1 - Thermal Effects in Buildings

8/3/2019 1 - Thermal Effects in Buildings

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THERMAL EFFECTS IN BUILDINGS

Composite Materials Performance

MHK221188


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Introduction

The Basics

transfer; gases and vapours

Thermal Effects

Thermal insulation; insulation values; thermal bridging;

structural temperatures


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Nature of Heat

THERMAL ENERGYTHERMAL ENERGY

Heat is form of energy ie thermal energy

SI unit of heat is the Joule (J)Joule (J)

Others sometimes used; Calorie; kilowatt hour; and BTU

Other forms of energy also measured in Joules

POWERPOWER

Rate at which energy is converted from one form to another

P= H / t where H is Heat energy and t is time

SI unit of power is the Watt (W)Watt (W)(1 watt = 1 J/sec.)


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Nature of Heat

TEMPERATURETEMPERATURE

Simply, heat flows from objects at high temperature to those at

low temperature

By definition, when no heat transfer between the two objects they

are at the same temperature. Think of a heated building, heat will tend to flow from the hot

building to the cooler outside air (in winter) and visa versa in

summer

SI unit of temperature is the Kelvin (K)Kelvin (K)

0C = 273K 100C = 373K


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Nature of Heat

HEAT CAPACITY

Ability to hold heat

Do not confuse with Thermal Conductivity

Specific Heat Capacity = Quantity of heat required to raise 1kg of the

material by 1K(or1C) (Measured in J/kg K)

Water4190 J/kg K

Concrete 3300 J/kg K

Copper390 J/kg K

So water is a very good medium for storing heat

Water around British Isles retains heat and creates a temperate climate


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Nature of heat

DENSITY

Relates the mass of an object to its volume

Density () = mass (m) / volume (v)

Masonry high density (small volume has large

mass) therefore high heat capacity within smallvolume

Electric storage heater uses cheap electricity to

heat bricks up, which then emitheat during the day.

Heat storage capacity of brick, concrete and stone

very relevant to thermal behaviour of buildings


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Nature of Heat

CHANGE OF STATE

Solid; liquid; gas

Material absorbs heat to change from solid to liquid then to gas

Material releases heat when change form gas to liquid then solid

SENSIBLE AND LATENT HEAT

Sensible heat heat energy absorbed or released during a changein temperature

Latent heat ditto during change of state

Enthalpy

steam @ 373K > energy than water at 373K


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Heat transfer

Heat will transfer between bodies until they reach an equilibrium

CONDUCTION;

CONVECTION;

RADIATION


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Heat transfer

CONDUCTION

Transfer of heat energy through a material without the molecules

changing their positions

Heat transferred as molecules in one part heated, then heat moves toother parts

Metals best conductors (high in free electrons)

Poor conductors include liquids and gasses, so porous materials with

high air content, good insulators in buildings


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Heat Transfer

Measurement of thermal conductivity

Conduction can be measured

ThermalConductivitymeasure of rate at which heat is conducted. Relates to:-

Coefficient of thermal conductivity - Usually k or

Measured in W/mK i.e. the coefficient of thermal conductivity is heat flow in Watts

across 1m thickness of material for temp diff. of1K(1 C) and a surface area of1m

Resistivity (r)reciprocal of k-value ie r = 1/k or1/

Because reciprocal, r measured in mK/W


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Heat Transfer

CONVECTION

Transfer of heat energy through a material by the bodily movement of particles

Will only happen in fluids (i.e. liquids and gasses). Will not happen in solids

Convection occurs when the fluid e.g. air, is heated;

It then expands Heated (expanded) air is less dense, so cooler fluid displaces the warmer air causing

the latter to rise

New air then also heated and process repeated

Gives rise to a convection current

Convection currents in a room

Air is poor conductor but whole room can be heated by a

single heater, using Convection as the mode of heat transfer


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Heat Transfer

RADIATION

In outer space, convection and conduction are not possible, so how is the Earth

warmed by the sun?

Radiation - Defined as the transfer of heat energy by electromagnetic waves

Simple rules

Dull black surfaces have the highest absorption and emission of radiant heat

Shiny silver surfaces have the lowest absorption and emission of radiant heat


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Heat transfer

THE GREENHOUSE EFFECT

Sun emits short wavelength radiation

It passes through atmosphere and glass

Inside Greenhouse, heat absorbed by plants which then re-radiate heat

Re-radiated heat is of longer wavelength, which dont easily penetrate glass

Re-radiated heat is therefore trapped, causing internal temp. to rise

Planet Earth behaves in the same way.

Increase in GH gasses eg C0

Implies greater level of re-radiated heat is retained

Contribution to global warming


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Thermal Effects

LEARNING

Need for building insulation

Types of materials used

Calculate R-values and U-values

Compare types used in different parts

Assess the building for insulation quality

Building codes related to insulation

Assess bldg elements for relative insulation values

Cause and effect of thermal bridging

Temp profiles and prediction of condensation Why different structures respond to temperature changes at different rates ?


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Thermal Effects CONTENTS THERMAL INSULATION

materials;

insulators;

thermal conductivity;

INSULATION VALUES -

thermal transmittance (U-value);

elemental U-values; thermal resistance (R- value);

u-value adjustment

THERMAL BRIDGING

Bridge effects;

pattern staining;

combining U-values

STRUCTURAL TEMPERATURES

response times;

temperature gradients


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Thermal insulation

General

Retain heat inside for as long as possible

Conserves energy & reduces costs

Less energy use implies less CO (and other)

Implies reduced global warming

Good insulation will achieve this

Will also reduce heating effect in the summer

Consider a tent no insulation: hot in summer & very cold in winter

Large buildings sometimes more costly to cool than heat


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Thermal insulation

General (contd)

Condensation is a significant problem in poorly insulated properties particularly

where surface temperatures are low

Good thermal insulation will keep internal surface temperatures above the dew-

point and therefore reduce the condensation effect

Well placed insulation reduces time for a room to heat up e.g. when unoccupiedduring the day


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Thermal Insulation

INSULATING MATERIALS DESIGN to oppose transfer of heat between areas at different temperatures

Vacuum is perfect insulator against conduction not practical, so consider gasses

Atoms spaced well apart and low densities almost as good

Air is the active ingredient used in many insulation materials

E.g. mineral wool; aerated concrete

Cannot use air alone since no strength and moving air would also carry heat through

convection. (Air is trapped within min. wool)


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Condensation of water vapour

poor ventilation of rooms/thermal bridges


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Thermal insulation

MATERIALS (contd)

To restrict radiantheat, use surfaces that do not absorb or emit

radiant heat

Which are??

Shiny surfaces that reflect electromagnetic waves E.g. Aluminium foil

But Aluminium is a very good conductor!

However foil is so thin, very little conductive effect


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Thermal Insulation

Rigid preformed materials

Flexible materials

Loose fill materials

Materials formed on-site

Reflective materials

e.g. aerated concrete blocks

e.g. mineral wool quilts

e.g. expanded polystyrene granules

e.g. foamed polyurethane

e.g. aluminium foil

TYPES OF THERMAL INSULATOR


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Thermal Insulation

PROPERTIES

Good insulator

Suitable strength

Moisture resistance

Fire resistance

Pest and fungi resistance

Harmless to humans and environment

Compatible with adjacent materials


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Thermal Insulation

THERMAL CONDUCTIVITY

Remember:

thermal conductivity

(rate of conduction of heat)

k or

Units: W/mK

Values ofkcan vary due to differences in density, thickness, moisture content,

degeneration of the material, but for our purposes, we will assume a set density for a

particular material, as shown in Table 2.1


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Thermal Insulation


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Thermal Insulation


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Thermal Insulation

RESISTIVITY

Remember:

Reciprocal of conductivity (an alternative measurement for conduction)

r = 1/k

NB: not to be confused with Resistance (R) see later slides

THERMAL TRANSMITTANCE (U-VALUE)

U-value is usually the performance value defined in the Building Codes

(Building Regulations)

Applied to walls; floors and roofs in their composite form

e.g. the U-value of walls < 0.25 W/mK


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Insulation

values


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Emissivity and Absorption

Ability of a material to give off or absorb radiant heat

Relates to the surface of the material

Rough black absorbs and emits most heat

Shiny silver absorbs and emits least heat

All materials compared with a black body

EMISSIVITY fraction of energy radiated by a body compared to that radiated

by a black body at the same temperature

ABSORPTIVITY fraction of radiant energy absorbed by a body compared with

that absorbed by a black body etc

A "black body" is a theoretical perfect

absorber, which absorbs radiation of all

wavelengths falling on it. It reflects no light at

normal temperatures and thus appears black.

However, like ideal gas in kinetic theory, it is a

theoretical model and we may find in reality

only "Almost perfect black bodies".


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Emissivity and Absorption

Examples: Aluminium Emissivity 0.05 Absorption 0.2

Dark bricks Emissivity 0.9 Absorption 0.6

Generally, colour has an important effect on heat absorbed by the building via the

high temp. radiation from the sun.

Colour has little effect on the heat emitted from buildings (low temp. radiation)

Low E Glass

Transmits maximum light; rejects max. Solar energy; and reflects max. room temp

energy back into the room


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Insulation Values

THERMAL TRANSMITTANCE U VALUE

Different materials conduct heat at different rates

In a cavity, also heat transfer via convection and radiation

Also have to account for surfaces, because radiation and convection will be

affected by surface colour and exposure to weather

Combination of all these factors provides us with the overall thermal

transmittance or U-value

The higher the U-value the more heat flows through so a good U-value is a

low one as you want to keep heat inside the building or outside depending on

the climate you live in.


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Insulation Values

U-Value is a measure of the overall rate of heat transfer, by allmechanisms under standard conditions, through a particular

section of construction

Unit: W/mK

U values have a linear relationship with heat loss ie wall with U-value 0.3 W/mK loses heat at half the rate of a

wall with U-value of0.6 W/mK

Also, cost of replacement heat will be half!!

The technical explanation of the U-value

The U-value physically describes how much thermal energy in Watts[W] is transported through a building component with the size of 1

square meter [m] at a temperature difference of 1 Kelvin [K] (=1C).


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Elemental U-values

Insulation properties will vary,

depends on moisture content

U-values calculated assuming

standard values for mc and rates of

heat transfer at surfaces and in

cavities see slide on Standard

thermal resistances Building Regulations and Scottish

Codes use U-values as targets and

limits for thermal insulation and

energy use

U-value standard values

common basis for comparison (seetable 2.3)


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Elemental U-values


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Elemental U-values

Note: Values indicative only manufacturers details must be consulted for accurate assessment


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Thermal Resistance (R)

U-values calculated from the R-values of the various parts

that make up an element e.g. a wall

Thermal resistance (R)is a measure of the opposition to

heat transfer by a component in say a wall

Unit: mK / W

Three types of thermal resistance

Material resistance

Surface resistance

Airspace resistance


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Thermal Resistances,R

Thermal Resistance, R [m2K/W] A measure of the opposition to heat transfer offered by a particular component in a

building element.

Thermal resistance of homogeneous layers

Design thermal values can be given as either design thermal conductivity or design thermal

resistance. If thermal conductivity is given, we can obtain the thermal resistance of the layer from

where

d = thickness of material (m)

k = thermal conductivity of material (W/mK)

R= thermal resistance (m2 K/W)

k

dR!


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Thermal Resistance

MATERIAL RESISTANCES

Resistance Rdepends on the thermal conductivity (k)

and its thickness (d)

R = d/k

Also, R= r x d

Where r is the resistivity (as opposed to Resistance)

Remember, r = 1 / k

i.e. the reciprocal of the conductivity value

(When calculating U-values, we are usually told the k value i.e.

ThermalConductivity)


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Thermal Resistance

SURFACE RESISTANCES

Have to be factored into the U-value calculations as well

Usually given as a set of standard values (see table 2.4 below)

AIRSPACE RESISTANCES

Ditto

And ditto


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Thermal Resistance


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Thermal Resistance

Total Thermal Resistance

Calculate individual resistance of various components

Add them together

Get Total Thermal resistance - RT or R

Analogy Total Resistance is similar to adding electrical resistance inseries


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Thermal Resistance

Once we have RT (or R), only a simple step away from establishingthe U-Value !!!!!!


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Calculation of U-values

U-value is the reciprocal of Total Thermal Resistance ie

U = 1 / R

Where

U = U-value (W / m K)

R= sum of thermal resistances of all components in the element (eg

surface resistances; air space resistances; brick resistance; plaster

resistance; all forming a solid brick wall)


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Insulation Values U-values

Where:

U - thermal transmittance of overall structure ( W/m2 C)

Rsi,Rso - inside and outside thermal resistances (m2 C/W)

R1,R2 - thermal resistance of structural elements (m2 C/W)

Ra - thermal resistance of airspace (m2 C/W)

si 1 2 a so

1U =

R + R + R + ..... + R + R


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Adjustments to U-values Typically we might have a 1930s as built cavity wall

We can calculate its existing U-value as described already,

perhaps 1.6 W/m K

What if we wanted to thermally upgrade the wall? What insulation would be needed to upgrade the U-value to a

target of say 0.3 W/m K

Calculation process is simply the reverse of what we have done

already

Documents

1 - Thermal Effects in Buildings