Energy efficiency terms explained

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.

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.

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.