Heat Flow Through a Typical Roof in Akure

HEAT FLOW THROUGH A TYPICAL ROOF IN AKURE

PREPARED BY:

ADEGBOLA ADEWALE OLADEJI

(ARC/06/8475)

APPLIED CLIMATOLOGY

(ARC 810)

SUBMITTED TO:

THE DEPARTMENT OF ARCHITECTURE

SCHOOL OF ENVIRONMENTAL TECHNOLOGY

THE FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF

MASTERS OF TECHNOLOGY (M.TECH) DEGREE IN ARCHITECTURE.

SEPTEMBER, 2011

TABLE OF CONTENT

Title pagepages

Abstract……………………………………………………………………………………ii

Chapter One

1.1Introduction…………………………………….……………………….……………. 1

1.2Aluminum………………………………….…………………….…………..………1

1.3Thermal conductivity…………………………….…………….………………………..1

1.4Reflectivity………………………………………...……….…………………………2

1.5Properties of aluminum……………………………….…………...………………2

Chapter Two

2.1Heat Flow through building materials…………………….….…….…..………..3

2.2Types of heat transfer through aluminum roofing…………….…….……….………7

2.3Effects of heat flow into the building…………………………….…..…………………9

2.4Control measures………………………………………….…..……………………9

Chapter Three

3.1Conclusion and Recommendations…………………………..….….……..…………10

Reference…………………………………………………………….……………………11

Abstract

Building materials have an important role to play in this modern age of technology. Althoughtheir most important use is in construction activities, no field of engineering is conceivable without their use. There are certain general factors which affect the choice of materials for the thermal comfortability of interior spaces. Perhaps the most important of these is the ‘climatic issues’. Obviously, building materials response to the solar radiation emitted through the depletion of ozone layers, radioactive decay of the unstable isotopes of nuclear fusion and fission. Composition of materials, physical properties, heat capacity, and its conductivity etc determine the resistivity of the ‘long span aluminum roof’.

CHAPTER ONE

1.1Introduction

Heat transfer in building and acceptable indoor climates is important to the success of a building not only in making it comfortable, but also in deciding its energyconsumption and ensuring its sustainability. The manufacturer of building materials has not seen it as part of their major task to consider the resistivity, adaptability, sustainability, depletion of ozone layer before the production of their materials in reference to the effects of heat transfer in our climate. With increasingdepletion of ozone layer, solar radiation and climate change, ability of the building materials to reduce the transmission of heat through radiation, conduction is subjective.

Thispaper introduces the transmission of heat flow through a typical corrugated aluminum roofing sheet in the tropical climate, the principle of heat flows and the properties of the material.

1.2Aluminum

Physically, chemically and mechanically aluminum is a metal like steel, brass, copper, zinc, lead or titanium. It can be melted, cast, formed and machined much like these metals and it conduct electric current. It is a very light metal with a specific weight of 2.7g/cm3, about a third that of steel.

1.3Thermal conductivity

Aluminum is an excellent heat and electricity conductor and in relation to its weight is almost twice as good a conductor as copper. This has made aluminum the most commonly used material in major transmission lines.

1.4Reflectivity

Aluminum is a good reflector of visible light as well as heat, and that together with its low weight, makes it an ideal materials for reflectors in, for example roofing, light fittings.

1.5Physical propertiesof aluminum

Phase : solid
Density : 2.70gcm3
Liquid density at m.p. 2.375gcm3
Melting point of 933.47, 1220.580F.
Boiling point of 2792k, 2519oC,
Heat of fusion 10.71kj/mol.01
Heat of vaporization is 294.0kJ/ mol01
Molar heat capacity of 24200 J/ mol01, K01.
Thermal conductivity of 237W/m01
Thermal expansion of (250C) 23.1µm/m01, K01.

CHAPTER TWO

2.1Heat Flow through building materials (longspan aluminum roofing).

The temperature in a building always involves controlling heat transfer. Heat transfer or heat exchange is the transition of thermal energy from hotter object or area to cooler object or area as described by the law of thermodynamics.Whenever there is a difference in temperature across a given area, heat naturally flows from warm areas to colder area.Regardless of direction it seeks a balance until the temperature is the same across the whole area. In the hot climate area, high heat from roofs travels from outside to inside. Heat transfer can never be stopped, it can only be slowed or reflect away. The purpose of insulation is to impede this natural process.Radiant Heat Barrier is used as insulation against radiant heat transfer and can be used to its greatest advantage in instances where radiation is the predominant means of heat transfer.

The medium of heat flows through a typical longspan aluminum roofing in the tropical climate of Ondo- State, Akure are through the normal means of heat transfer;conduction, convectionand through the emission of solar radiation.The intensity of the radiation on various building materials dependon the methods of construction, composition of the building materials (aluminum), and the roof carcass materials (timbers or steel). The common heat transfer of some building materials co-efficient are deduce based on; climatic factor, depletion of ozone layer, research on the composition of the materials and intensity of solar radiation.

Plate 1: A modern market at the central business district, Akure, Ondo State.

Source: Authors field survey, 2011.

Plate 2: A typical aluminum roofing.

Source: Authors field survey, 2011.

Figure 1: Heat flow through longspan roofing.

Source: Authors field survey, 2011.

Table 1:Common Heat Transfer Coefficients of some common Building Elements.

Building Element / Heat-Transfer Coefficient
(Btu/hr ft2oF) / (W/m2K)
Doors / Single sheet – metal / 1.2 / 6.8
1 inch – wood / 0.65 / 3.7
2 inches – wood / 0.45 / 2.6
Roofing / Corrugated metal - uninsulated / 1.5 / 8.5
1 inch wood – uninsulated / 0.5 / 2.8
2 inches wood - un-insulated / 0.3 / 1.7
1 inch wood - 1 inch insulation / 0.2 / 1.1
2 inch wood - 1 inch insulation / 0.15 / 0.9
2 inches - concrete slab / 0.3 / 1.7
2 inches - concrete slab - 1 inch insulation / 0.15 / 0.9
Windows / Vertical single glazed window in metal frame / 5.8
Vertical single glazed window in wooden frame / 4.7
Vertical double glazed window, distance between glasses 30 - 60 mm / 2.8
Vertical tripple glazed window, distance between glasses 30 - 60 mm / 1.85
Vertical sealed double glazed window, distance between glasses 20 mm / 3.0
Vertical sealed triple glazed window, distance between glasses 20 mm / 1.9
Vertical sealed double glazed window with "Low-E" coatings / 0.32 / 1.8
Vertical double glazed window with "Low-E" coatings and heavy gas filling / 0.27 / 1.5
Vertical double glazed window with 3 plastic films ("Low-E" coated) and heavy gas filling / 0.06 / 0.35
Horizontal single glass / 1.4 / 7.9
Walls / 8 inches - poured concrete 80 lb/ft3 / 1.5 / 8.9
12 inches - poured concrete 80 lb/ft3 / 1.1 / 5.9

Source:

U and R-values of building materials

U-value (or U-factor) is a measure of the rate of heat loss or gain through a construction of materials. The lower the U-factor, the greater the material's resistance to heat flow and the better is the insulating value. U-value is the inverse of R-value.

The U-value of a construction materials consisting of several layers can be expressed as

U = 1 / ∑ R

Where;

R = "R-value" - the resistance to heat flow in each layer (hr sq.ft oF/Btu)

The R-value of the single layer can be expressed as:

R = 1 / C = lt / K

Where;

C = layer conductance (Btu/hr ft2oF)

K = layer conductivity (Btu in/hr ft2oF)

lt = thickness of layer (inches).

2.2Types of heat transfer through aluminum roofing.

Conduction: is the rate of heat flows through a direct contact. This contact occurs within materials or through two materials in contact.(John Straube, 2006).In order for the heat to be conducted, there should be physical contact between particles and some temperature difference. When one part of an object is heated, the molecules within it vibrate against one another; begin to move faster and more vigorously, when these molecules hit other molecules within the object they cause heat to be transferred through the entire object.Aluminum is a good insulator.Wood falls somewhere in between.

Convection:is the transfer of heat by the movement or flow of molecules (liquid or gas) with a change in their heat content. (John Straube, 2006). Convection is the process of energy transport by the combined action of heat conduction, energy storage and mixing motion which results in conventional currents.The rate of heat flow will depend on the temperature of the moving gas or liquid and on its rate of flow. Circulatory air motion due to warmer air rising and cooler air falling is a common mechanism by which thermal energy is transferred.

Convection also occurs if air can circulate through the insulation - if insulation is to be effective, it must prevent air from flowing easily through it. Properly applied insulation reduces convective heat loss by resisting and minimizing air movement.

Radiation:is the transfer of heat by electromagnetic waves through a gas or vacuum. Heat transfer by this mode therefore requires a line of sight connection between the surfaces involved. (John Straube,2006).Radiation is the process by which heat flows from a body at a higher temperature to a body at a lower temperature when the bodies are separated in space depending on the nature of the surface. Heat transfer through radiation takes place in form of electromagnetic waves mainly in the infrared region. Therefore, the angle of incident is equal to the angle of reflection. The emissivity coefficient lies in the range 0 < 1 depending on the type of material and the temperature of the surface.

Figure 2: Heat flow through radiation in longspan roofing.

Source: Authors field survey, 2011.

The illustration above show the incident of heat from the sun on the aluminum roofing which absorbed the radiant heat. The reflected heat diffused due to the coarse surface of the materials which has density value of 2.70gcm3. The steel materialstransmit the absorbed heat gained into the interior of the building. However, the angle of incident on the roofing materials is not equal to the angle at which the heat diffused from the materials and the transmitted heat gained into the building.The rate of heat flow through a specific material will be influenced by the difference of temperature and its thermal conductivity.

2.3Effects of heat flow into the building

The absorbed heat through radiation, conduction etc causes discomfort such as;
Structural heating
Expansion and contraction of building elements
Thermal discomfort
Emission of unwanted gas from the ceiling materials which pollute the supply of fresh.
Body heating

2.4Control measures

Insulation of roofing materials
Use of insulating roof members
Reduction of the building U-factor.
Use of heat resistance ceiling materials.
Headroom of 3600mm for residential building reduces the thermal discomfort.
Landscaping of the building envelope with the soft and hardscape materials.

CHAPTER THREE

3.1Conclusion and Recommendations

The rate at which the radiation is militating against the building materials in the tropical climate is alarming.The building materials deteriorate faster due to the action of climate change. The heat flow through building materials depends on the temperature difference across it. The conductivity of the roofing materials and its thickness means less heat flow and so does a lower conductivity.

With all the aforementioned, the level of heat flows through typical roofing materials should be controlled during the composition of the materials before its consumption. Production of insulated coated roofing materials should be made to resist the radiant heat gain.. The composition of the raw materials should be environmental friendly and non-hazardous.Therefore, the manufacturing company of building materials should make it’s of priority to produce materials which is suitable and capable of resisting the penetration of solar radiation through the depletion of ozone layer.

Reference

Polmear, I.J. (1995). Light alloys: Metallurgy of the light metal .Arnold. ISBN.

Strauby, J. (2006).Thermal controls in Buildings: Building Science Digest 011.

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