Building Code compliance
A key feature of ALF is its ability to demonstrate that a house design meets the thermal performance requirements of the New Zealand Building Code.
The New Zealand Building Code sets out how a house has to perform, but not how it has to be designed or built. For example, Building Code clause H1 Energy efficiency states that "The building envelope enclosing spaces where the temperature or humidity (or both) are modified must be constructed to (a) provide adequate thermal resistance and (b) limit uncontrollable airflow".
Most Building Code clauses have Acceptable Solutions or Verification Methods that describe how the performance requirements can be met. These are not mandatory but, if they are carefully followed, they provide a means of compliance with the Building Code that building consent authorities must accept.
The Verification Method H1/VM1 states that ALF can be used to calculate the Building Performance Index (BPI) of a house design. If a design has a BPI of 1.55 or less, then it complies with clause H1. An ALF report setting out the details of a design and demonstrating its compliance can form part of a building consent application.
Using ALF to calculate the BPI of a design is not the only way of demonstrating compliance with H1. The schedule method, calculation method and modelling method set out in NZS 4218:2009 Thermal insulation – Housing and small buildings can be used to demonstrate compliance.
The schedule method sets minimum R-values for the roof, walls, floor and glazing. The total area of glazing using this method must be less than 30% of the total wall area and the combined area of glazing on the south, east and west walls must be no more than 30% of the combined area of these walls. Decorative glazing and louvres can be no more than 3 m².
The calculation method is more flexible. It calculates heat losses to allow a reduction of R-values in some parts of the envelope to be compensated for by an increase in R-values in other parts. The calculation method can be used where glazing is equal to 50% or less of the total wall area.
Where these two methods cannot be used, the modelling method uses computer simulation to calculate the energy use of a building.
There are some cases where another method may be more appropriate than ALF. For example, ALF's thermal mass calculations do not yet factor in the orientation or placement of thermal mass. If thermal mass is a significant component of a design, designers may consider using other tools under the modelling method from NZS 4218:2009. For apartment buildings and medium-density housing, modelling methods should be used.