Energy efficiency terms explained

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Not diabatic, occurring without gain or loss of heat.
For thermal bridge modelling, the cut-off planes of the model are assumed to be adiabatic. This can be controlled by checking the isotherms of the model: if they are perpendicular to the cut-off plane some centimetres before they meet the cut-off plane, no heat loss will occur through the cut-off plane.


An=not iso=equal tropic=directed.

Not isotropic; having different physical properties in different directions.

Used for materials where the thermal conductivity varies depending on the direction of heat flow; for example timber has an approximately 2.2 times higher thermal conductivity, when the heat flow is in line with the grain, than when it is perpendicular to the grain. End grain boards used as flooring will thus have a significantly lower thermal resistance than for example plywood boards, even when exactly the same timber is used. While most building materials can be viewed as isotropic, i.e. having a thermal conductivity not depending on direction, building materials other than wood may behave anisotropic as well, for example vertical coring lightweight bricks, or certain forms of reinforced concrete. When a material is not uniform, or extensively structured, anisotropy is likely.



The chi-value applies to a point thermal bridge. This can be a bolt or fastener - anything with not much of an extension in two of the three dimensions. Greek symbol χ. Unit: W/K. A single point thermal bridges can be neglected for the energy balance, but recurring point thermal bridges, as they occur e.g. in curtain wall facades, need to be accounted for.



Emissivity, symbol ε, is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature, which corresponds to a value of 1. Surfaces with a modest emissivity emit little, those with high emissivity radiate much energy. Emission levels of common construction materials vary from 0.80 to 0.95. An exception are coated glasses and polished metal surfaces, whose values may be considerably lower. If the emission level of a building material is unknown, 0.90 can be assumed.


Final Energy

Energy supplied to consumers, where it can be converted into useful energy, e.g. electricity into light or warmth, using respective devices.

Final energy is derived from primary energy, usually taking in transformation and distribution loss in the process.



The g-value gives the total solar energy transmittance, i.e. the fraction of solar energy (direct and indirect) that enters the building through a transparent element. The direct gains equal the total short wave transmissivity of the element. This fraction is zero with opaque elements. Indirect gains are obtained via absorption of solar energy, which is then radiated as heat. This fraction is > zero with both, opaque and transparent building elements. In steady state calculations, the g-value is usually only used with transparent elements, nevertheless. As input value for PHPP, the g-value needs to be assessed using EN 410.

The solar heat gain coefficient (SHGC) used in the United States and some other countries, for example New Zealand, describes the same physical property, but is assessed using deviating boundary conditions. Thus, g-value and SHGC can not be used interchangeably.

Glazing for Passive Houses

Physical properties for window glazing of interest for planning Passive Houses are total solar energy transmittance and U-value, predominantly.
NFRC 100 and 200 are the standards used in the United States and some other countries, while in Europe and some other parts of the world EN 410 and EN 673 standards are used to assess these values.
Using NFRC standards will result in U and g-values that differ significantly from those derived from the relevant European standards.
Certification criteria for Passive Houses require the use of EN 410 ( g-value) and EN 673 ( U-value) as assessment standards, and PHPP results may be inaccurate, when NFRC values are used instead.
Total solar energy transmittance ( g-value, Europe) or solar heat gain coefficient (SHGC, USA) give the proportion of solar energy that passes through the glazing.
Ug for glazing U-value gives the thermal transmittance of the glazing, in other words: the amount of heating energy that can escape through a square meter of this glazing unit. This is however determined at the centre of glass; with multi-pane units the influence of the material of the edge spacer can significantly add to the heat loss for the glazing unit.

Gross density

The formal definition of density is mass per unit volume. In some contexts the density is expressed in grams per mL or cc. In the building sector, kg/m3 is more commonly used. Mathematically a "per" statement is translated as a division. Density = Mass/Volume, in the building sector most commonly expressed in kg/m3.



Heat is the least "noble" energy form. According to the second law of thermodynamics, heat cannot be transformed into higher forms of energy - like mechanical or electrical energy - without a loss, whereas the reciprocal process can happen loss free. While heat is a scalar and thus undirected, heat transfer always occurs from warm to cold, and never the other way around, unless work is added. As heat is at the end of a chain of energy transformations, it can be the tip of an iceberg of energy usage. Using "nobler" forms of energy to generate heat, e.g. using electricity to generate space heating, should be minimised, and waste heat from mechanical or electrical processes should be utilised to preserve energy sources.

Heat flow

Heat flow describes heat per unit of time in J/s or W; it is also a measure of power. Heat flow is a scalar, therefore not directed and not affected by changes of the co-ordinate system.

Applied to a building element, however, heat flow turns into a heat flow rate in W/m². It looses is undirectedness, becoming a vector. The heat flow rate is proportional to the temperature difference at both ends of the system, for example at building element surfaces.

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