A guide to Thermal Mass and Energy Efficiency in Buildings by Bruce Spenser MSc MCIOB

THERMAL MASS AND ENERGY EFFICIENCY IN BUILDINGS - Bruce Spenser MSc MCIOB

Abstract:         The built environment uses 50% of the UK’s energy, 90% of which comes from fossil fuels.  The reduction of emissions  from buildings can provide high proportionate savings and legislation has been enacted to this effect.  The physics of thermal energy is explained and how these physical properties can be harnessed as tools by the designer to conserve energy utilising Thermal mass.  Glazing is explained as a modulator of thermal mass.  Savings from the use of thermal mass is reviewed. 

Key words:  Thermal Mass, sustainable construction, U Values, Renewables, specific heat capacity, emissivity, thermal conductivity, thermal resistance, surface resistance, Phase change products. 

Background: The earth is warming because of energy production by burning fossil fuels.  If this continued it is predicted that the concentration of greenhouse gasses will double by 2035.  (Stern, /Blundell/King). 

The world summits commenced in Stockholm in 1972.  The Kyoto protocol agreed methods to tackle the problem in 2005. 

The UK is committed to a 12.5% reduction of greenhouse gasses from 2008-2012.  It has set itself a voluntary reduction target of 20% by 2010 and a long term aim of 60% by 2050.  Each sector of the UK economy will be required to reduce their CO2 emissions. 

50% of the UK’s energy is used in the built environment; 29% in dwellings and 53% of that is used to heat the dwellings. 

The emissions of the built environment will be reduced by regulation.  Part L of the building regulations has been framed to reduce current emission by 20% within new builds.  This is achieved by a notional dwelling emission rate (DER) being reduced by 20% which is the Target emission rate (TER) the new building must meet.  SAP and SAP2005 are the procedures and methodologies for rating and demonstrating compliance.   (BREDEM (Anderson et al 1985).   These standards require:

·         High efficiency boilers with modern controls

·         Insulation of cavity walls, lofts and cylinders

·         Modern compact fluorescent lights

·         Accredited construction details

·         Draught proofing

·         Maximum glazing dependent on floor area

·         These standards reward the use of renewable energies: 

·         Wind

·         Solar Panels

·         Photo Voltaic Cells

·         Ground Source Heat Pumps

·         Local level energy production – distributed or decentralised – Microgeneration - District heating – CHP –Biomass - Energy crops – willow coppice 

Legislation will encourage their10% growth as a proportion of supply by 2010 and to twice this by 2020. Within the standards freedom is given to designers to reduce co2 emissions utilising the physical characteristics of heat. 

The EEC made EPCs mandatory via Directive 202/91/EC - Energy Performance of Buildings subsequently clarified by Directive 2010/31/EU 

The Physical Characteristics of Heat: 

The force known as cohesion holds materials together.  Brownian motion describes the molecular movement within materials.   This movement is Kinetic energy. As energy is applied to a material potential energy will be added.  These total energies, kinetic and potential are known as heat or thermal energy. 

The quantity of thermal energy alters the position of the molecules and their distance apart and determines the phase of their mass: solid, liquid, vapour or gas. 

Temperature is the measurement of heat.  The Celsius or Centigrade scale is gradated on the melting point of water (0) and its boiling point (100) – the Kelvin scale uses absolute zero as its zero and uses the same Celsius unit. 

Thermal energy is transferred by conduction, convection and radiation. 

Conduction is the transference of heat from hot to cold until equilibrium is reached. 

Convection is the movement of heat (thermal energy) within a material which is free to travel such as a liquid or a gas. 

Radiation is heat transference by electromagnetic waves.  This does not require a transfer medium and is effective over long distances. 

The sun radiates heat which is transmitted, reflected or absorbed by terrestial materials.  Some of this absorbed heat is radiated out.    The radiated heat of the sun is in the spectrum of ultra-violet,  visible and infra-red which  known as  short wave radiant energy.  The radiated heat of the terrestial materials is in the far infra red and is known as longwave radiant energy. A wavelength is measured in namometers which is .000000001 Meter.

Emissivity - Materials have different capacities to absorb or release heat which is known as their specific heat capacity (j/Kg).  The release of this energy is known as emissivity. 

The thermal conductivity (λ)of a material is the amount of energy in watts which is conducted through 1M2 with a temperature difference of 1°K

A material’s thermal resistance (R) is its resistance to the passage of heat.  It is determined by dividing its thickness in meters by its thermal conductivity.

Its thermal capacity is its capacity to hold heat and is the product of its density and specific heat capacity.

A u value is the reciprocal of the amalgam of the thermal resistances within a structure.  It is used as an insulant measurement scale.

The surface resistance is dependent on emissivity, orientation, exposure etc.

Standard values for surface resistances are

·         Rs1 (inside) .12 m2 K/W

·         Rso (Outside) .06

·         Ra – Air space of cavity 

Evaporation occurs when a liquid releases particles (affected by Brownian motion) as a vapour.  Evaporation is speeded by an increase of the surface area or temperature, wind across the surface or reduced humidity.  Evaporation removes thermal energy and thus has a cooling effect on the material.

Condensation is the release of water vapour by air cooled below the dew point.

The use of the physical characteristics of heat: 

The thermal mass of materials can be used to conserve thermal energy:

Excess energy from solar heat and internal energy sources can be stored by selecting and distributing materials with the highest Thermal capacity to promote an even distribution of heat.

This stored energy can harmonize external temperature fluctuations.

This stored energy can be retained and released as required reducing heating and cooling requirements.

·         Utilising Phase Change Materials (PCMs) to:

o   “store heat energy in a latent, as well as sensible fashion, leading to greater heat storage capacity per unit volume than that of conventional materials” ( Kelly)

·         Utilising PCMs  with a high thermal capacity such as Dupont`s energain (Parafin wax) when refurbishing low mass structures

·         Orientation of the dwelling to maximise solar gain

·         Proper sizing and locating of building overhangs and shading elements to avoid overheating in the summer

·         Correct detailing of mass and insulation, i.e. do not insulate the thermal mass from the interior of the dwelling.

·         This heat storage, light and air may be modulated by:

·         Insulation of the external structure

·         Mechanical and manual ventilation

·         Use of energy-efficient glazing and careful window sizing to reduce heating costs in the winter, avoid overheating in summer and promote thermal comfort year round

Glazing: 

Windows are the modulator of heat, light and air.  The window can be used to modulate heat gain and heat loss dependent on the climatic requirements. 

Ordinary glass is a good transmitter of solar radiation (84% of the short wave) and a poor transmitter of terrestrial radiation (long wave).  This property is ideal for cold climates.   

Short wave and long wave radiation may be selectively controlled by treating the glass so that it absorbs, reflects or transmits the short or long wave radiation.  Absorption and reflection is achieved by tinting the glass utilising the spectrum, reflection is achieved by applying coatings known as low emissivity - high-solar-gain low-E coatings for cold climates, and for low-solar-gain low-E coatings for hot climates. 

Absorption can act as a solar store. 

Glass has a thermal conductivity of 1.01 (CBSE Guide).  It is used in thicknesses generally of  4 – 6mm in dwellings.  At 6mm its TC is .0057 which would translate into a u value of  168.  It is its surface resistance which makes it a useful construction material – this gives a 6mm pane of glass a u value of 5.51.  There is no appreciable added insulation value in thickening the glass.  Utilising the resistance of air and suitable gasses, such as  Argon, Krypton and Xenon as a filler between  panes of  glass can make double, triple etc glazing  units excellent insulators.   For example a triple glazed xenon unit (6mm glass) will give a u value of .118.  

Further advances in technology are allowing the, “smart” control of the window as modulator of the internal climate by the use of chromogenic materials.

Thermal energy can be swiftly removed and modulated by creating through ventilation by opening windows on opposite external faces of the building. 

CONCLUSION 

A reduction of CO2 emissions is not the same as a reduction in energy use it is minimising the use of Carbon based energy forms, maximising energy from renewables and minimising heat loss by insulation and thermal storage within the mass of the building and within Phase Change Materials. 

Research shows that utilising thermal mass can reduce C02 emissions from 5 – 20%.  Data from NAHB suggest energy savings of 11% were possible utilising thermal mass as opposed to timber framed light weight construction. (Kosny et al).  

Utilising PCMs can increase these energy savings by 15% in a thermally moderate environment (Kelly 1999). 

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Areas covered

London, East London, North London, West London, South London, E1, E2, E3, E4, E5, E6, E7, E8, E9, E10, E12, E13, E14, E15, E16, N1, N2, N3, N4, N5, N6, N7, N8, N10, N11, N12, N13, N14, N15, N16, N17, N18, N19, N20, N21, N22, N23, N24

Stoke Newington, Highbury, Islington, Shoreditch, Newington Green, Hackney, Tottenham, Green Lanes, Manor House, Turnpike Lane, Enfield, Wood Green, Highgate, Hampstead, Muswell Hill, Hornsea, Arsenal, Finsbury Park, Camden Town, Holloway, Kings Cross, Caledonian Road, Stamford Hill, Clapton Common, Upper Clapton, Lower Clapton, Seven Sisters, Tottenham Hale

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