strength and allow for rot and decay (Winter et al., 1999). Besides that, fire damage

and termite problem due to improper chemical treatment will also bring major destruction of

the roof. For steel roof truss, it is higher in both compressive and tensile strength, fireproof

Tapered firing pieces

Parallel firring pieces

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and prevent termite problem. The main disadvantage for using steel is that it is more

expensive. In any construction, cost is an issue and despite some of the disadvantages of

timber possesses, it is still widely used.

2.3.2 Type of Roof Truss

There are actually many truss types. Some of the truss types are shown in Figure 2.4.

Most roof truss will leave an empty space between the roof section and the ceiling. The

construction of roof trusses are mainly meant for pitched roof with an angle of more than 10˚.

Four Panel Flink Truss

Six Panel Fan Truss

Four Panel Pratt Truss

Four Panel Howe Truss

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Six Panel Howe Truss

Figure 2.4 Type of Roof Truss (Source: Fleming, 2005)

2.4 Types of Roof Ventilation

Roof spaces can be ventilated in many ways to keep the heat from entering the living

areas. When the radiant heat from the sun hit the surface of the roof, the temperature increases

at the surface of the roof. Conduction of heat through the roof material will bring the heat

within the roof spaces. As the heat builds up within the spaces, it will radiate to the ceiling

and eventually to the living areas. Ventilation of roof plays a very important part in

maintaining a suitable temperature for occupants to live in. Figure 2.5 will show the type of

ventilation that are currently used. Study done by Patania et al., 2011 showed that the

performance of ventilated roof is much better than unventilated roof with the same total

thermal resistance. Besides that, the study also shows that a tilt ventilated roof permits an

energy saving of 60% compared to unventilated roof. In addition to that, study done by Zain

et al., 2007 also showed that a cross ventilating roof attic or gable vent can improve comfort.

13

Soffit Vent

Soffit vents are installed under the roof

overhang which are either continuous or

consist of individual spaced vents. The

passage of air from the soffit to the roof

space will bring hot air out of the other side

of the soffit. However, if there are

surrounding houses, it would be difficult for

the wind to pass through the soffit.

(Source: Forgues, 1985)

Ridge Vent

Ridge vents are designed to bring heated air

from an attic regardless of wind direction or

force. Ridge vents usually use an external

baffle. When the wind is perpendicular to

the ridge, it hits the external baffle and

crosses over the ridge. This movement will

create a Bernoulli effect causing the low

pressures to develop on both sides of the

vent. This effect will cause the air within

the roof space to be lifted out. Even if little

wind exists, warm air will rise and exhausts

through the ridge vents.

(Source:

http://www.airvent.com/professional/resources

/literature.shtml)

Gable Vent

Gable vents are louvers which wind will

travel through it bringing unwanted air out

of the roof space. However, it is not as

effective because it will still leave warm air

under the sheathing

(Source:

http://www.airvent.com/professional/resources

/literature.shtml)

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