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Salford City College

Construction&

Engineering

Nationals

In

Construction

Construction science and materials After this next session is over you will be able to :

• Calculate the "U" value of a concrete roof slab 150 mm (0.15m) thick, asphalted 30 mm

(0.03m) for rainwater protection and the soffit faced with plasterboard 12 mm (0.012)thick on

50 mm softwood battens, the void filled with 50mm (0.05) Glass fibre quilting .

• K Concrete = 1.0 W/m² °CK Asphalt = 0.25 W/m² °C

K Plasterboard = 0.35 W/m² °CK Glass fibre Quilt = 0.035W/m² °C

You will also…..

• Know about heat gains and losses in buildings

Where is heat lost in a dwelling?...

Next – Heat Losses in Homes

Heat Losses In

Homes

Coming Next – Construction Techniques

Give an example of steps you can take to save energy …

Construction Techniques

DG

Next note taking …Factors affecting energy use

Double galzing…

Next:Note taking…

Name things which can increase or decrease energy usage …

• climate: • Location • solar radiation • Wind• degree of exposure

• Building Characteristics

• shape and size

• Usage: • the way that the property is

used including space and heating requirements

• internal temp causes :• domestic appliances• desired internal temp.• heating system and its

efficiency

Coming Next – the Total Heating Load

The Total Heating Load…

The Total Heating Load

Boiler

Ventilation

Roof

Electrical appliances

Floor

Windows

Walls

Solar Gains

Fuel Supply

Flue losses

Hot water

Cooking

Next slide…FHL + VHL = SHL

Total Heat Load• Gains

• What types of things heat a house up?

• • Solar effect

• Cooking

• Hot water

• Electrical appliances

• Losses

• How is heat lost from a

building?

• Fabric Heat Loss

• Ventilation Heat Loss

• Specific Heat Loss

• In what ways does heat flow? ...

THERMAL STUDIES• HEAT FLOW

Radiated heat flow through air such as the rays from the sun

Convection as fluid, gas or liquid. It expands on heating becoming lighter and is forced upwards by cooler fluid taking its place and therefore creates circulation

conduction by passing heat from one molecule to another, this is the source of most heat loss in buildings.

• The current Building Regulations carry a section relating to the conservation of energy

• Someone has to know how to add up the amount of heat loss

• So who does it and how is it done?..

K…

What is the standard unit of THERMAL CONDUCTANCE?

("K" value)• In physics, thermal conductivity, k, is the property of a

material that..

• …indicates its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction.

Coming Next – values for common materials

• K values vary with good insulators having a value of around 0.003 W/m² °C and up to 400 W/m² °C for good conductors.

• (Don’t try to remember these numbers they are set out in industry tables)….

K values for

common building

materials: adapted from the CIBSE

chartered institute of building service engineers

guide

Material K value (W/m2 0C )

• Aluminium alloy 160 • Asbestos cement sheet 0.4• Asphalt roofing 0.8• Brickwork 0.84• Concrete (regular) 1.83• Concrete (lightweight) 0.19• Copper 160• Corkboard 0.042• Fibre insulating board 0.05• Glass 1.0• Glass wool (mat or fibre) 0.04• Hardboard 0.13• Mineral wool 0.039• Gypsum plaster 0.46• Plasterboard 0.16• Polystyrene (expanded) 0.033• Polystyrene (solid) 0.17• Polyurethane foam 0.026• PVC flooring 0.04• Carbon steel 150• Sandstone 1.3• Softwood timber 0.13• Hardwood timber 0.15• Woodwool slab 0.085 Reciprocal of K is R…

Resistivity & Conductivity• K values are good but

so is the opposite of conductance,which is…

• Resistance

• to show how a material resists heat a Resistivity (r) value can be found. This is the reciprocal (opposite) of a K value

• How do you find resistance?

• Resistivity =1/K value• materials are not always

supplied in one-metre blocks, using the resistivity you can look at the thickness of the material and calculate the "resistance" (R)

What if you have lots of Rs …

HEAT LOSS FROM BUILDINGS

• Finally to find how much heat energy is being conducted we use a thing called the unit of "Thermal Conductance" used …

• U = 1/total resistance expressed in Watts per metre²

What does the U-value represent?…..

• The term ‘U’ represents overall thermal conductance from the outside to inside covering all modes of heat transfer.

• What use is this in relation to the whole?

• An average U-value can be calculated

• The U-Value is an important concept in building design.

• It represents the air-to-air transmittance of an element (part or fabric). This refers to how well an element conducts heat from one side to the other, which makes it the reciprocal of its thermal resistance.

• So if you calculate the thermal resistance of an element, we can simply invert it to obtain the U-Value..

• U = ?

•U = 1/total resistance expressed in Watts per metre²

Lets work some out…

Example • Calculate the "U" value of a brick wall, plastered one side

with 20 mm background and 3 mm of hardwall gypsum finish, where "K" values for Brickwork = 0.55 W/m² °C,

background = 0.6 W/m² °C, hardwall plaster = 0.9 W/m² °C.

Material L (Thickness) K Value R=(1/k) x L

Brickwork 0.103 0.55 0.187

Background 0.020 0.6 0.033

Gypsum 0.003 0.9 0.003

Total Resistance 0.223

U value = 1/Total Resistance = 1/0.233 = 4.484 W/m² °C

Coming Next – further examples…

Example

• Calculate the "U" value of a concrete roof slab 150 mm (0.15m) thick, asphalted 30 mm (0.03) for rainwater protection and the soffit faced

with plasterboard 12 mm (0.012)thick on 50 mm softwood battens, the void filled with 50mm (0.05) Glass fibre quilting .

• K Concrete = 1.0 W/m² °CK Asphalt = 0.25 W/m² °C

K Plasterboard = 0.35 W/m² °CK Glass fibre Quilt = 0.035W/m² °C

• Work to 3 decimal places.

Material L K R=(1/k) x L

Concrete 0.15 1.0 0.15

Asphalt 0.03 0.25 0.12

Glass Fibre 0.05 0.035 1.426

Plasterboard 0.012 0.35 0.034U value = 1/Total Resistance = 0.58 W/m² °C

Total R

1.73

Coming Next – air layers & cavities

BOUNDARY STILL AIR LAYER & CAVITIES

• In any construction between the fabric and the internal/external environment there will be a layer of air which is "trapped" by the surface roughness of the materials used

• This boundary still air layer has an insulating quality that can be included in the heat loss calculation.

• The exterior surfaces of buildings are usually rougher than the internal faces

Coming Next – thermal properties of cavities

CAVITIES

• cavities are used to keep heat in • To increase the width of a cavity will not necessarily

produce any significant improvement in heat retention-meaning…

• You only need a small gap

Coming Next – standard values…

BOUNDARY STILL AIR LAYER & CAVITIES

• Standard Internal Resistance value 0.123 W/m²

°C

• Standard External Resistance value 0.055

W/m² °C

• Standard Cavity Resistance value 0.18 W/m²

°C

Coming Next – further standard values

HEAT AS A FORM OF ENERGY

• Total heat loss per second =

Area x "U" x Temperature difference.

• Calculations to determine the total

heat loss for a particular building for

a particular period for a particular

temperature difference can now be

made

This session completed…

• Average U value = total exposed surface area / total exposed surface area

Heat load calculator

• http://www.shophmac.com/info-center/hvac-calculators/heat-load-calculator.php

• STOP

Material L (Thickness) K Value R=(1/k) x L

Brickwork 0.103 0.55 0.187

Background 0.020 0.6 0.033

Gypsum 0.003 0.9 0.003

Total Resistance 0.223

U value = 1/Total Resistance = U value = 1/Total Resistance = 1/0.233 = 4.484 W/m² °C

Coming Next – further examples…