To maintain a constant comfortable internal temperature known as the Steady State, building should achieve Thermal Balance.
(Balancing Heat Loss and Heat Gain)
- 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
A building envelope has any potential gaps. Warm air can leave the fabric or cold air can penetrate from outside.
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
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
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
- 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.
- 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
- 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)
- 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
Quilt: In the form of Batts, Rolls, or Blankets. These are flexible products made rom glass, natural fibres or mineral fibres.
Blown: Loose-fill insulation includes loose fibres or pellets that are blown into building cavities using special pneumatic equipment.
Rigid: Fibrous materials or plastic foams that re pressed or extruded into board-like forms and moulded pipe-coverings.
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