The BPI calculation​

The BPI is a crucial measure of heating energy efficiency in houses.

New Zealand Building Code clause H1 - Energy efficiency (4th edition, amendment 3, 1 January 2017) states that: "Buildings must be constructed to ensure that their building performance index does not exceed 1.55." (H1.3.2E). Because the BPI is divided by a climate severity factor (degree months) it is largely independent of climate. The BPI applies only to houses.

The Verification Method H1/VM1 states that ALF calculates the BPI for detached dwellings. ALF’s thermal performance calculations can be used to demonstrate that a house design complies with H1, and is another method to the schedule and calculation methods in NZS 4218:2009 Thermal insulation – Housing and small buildings.
H1 explains the BPI calculation as: "The heating energy of the building divided by the product of the heating degrees total (the minimum of 12 and the degree months to a base of 14°C) and the sum of the floor area and the total wall area."

The BPI requirement in its complete form is: 1.55 kWh/(m².°C.month).

For example, assume ALF calculates that a Tauranga house of 150m² with 120m² walls and a degree months (base 14°C) total of 15 requires heating energy of 5750 kWh per year.

The BPI = 5750 / (150 +120) x 15 = 1.42. This house complies because 1.42 is below 1.55.

To put the calculation in more technical form:

\begin{equation} BPI=\frac{Net Heating Energy Use [kWh]}{Degree Months(base14ºC)[DM_{14}] \times Total Floor Area [m^2] \\ + External Wall Area [m^2] + External Window Area [m^2]} \end{equation}

A number of standard conditions apply to maintain the building at a constant internal temperature for the BPI heating energy calculation. These are:

  • A continuous temperature of 20°C throughout the building;
  • An air change rate of 1 change per hour or the actual air leakage rate, whichever is greater;
  • A heat emission contribution arising from internal heat sources of 1000 kWh for the first 50 m² of floor area, and 10 kWh for every additional square metre of floor area;
  • No allowance for carpets on floors or curtains/blinds on windows;
  • Windows to have a shading coefficient of 0.6 (made up of 0.8 for windows and recesses and 0.75 for site shading);
  • Occupant number assumptions based on total floor area:
    • <120m² – 3 occupants;
    • 120 m² to <180m² – 4 occupants;
    • >180m² – 5 occupants.

ALF applies these conditions automatically when calculating the BPI.

Even when ALF calculates a BPI of 1.55 or less for a design, the minimum R-values required by New Zealand Building Code clause E3 - Internal moisture must also be met.