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Energy Balance

To maintain a constant comfortable internal temperature known as the Steady State,  building should achieve Thermal Balance.

(Balancing Heat Loss and Heat Gain)

Heat Loss:

  • The major heat loss channels from a building are ventialtion, and transmission infiltration through e construction fabric
  • Loss of warm, moist air by: Ventilation through ducts & flues, ventilation through gaps at doors and windows frames
  • Transmission heat loss via: walls, floors, roofs, windows & doors



Energy Balance:


A building envelope has any potential gaps. Warm air can leave the fabric or cold air can penetrate from outside.


Energy Balance: 

Heat Loss

  • Conduction Loss through the building is measured as a function of the material's conductivity
  • Convection Losses on both the inside and outside surfaces of any building layer is measured as the surface resistance
  • Radiation heat loss is measured as the energy transmission at the surface resistance of an object


Energy Balance:

Heat Gain

The major heat gains channels into a building are solar gain, incidental gains and heating inputs

  • Sensible and Latent Gains by: Direct solar gain, Appliances, Lighting and Heating, people
  • Transmission Heat Gain via: Walls, Roofs, Windows and Doors due to ambient air temperature


Energy Balance:

Solar and Metabolic Gains

  • Solar Gain: It is possible to predict passive solar gain by referring to Solar Flux values for your geographical region. (Solar Gain = 0.52 x Window Area x Solar Flux)
  • Metabolic Gain: If a building is continually occupied, heat gain can be generated by users; this is referred to as Metabolic Gain


Thermal Analysis:


  • Lack of precision in the way buildings are made mean that we must look to robust benchmarking methods to assess how thermally efficient buildings are
  • one method is U-Value Calculation
  • U-Value: establishes the rate of heat loss through walls, roofs and floors using the thermal conductance of building elements.


Thermal Analysis:


  • The conductivity of a material is expressed as its K-Value
  • It is determined by a calculation of the amount of heat transmitted through 1m2 of the surface of a material that can cause a temperature change of 1ºC from one side of the material to the other
  • The lower the K-Value for a material, the better it insulates
  • K-Values are predetermined and an be found in reference tables or manufacturers data, Units are W/mK


Thermal analysis:


  • The Resistivity of a material is expressed as its r-Value
  • Resistivity is the ability of a building material to resist the flow of heat. It is the inverse of conductivity and is expressed as the reciprocal of conductivity
  • The formula for calculating Resistivity is: r = 1/K
  • r = The Thermal Resistance per unit area of a piece of material (m2K/W)
  • K = represents the conductivity of the material (W/mK)


Thermal Analysis:


  • The resistance of a material is expressed as an R-Value, it is a measurement of a building materials resistance to heat flow
  • The higher the R-Value, he greater the insulation
  • Resistance is calculated as the inverse of Conductivity x the actual thickness or depth of the material
  • R = L x r  
  • R = Resistance measured in m2K/W
  • L = Thickness of material (always in meters)
  • r = Resistivity of a material in mK/W


Thermal Analysis:

Insulation Types

  1. Quilt: In the form of Batts, Rolls, or Blankets. These are flexible products made rom glass, natural fibres or mineral fibres.
  2. Blown: Loose-fill insulation includes loose fibres or pellets that are blown into building cavities using special pneumatic equipment.
  3. Rigid: Fibrous materials or plastic foams that re pressed or extruded into board-like forms and moulded pipe-coverings.
  4. Reflective Foil: Films fabricated from Aluminium foils with a variety of backings e.g. strong paper, plastic film, Polyethene bubbles or cardboard


Measuring Thermal Performance

  • Housing in Scotland governed by the Building Standards
  • They require all new dwellings to meet a set target for carbon dioxide emissions known as the Target Emission Rate (TER). This includes calculating for heat loss and the efficiency of heating systems.
  • To avoid excessive heat loss the following U-Values need to be achieved: External Walls (0.19 W/m2K), Floors (0.15 W/m2K), Roofs (0.13 W/m2K), Windows and Doors (1.50 W/m2K)