Using ALF for existing houses

ALF can be used to assess existing houses and improve their thermal performance, but some adjustments may be necessary.

Although ALF is typically used at the design stage for new homes, ALF can also be used for existing homes. The performance of an existing home can be determined using the Building Performance Index (BPI) or the overall heating requirements for the design. The potential energy use for different upgrades - such as to glazing systems - can be measured. Designers can use the tool to find the best solution from a range of options and demonstrate these benefits to clients.

Many building techniques used in the past no longer meet current standards or the New Zealand Building Code and are not included in ALF today. If a building element construction type in an existing house differs from the options offered in ALF, select the ALF option that most closely matches the construction R-value. Make sure that the option you choose does does not exceed the R-value of the existing element.

In several areas, ALF offers a Custom R-value option. In these cases, the overall construction R-values can be entered. Current or past editions of the BRANZ House Insulation Guide may be useful to help find appropriate figures. The current version is the House Insulation Guide, fifth edition, July 2014.

Installed insulation R-values may change over time. Wall insulation may slump, ceiling insulation can be moved by wind or be compressed, foil under a suspended timber floor may be dusty, torn or missing, which reduces their effectiveness. To give a more accurate calculation for existing construction you may reduce the R-value used in the calculation to better reflect actual values and see how this will affect the energy use of the building. However, if you are planning on using ALF as part of consent application, you will need to use the full design R-value for all new construction.

Selecting the appropriate airtightness classification under the Ventilation tab is important for houses of different ages. A house that has not been lined with plasterboard and still has its original strip timber floors and timber windows will be draughty, while a more recent house is more likely to be regarded as airtight. Under the Ventilation tab there is also an option to consider past retrofits that may have changed the airtightness of a house.

Fill in all the required boxes just as with a normal ALF calculation. If areas of the calculator are ignored it will affect the results. Where boxes ask for data that does not apply, leave them blank – do not put zeros in them.

As with a typical ALF calculation, different design options can be entered to determine which offer the best potential improvement in thermal performance. For existing houses these are likely to include glazing alternatives and adding to insulation where that can easily be accessed (such as in a roof space or under a suspended floor).

Some options that ALF offers, such as rotating the house to examine the impact of different building orientations, are unlikely to apply to existing houses and can be left alone.

ALF can also be used to assess potential improvements that will reduce the likelihood of mould forming in a house.