build thermally efficient and sustainable structures · web viewbuild thermally efficient and...

Build Thermally Efficient and Sustainable Structures

Chapter 4 What is Thermal Performance?

Produced by Pointsbuild in partnership with the Master Builders Association of NSW

Supported by the NSW Government as part of the Energy Efficiency Training Program — visit savepower.nsw.gov.au

Copyright and disclaimer The Office of Environment and Heritage and the State of NSW are pleased to allow this material to be used, reproduced and adapted, provided the meaning is unchanged and its source, publisher and authorship are acknowledged. The Office of Environment and Heritage has made all reasonable effort to ensure that the contents of this document are factual and free of error. However, the State of NSW and the Office of Environment and Heritage shall not be liable for any damage which may occur in relation to any person taking action or not on the basis of this document. Office of Environment and Heritage, Department of Premier and Cabinet59 Goulburn Street, Sydney NSW 2000PO Box A290, Sydney South NSW 1232Phone: (02) 9995 5000 (switchboard)Fax: (02) 9995 5999TTY: (02) 9211 4723Email: [email protected]: www.environment.nsw.gov.au

Table of contents4.1 Introduction...............................................................................................................................................4

4.2 Heat Flow between a Building and its Surroundings.................................................................................4

4.2.1 Winter........................................................................................................................................................54.2.2 Summer.....................................................................................................................................................5

4.3 Building Design and Thermal Performance...............................................................................................6

4.3.1 Orientation................................................................................................................................................64.3.2 Insulation...................................................................................................................................................74.3.3 Glazing.......................................................................................................................................................74.3.4 Thermal Mass............................................................................................................................................84.3.5 Ventilation.................................................................................................................................................9

4.4 Definitions of Building Thermal Performance............................................................................................9

4.5 Solar Radiation........................................................................................................................................10

4.5.1 Absorption by the Atmosphere................................................................................................................104.5.2 Components of Solar Radiation...............................................................................................................114.5.3 Colour and Absorptance..........................................................................................................................11

4.6 Calculation of Heat Flow..........................................................................................................................12

4.6.1 R-Values...................................................................................................................................................124.6.2 R-value of Bulk Materials.........................................................................................................................134.6.3 Resistance of Air Films.............................................................................................................................144.6.4 Resistance of Air Gaps.............................................................................................................................144.6.5 Resistance of Roof Spaces........................................................................................................................154.6.6 Total R-value of a building element.........................................................................................................16

Building Thermally Efficient and Sustainable Structures: Chapter 4 – Thermal Performance____________________________________________________________________________________________

4. What is Thermal Performance?4.1 IntroductionA building with good thermal performance is one that modifies or filters its surrounding climate using good design principles and selection of materials, fittings and fixtures that

Significantly improves comfort within the building Reduces or eliminates electricity and gas bills by reducing heating and cooling loads Reduces greenhouse gas emissions from heating, cooling, mechanical ventilation and lighting.

Passive design heating and cooling principles incorporated into a building may result in a building that does not require artificial heating or cooling and maintain comfort by taking advantage of the natural climate around it.The building envelope i.e. the roof, walls, windows, floors and internal walls of a home, is the primary focus of controlling the thermal performance of a building as it impacts on heat gain in summer and heat loss in winter. Well designed building envelopes promote cooling air movement and exclude sun in summer. In winter, they collect and store heat from the sun and minimise heat loss to the outside. But how does the building envelope work? How does it work when it is cold or hot outside and what impact does it have on the internal temperature and comfort?This chapter answers these questions and explains how heat flow through the building envelope works and provides a better understanding of how R values, the most commonly used description of the thermal performance of a building material, insulation etc, are calculated.Firstly, we look at the science that explains how heat flow works and the terms used.

4.2 Heat Flow between a Building and its SurroundingsHeat flows out of a building if the air is cooler outside and into the building if the air is hotter. The rate of heat flow depends on the temperature difference and the insulating properties of the building envelope.

When the exposed mass of the floor and walls is cooler than the air, it absorbs heat from the air. This heat is stored and released again when the air temperature drops below that of the mass. This reduces temperature swings in the room.

Heat from the sun (solar radiation) entering the house is absorbed by the surfaces of the room and is subsequently released as heat when the air temperature falls below that of the surface.

Infiltration, or uncontrolled ventilation through cracks and permanent vents, will cause the building to heat up when it is hot outside, and to cool down when the outside air is cold.

___________________________________________________________________________________________

Unit of Competency: CPCCBC4021A 4


Figure 1 Heat flow interactions between a building and its environment

4.2.1 WinterThe internal temperature maintained by a building in winter, depends on the

heating or cooling provided, if any heat from the Sun entering through the windows heat generated by appliances and the activity of the occupants heat stored in the mass contained within the insulation, including the floor heat lost or gained through the roof, walls, floor and windows loss or gain of heated air moving through cracks and vents.

4.2.2 SummerHeat flow into the building in summer can be reduced by

shading external walls and windows use of light external colours insulating the walls and ceiling

Heat can be removed through ventilation by replacing hot air with cooler air ventilation by cooling the mass of walls and floor in the evening and at night conduction to the cooler earth below the floor use of a heat pump i.e. air conditioner, evaporative cooling, although the humidity will increase,

___________________________________________________________________________________________



and could contribute to discomfort

4.3 Building Design and Thermal PerformanceTo better understand and explain thermal performance of a building, it is useful to have a look at the key elements of the building envelope.In Chapters 2 & 3 we expanded our understanding of “Climate” and “Comfort”. In the following chapters we look at a number of key elements of a building in detail that have an impact on a building’s thermal performance.

4.3.1 OrientationA home that is well positioned on its site delivers significant lifestyle and environmental benefits. Correct orientation assists passive heating and cooling, resulting in improved comfort and decreased energy bills.

Figure 2 Good orientation benefits the design

Passive solar heating is about keeping the summer sun out and letting the winter sun in. It is the least expensive way to heat a building.

Issues that impact on passive heating include :

Northerly orientation of window areas. Passive shading of glass. Thermal mass for storing heat. Minimising heat loss with insulation, draught sealing and advanced glazing. Using floor plan zoning to get heating to where it is most needed and keeping it there.

___________________________________________________________________________________________



Passive cooling is the least expensive means of cooling a building and is suitable in most Australian climates. Correct positioning of thermal mass, maximising the design of the building envelope and correct building orientation to the prevailing cooling breezes are essential to reducing the energy required for comfort.Chapter 5 looks at the orientation of a building, the impacts of shading to walls and glazing and how passive heating and cooling principles reduce the energy required to maintain comfort.

4.3.2 InsulationInsulation is an essential component of passive design. It improves building envelope performance by minimising heat loss and heat gain through walls, roof and floors.

Figure 3 Examples of bulk and reflective foil insulation

In Chapter 6 the topics covered include :

Insulation types and their applications. Recommended insulation levels for different climates. Strategies for cost effective insulation solutions.

4.3.3 GlazingWindows and glazing are a very important component of passive design because heat loss and gain in a well insulated home occurs mostly through the windows.

Figure 4 Glazing impacts on the thermal performance of a room

___________________________________________________________________________________________



With good passive design, this is used to advantage by trapping winter heat whilst excluding summer sun. Cooling breezes and air movement are encouraged in summer and cold winter winds are excluded.Well positioned and high quality skylights can improve the energy performance of your home and bring welcome natural light to otherwise dark areas. Chapter 7 examines the role of glazing and its impact on the thermal performance of a building.

4.3.4 Thermal Mass Materials like concrete, bricks and other masonry are used in passive design to absorb, store and re-release thermal energy. This moderates internal temperatures by flattening out the up and downs in the outdoor temperatures, therefore increasing comfort and reducing energy costs.

Figure 5 Good use of thermal mass can improve the thermal performance of a room

Chapter 8 looks at thermal mass in buildings and :

Where and how to use thermal mass. Thermal mass solutions for different climates and construction types. How much thermal mass to use.

4.3.5 Ventilation Air movement through a building is the main element of passive cooling in many Australian climates as the moving air cools the occupants through increasing evaporation rates.

It is the least expensive means of cooling a building and the lowest environmental impact and is examined in Chapter 9.

4.4 Definitions of Building Thermal PerformanceHeat is a form of energy appearing as motion of atoms, molecules and ions, or as radiation travelling through space. The metric unit of heat is the Joule (J).

The units kilojoule (kJ), megajoule (MJ) and kilowatt-hour (kWh) are used on electricity and gas bills to describe the total amount of energy consumed over a period of time.

___________________________________________________________________________________________



1 kWh = 3.6 MJ.

Temperature (T) is a measure of the thermal state of a material due to the presence of heat. Degrees Celsius, (oC), is commonly used for everyday measurement of temperatures although the S.I. unit is the Kelvin. The Kelvin scale has its zero at “absolute zero” when all atoms, molecules and ions cease to move. This occurs at a temperature of -273.16oC.

The Celsius scale uses the freezing point of water as its zero, with the boiling point of water occurring at 100oC. A change in temperature of 1K is the same as a change in temperature of 1oC.

To convert from one scale to the other, the following formula is used: T (K) = T (oC) + 273.16

Heat flow will occur between two bodies if there is a difference in their temperatures. This exchange of energy always occurs from the hotter to the colder surface.

Heat flux is the rate of heat flow, i.e. the total heat flow per second, (s), through a specific area. The S.I. unit of measurement is Joule per second, (J/s), which is defined as the Watt, (W).

Heat flux density is the intensity of the rate of heat flow, i.e. the heat flux through an area of 1 square metre, (m²). The unit of measurement is Watt per square metre, (W/m²).

Specific heat is a measure of the ability of substance to change its temperature as it absorbs heat.

Latent heat is a measure of heat absorbed or emitted as the material changes its state from solid-liquid-gas. This phase change takes place without a corresponding change in temperature.

Objects at different temperatures will exchange energy via three thermal mechanisms, conduction, convection and radiation.

Thermal conduction occurs whenever energy transfer is due to the exchange of kinetic energy between particles at the atomic or molecular level.

Thermal convection involves energy transfer from a solid to a fluid such as air or water, or energy transfer within a fluid due to the movement of fluids at higher or lower temperature than the surrounding fluid. Convection heat transfer often refers to the flow of heat from one surface, through a fluid, to another surface.

Natural convection occurs when fluid (liquid or gas), in contact with a hot surface is heated, rises due to its expansion and resultant lower density.

Forced convection heat transfer occurs when fluid currents are produced by some external source such as the wind, blowers or pumps.

___________________________________________________________________________________________



Ventilation can transfer heat by the movement of matter alone when the air being replaced is at a different temperature.

Thermal radiation is energy emitted by a body as a consequence of its temperature alone. Radiant energy takes the form of electromagnetic waves or photons. Unlike conduction or convection there is no heat transfer due to the movement of matter. In a vacuum all radiant energy travels at a constant speed, (the speed of light), irrespective of its energy or wavelength.

4.5 Solar RadiationThe heat from the sun that falls on the building envelope is a major factor to the thermal performance of a building.It is useful to review the impact of solar radiation and its components.

4.5.1 Absorption by the AtmosphereThe solar radiation received by the surface of the earth is considerably reduced by atmospheric absorption. This is caused by ozone, water vapour, carbon dioxide and oxygen and is wavelength dependent. In the upper atmosphere ozone removes virtually all the short wavelength ultra-violet radiation reaching the earth's surface thus protecting organisms from its lethal effects. Water vapour and carbon dioxide in the lower atmosphere absorb some radiation.

4.5.2 Components of Solar RadiationThe solar radiation (heat from the Sun expressed in Watts/m²) incident on a surface such as a wall is made up of three components, as shown in diagram below.

Beam radiation travels directly from the sun to the surface. Diffuse radiation is the radiation that reaches a surface after being scattered by the atmosphere

and thus comes from all directions of the sky hemisphere. Reflected radiation is the radiation that has reached a surface by reflection from the ground or a

nearby surface.

___________________________________________________________________________________________



Figure 6 Components of Solar Irradiance

4.5.3 Colour and AbsorptanceSolar absorptance refers to the ability of a surface to absorb the sun's emission spectrum. Absorbed light is converted to heat at the absorbing surface. Light that is not absorbed by an opaque surface is reflected.

Dark external colours increase the absorptance of solar energy and increase surface temperature. The amount of this energy that is transmitted into the house depends on the amount of insulation and mass in its path.

Insulated houses are less affected by colour than uninsulated houses of similar construction.

The diagram below shows the relative temperatures in the cavity below the roof. The cavity below the dark uninsulated roof has the highest temperature, followed by the cavity below the light uninsulated roof. When both the dark and the light coloured roofs are insulated there is a dramatic drop in temperature that reduces the importance of external colours in determining performance.

Figure 7 Colour and roof space temperature

Light external colours decrease winter performance but improve performance in summer.

Since the thermal performance of a house in a warm climate is generally dominated by how it performs in summer, a better overall performance results with the use of light colours.

___________________________________________________________________________________________



4.6 Calculation of Heat Flow4.6.1 R-ValuesIn the building industry today, the thermal performance of building materials and insulation products is described by its R Value.

The R-value of a building material describes its ability to resist heat flow. The higher the R-value, the greater the resistance to heat flow due to the temperature difference from one side to the other and the less the actual heat flow.

Figure 8 Heat Flow through bulk insulation reduces as material thickness increases

Materials, air gaps and air films on material’s surface can all be assigned R-values.

4.6.2 R-value of Bulk MaterialsBulk materials have R-value directly proportional to their thickness.

Below is a table listing the R-values of some common building materials. Most building materials have low R-values that need to be boosted with the addition of insulation if they are to reduce heat flow. Only a few building materials like Autoclaved Aerated Concrete e.g. CSR Hebel, have an intrinsically high R value.

MaterialR

(m2K/W)

10 mm plasterboard 0.06

200 mm concrete 0.14

___________________________________________________________________________________________



100 mm brick extruded clay

0.16

20 mm pine weatherboard 0.20

300 mm mud brick 0.39

20 mm carpet and underlay

0.43

90 mm timber stud 0.90

200 mm AAC block 1.54

90 mm glass fibre batt 2.05

Table 1 R-values of some common building materials

4.6.3 Resistance of Air Films The air film on the surface of materials has a very small resistance to heat flow. The R-value of this air film resistance depends on the type of surface of the material.

Generally, there are two main groupings of materials for the calculation of air film resistances, those with clean bright metal surfaces and those that do not (the rest).

The air film resistance of clean bright metal surfaces such as that of aluminium foil will have a higher R-Value than most other common building materials.

The movement of air over the surface impacts on its ability to resist heat flow as well i.e. ventilated cavities perform worse than well sealed cavities with no air movement.

4.6.4 Resistance of Air GapsBuildings often incorporate air gaps into the building element design i.e. a cavity in brick veneer or cavity brick construction. Air gaps resist heat flow and it is possible to have a R-Value applied to it.

The heat transfer across an air gap in a wall or roof space depends on the types of surfaces on either side of the air gap, but otherwise does not depend on gap width. When either one or both sides of an air gap are clean shiny foil products like aluminium foil, the heat flow across the gap is reduced significantly with a higher R-Value.

Air gaps between two materials that are not shiny foil (such as the cavity in a cavity brick wall), the heat flow is larger and the R-value lower.

___________________________________________________________________________________________



Air gaps that have one or both surfaces of shiny aluminium foil are referred to as reflective air gaps or spaces while those that contain no foil surfaces are referred to as non-reflective air gaps or spaces.

Resistance to heat flow increases as the gap width increases to about 30mm, after which it remains nearly constant.

Figure 9 Resistance of airspaces compared to brick and glass wool

For non vertical air gaps, such as the air gap between roof sheeting and sarking under, the R-Value is split into an up and down value.

Heat flows upwards more easily than downwards (warm air rises) and therefore, air gaps are given both an Rup and an Rdown value.

The Rup value is always less than the Rdown value for non vertical air gaps or tilted gaps such as in a roof.

Reflective aluminium foil itself has no R-value as such and requires an air gap to be effective. If no air gap exists such as when foil is placed directly behind or in contact with cladding and the wall cavity is filled with bulk insulation, the foil acts as a vapour barrier only and does not contribute to the Total R-value of the building element.

4.6.5 Resistance of Roof SpacesHeat flow in roof spaces is more complex in nature than the simple heat flow across unventilated parallel sided air cavities. The table below lists some R-values in roof spaces calculated from experiments.

___________________________________________________________________________________________



Type of roof space R (m2K/W)

Up Down

Reflective Unventilated 0.56 1.09

Reflective Ventilated 0.34 1.36

Non Reflective Unventilated

0.18 0.28

Non Reflective Ventilated 0.11 0.46

Table 2 Experimental R-values of pitched roof spaces

Roof spaces are more effective at reducing heat flow into the building in summer than they are at retaining heat during winter. Reflective sarking under the roof increases the R-value of the roof space substantially. Ventilated roof spaces perform better in summer but worse in winter than unventilated spaces.

4.6.6 Total R-value of a building element A building element is made up of a number of materials, some of which may include air gaps or spaces. The various modes of heat transfer that take place are illustrated in the diagram below that uses an insulated brick veneer wall as an example. Here it is assumed that the inside of the building is at a higher temperature than the outside air.

___________________________________________________________________________________________



Figure 10 Example of heat transfer through an insulated brick veneer wallThe total R-value of the wall is calculated by adding up all of the individual R-values of individual materials, air gaps and air film resistances.

TO is outside temperature RO is outside air film resistance 0.05RB is brick resistance 110 mm 0.18RAG is air gap resistance 75 mm 0.16RIns is insulation resistance 50 mm 1.14RL is lining resistance 10 mm 0.06RI is inside air film resistance 0.105TI is inside temperature

The equivalent thermal circuit for the wall is shown below.

Figure 11 Thermal resistance network for a brick veneer wall

___________________________________________________________________________________________



This is mathematically equivalent to having a single resistance, RT, equal in value to the sum of the individual resistances.

The actual heat flow is calculated, or U Value of the building element, is calculated as the reciprocal of the equivalent total resistance.

U value is best referred to as thermal transmittance.

The U value represents the rate of heat flow or transfer through a 1 m2 area of the building element when there is a temperature difference of 1 degree between the air on either side.

The U value of a building element on its own does not determine thermal performance. The accessibility of any potential thermal mass is also important and is demonstrated in the graph below.

The thermal performance of an externally insulated tilt concrete wall is compared to internally insulated tilt concrete wall. The performance of the externally insulated wall is much better and when well insulated approximates that of a cavity brick wall due to the thermal mass being more accessible.

Figure 12 The positioning of the mass is just as important to performance as insulation level

___________________________________________________________________________________________



Building thermal performance is affected by many factors.

To optimise the thermal performance of a building, the building design must embrace passive heating and cooling principles, by using insulation carefully, correctly position thermal mass, shade walls and glazing when required and use orientation to access cooling breezes in summer that when designed well, will minimise the energy required to maintain comfort and to reduce electricity and gas bills.

___________________________________________________________________________________________



Acknowledgements

Figure 1 Dr Holger Willrath - The Thermal Performance of Buildings - Short Course Notes

Figure 2 Your Home Technical Manual - Department of Climate Change and Energy Efficiency -

Fourth Edition as amended - published 2010

Figure 3 Education in Building - Insulation Workbook - Paul Kearney and Associates

Figure 4 G James 131 Series Brochure

Figure 5 Your Home Technical Manual - Department of Climate Change and Energy Efficiency -

Fourth Edition as amended - published 2010








Table 1 Dr Holger Willrath - The Thermal Performance of Buildings - Short Course NotesTable 2 Dr Holger Willrath - The Thermal Performance of Buildings - Short Course Notes

___________________________________________________________________________________________



___________________________________________________________________________________________


Questions – Chapter 4: What is Thermal Performance?

1. The rate of heat flow through a building element is best described asA. The air film resistance on the surfaces of the building elementB. The temperature difference and the insulating properties of the building envelope C. The colour of the surfaces of the building elementD. The R-value of insulation installed in the building element

2. Passive solar heating is aboutA. Keeping the summer sun in and letting the winter sun outB. Keeping the summer sun out and letting the winter sun outC. Keeping the summer sun out and letting the winter sun inD. Keeping the summer sun in and letting the winter sun in

3. Which if the following is not a unit of heat?A. Joule B. Watt C. MJ D. kWh

4. The R-value of a building materialA. Measures heat flow through a building material B. Is not important in measuring the thermal performance of a building material C. Includes air film resistance of a building material D. Describes its ability to resist heat flow

5. Air film resistanceA. Is the same as air gap resistance B. Is included when calculating the R-value of a building material C. Increases with wind speed D. Is included when calculating the total R-value of a building element

6. Reflective foil insulationA. Reduces heat flow through its thickness B. Provides the same added R-value in a weatherboard wall as in a brick veneer wall C. Has a high resistance to heat flow from one side of the foil to the other D. Requires an air gap to be effective

7. To be most effective, bulk insulationA. Should be placed on the outside of a mass wall B. Should be placed on the inside of a mass wall C. Can be placed on the inside or outside of a mass wall D. Should be evenly distributed to both sides of a mass wall

build thermally efficient and sustainable structures · web viewbuild thermally efficient and...

Documents