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Build Thermally Efficient and Sustainable Structures

Chapter 8: Thermal MassProduced 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 contents8 Thermal Mass.............................................................................................................................................4

8.1 Introduction...............................................................................................................................................4

8.2 What is Thermal Mass?.............................................................................................................................5

8.3 Properties of Thermal Mass......................................................................................................................6

8.4 Classifying Thermal Mass..........................................................................................................................7

8.5 Temperature Swing and Time Lag.............................................................................................................8

8.6 Location of Thermal Mass.......................................................................................................................10

8.6.1 Contact with the Ground.........................................................................................................................108.6.2 Earth Bermed Walls and Earth Covered Roof..........................................................................................118.6.3 Single Level versus Two Storeys...............................................................................................................11

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Building Thermally Efficient and Sustainable Structures: Chapter 8 – Thermal Mass ____________________________________________________________________________________________

8 Thermal Mass8.1 IntroductionHomes that are designed with elevated floors and lightweight framed walls with weatherboards or FC sheet cladding do not perform the same way as cavity brick homes with slab floors.

Modern homes have well insulated walls, roof and ceiling spaces. Which is better? A house on a slab or one with insulated light weight walls, floors and roof/ceiling?

The answer is dependent on the location of the home and how the occupants live in the home but generally homes with well located thermal mass perform better in most climate types in Australia than insulated lightweight homes with little or no thermal mass.

The difference in the performance of the home and the occupant’s comfort levels is often due to the extent and location of the thermal mass within the home.

Today’s homes often have large areas of glass. With high levels of thermal mass, the performance of the home is much less sensitive to the problems caused by too much glass than those with little or no mass (lightweight buildings), where rooms can get too hot very quickly if they have cannot get rid of the excess heat quickly through ventilation.

Thermal mass delays and reduces the transfer of heat. The absorption of heat by mass in the walls or floor prevents the rapid rise in air temperature that could otherwise lead to overheating of a room.

There will be times when the presence of thermal mass can be a disadvantage. In summer there may be periods during a heat wave when the night-time temperatures do not drop sufficiently to cool the mass in the building. When the temperature does eventually drop it will take a high mass house longer to cool down than a low mass house.

Taken over the whole year however, high thermal mass buildings perform much better in temperate and subtropical climates.

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Unit of Competency: CPCCBC4021A 1


Figure 1 Thermal Mass used correctly enhances comfort

8.2 What is Thermal Mass? Thermal mass is the ability of a material to absorb heat energy. This energy can be both hot and cold.

In summer thermal mass can absorb the heat from the hot surrounding air keeping the home comfortable. This is referred to as passive cooling.

Figure 2 Use of thermal mass in summer

In winter, the same thermal mass can store heat from the sun or provided by a heater and release it a night when it is cold outside and help keep the home warm. This is referred to a passive heating.

Figure 3 Use of thermal mass in winter

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Thermal mass is beneficial in climates where there is a large differences between the daytime and night-time temperatures, referred to as the diurnal temperature range.

Where the difference between the maximum and minimum temperatures over one day is less than 6°C, thermal mass may not be beneficial. In climates where the difference is between 7°C and 10°C, thermal mass may be beneficial whereas in climates with a difference greater than 10°C, thermal mass is desirable.

Adding insulation to a home design will improve its thermal performance but it will not reduce the potential temperature swings within a room.

Insulation reduces the heat flow through a building element but does not stop it or store it whereas thermal mass will absorb heat from the surrounding air and release it slowly at a later time in the day when required.

Good passive design of a home will incorporate well insulated building elements and utilise thermal mass correctly to enhance comfort and minimise energy bills.

8.3 Properties of Thermal MassFor thermal mass to behave correctly, it needs to have high density, good thermal conductivity and low reflectivity.

High density materials generally have good thermal mass. Concrete is a good example of a high thermal mass material.

The type and colour of the surfaces of thermal mass are important. Dark, matt or textured surfaces will absorb and radiate heat better than smooth, light or reflective surfaces.

Good thermal mass must allow heat to flow through it and store it without releasing it too quickly.

One measure used to describe the properties of thermal mass is Volumetric Heat Capacity.

Volumetric Heat Capacity (VHC) is a way of expressing the heat capacitance in terms of volume rather than mass.

The VHC of some common materials are shown below.

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Material VHC (kJ/m3K)Air 1.2Glass wool (12 kg/m3) 11Aerated autoclaved concrete block 525Brickwork: generic extruded clay brick 1454

Adobe (Mud brick) 1500Concrete (2400 kg/m3) 2112Glass 2108Water 4192

Table 1 Volumetric Heat Capacity of some common materials

Generally, materials with a lower VHC are good insulators e.g. glass wool insulation, and those with a higher VHC perform better as thermal mass e.g. concrete.

The extent and volume of these materials in the home is important. If a small amount is used, the benefits will be negligible.

A brick wall that is only 110mm thick will not work as well as a 230mm thick wall.

Concrete is a good example of thermal mass in a typical home but care needs to be taken with using applied finishes like carpet or floating timber floors that insulate the concrete and reduce its effectiveness as thermal mass.

8.4 Classifying Thermal MassBuildings incorporating large amounts of thermal mass behave differently to buildings having only a small amount of mass.

It is useful to refer to buildings as having low, medium or high thermal mass.

The following diagrams give examples of each mass classification. Please note that the concrete block, slab on ground construction is classified as being high mass if built on a single level, but is considered to be medium mass in the two storey version if the intermediate floor is timber rather than suspended concrete slab.

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Figure 4 Examples of high mass construction

Figure 5 Examples of medium mass construction

Figure 6 Examples of low mass construction

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8.5 Temperature Swing and Time LagThermal mass that is accessible has a stabilising effect by reducing the daily temperature swing. This is illustrated below using graphs generated from Solar Logic’s BERS Pro software. The delays in temperature peaks depend on the area and to a lesser degree the thickness of the exposed mass.

Figure 1 High thermal mass reduces temperature swings and delays peaks

Thermal mass in this example can be of benefit in winter by absorbing solar radiation coming through windows and making heat available later in the day when indoor temperatures drop due to the cooling of the outdoor air around the building.

In summer, if the building is ventilated in the evening, (when it is cooler outside), the mass of the walls and floor will release heat, giving lower internal temperatures the next day.

Figure 8 Winter temperature profiles for high and low mass houses

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Figure 9 Summer temperature profiles for high and low mass houses

8.6 Location of Thermal MassTo be useful thermal mass must be located to ensure good exposure to the air in the room. The effectiveness of storing or releasing heat is greatly reduced if the mass is insulated from the room in any way.

A brick veneer wall provides no useful thermal mass because the bricks are thermally isolated from the room by an insulating air gap and often insulation.

The usefulness of a concrete slab floor is reduced when it is covered by carpet. Internal walls are twice as useful as high mass externally insulated external walls because there

is twice the heat exchange area in contact with internal air. Walls thicker than about 100 mm cannot fully take advantage of the extra mass. Surface area of

mass exposed to the air in the room is more important than total mass. There are diminishing returns for extra thickness, depending on the climate. Thermal mass should be exposed to incident sunlight, if possible. If exposed to sunlight or reflected light the thermal mass should be a dark colour. Areas of heat transfer should be well ventilated Ventilation corridors through the building should contain thermal mass Thermal mass must be insulated from the outside i.e. reverse brick veneer is better than

standard brick veneer

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Figure 10 Examples of well positioned thermal mass

8.6.1 Contact with the GroundSuspended timber or concrete suspended floors do not perform as well as a slab on ground floor. This is due to a combination of thermal mass differences and the degree of contact with the ground. The temperature of the ground beneath a house undergoes only small daily temperature swings and has less seasonal temperature variation than the outdoor air. In summer, the temperature of the ground is lower than the average air temperature, so having a floor that is in contact with the ground is an advantage where the main concern is with the reduction of overheating in summer.If the floor has to be elevated for some non thermal reason, then enclosing the underfloor space reduces heat gains and losses via ventilation and results in performance closer to a slab on ground.

Elevated unenclosed floors perform poorly in summer and winter because the higher ventilation rates past the underside of the floor result in greater exposure to adverse external temperatures compared to the more stable temperature of the ground beneath the house.

8.6.2 Earth Bermed Walls and Earth Covered RoofJust as the building benefits from the contact between the floor and the ground, so will it benefit from earth bermed walls or even an earth covered roof.

The diagrams below are ranked from the lowest to the highest thermal performance.

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Figure 11 Thermal performance improves as the contact with the earth increases

8.6.3 Single Level versus Two StoreysDifferences in thermal performance between single storey and two storey houses which have been constructed using similar materials are due in part to the type of floor between levels, and the degree of shading provided for the lower storey windows. The use of a high mass between storey floor, and the provision of eaves or shading devices for the lower floor can improve the performance of the two storey building.

In a warmer climate it is advantageous to have contact with the ground for good summer performance. This makes the single level slab on ground house perform better than an equivalent design on two levels.

Figure 12 A single level home has better ground contact than a two level home

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Acknowledgements

Figure 1 Your Home Technical Manual - Department of Climate Change and Energy Efficiency -

Fourth Edition as amended - published 2010





Figure 4 Dr Holger Willrath - The Thermal Performance of Buildings - Short Course Notes


Figure 6 Dr Holger Willrath - The Thermal Performance of Buildings - Short Course Notes.

Figure 7 Solar Logic - Bers Pro V4.2 software tool







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

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Questions - Thermal Mass

1. A cavity brick wall with a timber floor is an example ofA. No thermal massB. Low thermal massC. Medium thermal massD. High thermal mass

2. In a brick veneer wall, the brick leaf is best located where?A. On the outside of the wall elementB. In the inside of the wall elementC. In the middle of the wail elementD. Doesn’t matter

3. A home with high thermal mass typically has a time lag of A. 0B. 30 minutesC. 2 hoursD. 6 hoursE. 12 hours

4. Which statement is correct? A. Single storey houses perform better thermally than two storey homes with the same floor areaB. Two storey homes perform better thermally than single storey homes with the same floor areaC. Single storey houses perform worse thermally than two storey homes with the same floor areaD. Single storey and two storey homes perform thermally the same.

5. In summer on a hot day an insulated lightweight house will have a maximum temperature that is A. Much higher than the maximum outdoor ambient temperatureB. Somewhere near the maximum outdoor ambient temperatureC. About halfway between the maximum and minimum outdoor ambient temperaturesD. Somewhere near the minimum outdoor ambient temperatureE. A bit below the minimum outdoor ambient temperature

6. In summer on a hot day an insulated lightweight house will have a minimum temperature that is A. Much higher than the maximum outdoor ambient temperatureB. Somewhere near the maximum outdoor ambient temperatureC. About halfway between the maximum and minimum outdoor ambient temperaturesD. Somewhere near the minimum outdoor ambient temperatureE. A bit below the minimum outdoor ambient temperature

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