Energy efficiency terms explained

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P

Passive House

A very comfortable and healthy house that needs very little energy.

Quantitative performance benchmarks include:

heating energy demand below 15 kWh per year or
heating load below 10 W per square metre treated floor area
airtightness below 0.6 air changes per hour at 50 Pascal pressure differential
primary energy demand requirements depending on certification class

PHPP

PHPP is the Passive House Projecting Package, a spreadsheet based tool for planning and optimising highly energy efficient buildings. It can be obtained from the Passivhaus Institut in Germany and various national bodies.

Primary Energy

Primary energy is energy contained in raw fuels or other naturally occurring forms of energy before it undergoes transformation or is put to a use. It is a measure of the energy potential of a source, what could be used if conditions were ideal - which they are however not, for the largest part. Electricity derived from fossil fuels e.g. loses around 2/3 of its energy potential in the transformation process. In other words: it needs roughly 3 parts fossil fuel to generate one part of electric energy.
After transformation, energy is delivered to consumers as consumer or final energy, with additional losses due to transportation. Primary energy can be re-calculated from consumer or usable energy with the help of primary energy factors, if these are known. PHPP contains a list of primary energy factors for non-renewable energy services. The new certification categories are among other things concerned about renewable primary energy.

Psi-value

Coupled 2D thermal bridge coefficient, used to gauge the numeric impact of thermal bridges. Ψ, upper case psi (English: sigh), is the most commonly used symbol. The unit for psi is W/(m K). It applies to a length, i.e. the length of the thermal bridge. It denotes the thermal conductivity of an assembly of materials. The higher the value, the higher the additional heat loss through that joint.
When external dimensions are used to assess the heat loss through the thermal envelope, psi values can be negative (=calculatory heat gain), as with external dimensions convex corners are considered twice (or even thrice, if assessment is made in 3 dimensions). When internal dimensions are used for heat loss calculations (like it is done by default in NZ), every convex corner has to be analysed for thermal bridging effects. Using external dimensions overestimates heat loss, and thus compensates for some geometrical thermal bridge effects. Thus, when external dimensions are used, an assessment of thermal bridge effects at convex corners is usually not necessary.