c&g powerpoint thermal values

City & Guilds Construction

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PowerPoint presentation

Thermal values

Unit 301: Principles of organising, planning and pricing construction work



Aims and objectivesAim:

• Introduce learners to thermal values.

Objectives:

• Explain what thermal values are.

• Compare thermal values of wall construction.

• Explain the purpose of an Energy Performance Certificate (EPC).

• Research and suggest how to reduce the U-value of a room. This could take the form of a group presentation.

• Presentation on how to reduce the U-value of a room.



Thermal valuesHeat and the effects of heat are major factors in the provision of a quality environment.

A satisfactory thermal environment is usually the most important aspect in the design and use of a building.



Heat lossHeat will flow when there is a temperature difference.

It will flow from the hotter area to the cooler area until there is equilibrium (balance).



Heat and temperatureHeat is different from temperature.

• While two substances may be at the same temperature, one may have required more energy in the form of heat to reach that temperature.

• For example, more energy is required to heat water than oil.

• This is because some materials have a greater heat capacity than others.



Heat and temperature continuedBoiling water will retain its temperature level much longer than metal and sparks emitted by the grinding process because water has a higher heat capacity than metals.



Heat transferThe transfer of heat can occur in three ways:

• Conduction

• Radiation

• Convection



Heat and thermal power• Temperature is a measure of the ‘hotness’ of a substance, and is

measured in Celsius (˚C) or kelvin (K).

• Heat measures the thermal energy which flows into or out of buildings. Heat is measured in joules (J).

• Power is the rate at which heat flows, or the heat flow per second. Power is measured in joules per second (J/s) or watts (W).



Energy lossEnergy released by poorly insulated buildings can have a number of detrimental affects. (What are these?)

Highly insulated structures will help to:

• prevent heat loss

• reduce the size of heat-providing appliances

• reduce costs to the user

• help the environment

• reduce the country's energy demands.



Cooling and heatingSome structures can be more costly to cool than they are to heat.

Buildings with large amounts of glass in particular can consume more energy to cool the internal environment than they do to heat it.



Thermal conductivityHeat flows through solid materials by thermal conduction.

• Thermal conductivity is a measure of how good the material is at conducting heat.

• Copper has a high thermal conductivity value (it is classed as a good thermal conductor).

• Polystyrene and glass fibre have low levels of thermal conductivity (they are classed as thermal insulators).



Thermal conductivityThis chart shows the thermal conductivity of commonly used building materials.

Density (kg/m3) Thermal conductivity λ (W/m2 K)

Aluminium alloy 2700 190. 000Brickwork (exposed)

1700 00. 770

Reinforced concrete (1% steel)

2300 02. 300

Timber (softwood) 500 00. 130Insulation (mineral wool quilt)

12 00. 042



Reduce heat loss – materialsWhat types of material are available to designers to reduce heat loss?

• Reflective materials (foil)

• Loose fill materials (polystyrene granules)

• Flexible materials (fibreglass quilts)

• Materials formed on site (expanded foam)

• Rigid preformed materials (aerated concrete blocks)



Thermal conductivityThe term thermal conductivity is often abbreviated to k. The unit for k is W/m²/˚C.

This is because thermal conductivity is defined from the equation used to calculate the thermal power (P) which flows through a material.



Thermal powerFormula for calculating thermal power (P):

P = kA(T1 – T2) x

P = thermal power

A = area

X = thickness in m² and m respectively

(T1 – T2) is the temperature difference (in ˚C or K) between parallel surfaces which are at 90˚ to the direction of the heat flow.



Thermal power continued



Calculating thermal powerIf the internal temperature of a room is 21˚C and the external temperature is 6˚C, find the power loss through a pane of glass 2m x 1.5m, and 6mm thick.

P = kA (T1 – T2)x

P = 1.5 x 2 x (21 - 6) 0.006

P = 7500w



U-valuesStructural elements are often made from a number of different materials.

Heat will travel through each material at a different rate.



U-values continuedDesigners combine differing thermal factors when designing the separate elements of a building.

It is convenient to combine these factors into a single measurement – the U-value.



U-values continued• The U-value measurement describes a building’s ‘overall thermal

transmittance coefficient’ (no wonder they use the term U-value).

• A U-value is a measure of the overall rate of heat transfer, by all mechanisms, under certain standard conditions, through a particular section of construction.

• Unit: W/m²K



U-values continued

Examples of typical U-values in a modern domestic building.



U-values continuedElement Construction type U-value (W/m2K)Solid wall Brickwork 215mm, plaster

15mm2.3

Cavity wall Brickwork 103 mm, clear cavity 50mm, brickwork 103mm

1.6

Cavity wall Brickwork 103mm, clear cavity 50mm, lightweight concrete block 100mm, lightweight plaster 13mm

0.58

Cavity wall Brickwork 103mm, insulation 50mm, lightweight concrete block 100mm, lightweight plaster

0.48

Pitched roof Ties on battens, felt, ventilated loft airspace, 100mm mineral wool on joists, 100mm mineral wool between joists, plasterboard 13mm

0.20



U-values continued• The U-values on the previous slide were calculated using the

thermal resistances of the materials involved in making a particular part of the structure (the external walls for example).

• The thermal resistance of a material will depend on the thickness and the rate at which the material conducts heat.

• Thermal resistance = thickness / thermal conductivity



Surface resistanceAir in contact with a surface forms a stationary layer which opposes the flow of heat. This factor will have to be taken into account when calculating U-values.

Standard values for surface resistances can be found on the following slide.



Surface resistance continued

Type of resistance Construction element

Heat flow Surface emissivity

Standard resistances (m2K/W)

Inside surface Walls Horizontal HighLow

0.120.30

Roofs (pitched or flat) Upward HighLow

0.100.22

Ceilings/Floors Downward HighLow

0.150.56

Outside surfaces Walls Horizontal HighLow

0.060.07

Roofs Upward HighLow

0.050.05

Airspace (including boundary surfaces)

Unventilated, 5mm Horizontal or upward

HighLow

0.110.18

Unventilated, 20mm or greater

Horizontal or upward

HighLow

0.180.35

Ventilated loft space with flat ceiling, unsealed tiled pitched roof 0.11



Total thermal resistance The total thermal resistance of a building element can be calculated in the following way:

RT = Rsi + R1 + R2 + Rso = ΣR



U-value calculationThe U-value is a calculation of the total thermal resistance provided by the materials used to form a structural element.

or U= 1

ΣR



Energy Performance Certificates What is an EPC?

Energy Performance Certificates (EPCs) are needed whenever a property is:

• built

• sold

• Rented.



Energy Performance Certificates continued Energy Performance Certificates (EPCs) were introduced in England and Wales on 1 August 2007 as part of Home Information Packs (HIPs) for domestic properties with four or more bedrooms. Over time this requirement was extended to smaller properties. While the requirement for HIPs was removed in May 2010, the requirement for EPCs continued.



Energy Performance Certificates continued To view an example EPC, click on the link below:

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/49997/1790388.pdf






Any questions?

c&g powerpoint thermal values

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