ALF calculation of heat loss from ventilation.

Ventilation and airtightness

The heat loss due to ventilation can be calculated for passively ventilated homes, which can include mechanical extraction in the bathroom and kitchen. ALF cannot calculate the energy loss for whole house mechanical ventilation systems.

Details entered on the Ventilation page are used to determine the air leakage heat loss of the building. ALF starts with a basic measure of airtightness and then applies correction factors based on building features. The air leakage rate is then adjusted taking account of site exposure and windiness of the climate to give the local air leakage rate.

Airtightness and air leakage are measured in air changes per hour (ac/h).

The air leakage component of building heat loss depends on the airtightness of the building and whether occupants open windows or use other types of ventilation. These factors are unknown at the early design stage, so assumptions are made about air leakage and occupant behaviour.

General construction changes have resulted in homes becoming more airtight over time. A first approximation to the airtightness of a home depends on the year of construction of the home. Changes over time have included using less strip materials and more sheet materials, using windows that are more airtight, square stopping and airtightness barriers in the wall assemblies, and since the mid-2000's, greater airtightness detailing in New Zealand Building Code clause E2 - External moisture.

In addition to the age of construction, the airtightness can be influenced by whether the design of the house is "simple" or "complex" from an airtightness point of view.

A "simple" house will have a squarish boxy shape with few external joints and a moderate number of windows and doors.

A "complex" house will have a more elaborate shape with more windows and doors.

Use the table below to determine an airtightness level for the design being examined. Note the names of the airtightness levels are the same as those used for previous versions of ALF (with an addition) from the early 1990's and so "average" does not reflect average airtightness achieved today but "average" for the 1990's. A new increased airtightness level of "modern" has been introduced with a base air-infiltration rate of 0.15 ac/h.


Year of construction Complexity of design Common features Base airtightness rate (ac/h)

Base airtightness level

2005+ Simple Square stopped linings, E2 airseals, airtightness layers 0.15 Modern
2005+ Complex Square stopped linings, E2 airseals, airtightness layers 0.25 Airtight
1980-2005 Simple Aluminium joinery, sheet linings 0.25 Airtight
1980-2005 Complex Aluminium joinery, sheet linings 0.50 Average
1960-1980 Simple Some strip linings 0.50 Average
1960-1980 Complex Some strip linings 0.75 Leaky
-1960 All High ceilings, strip floors, strip linings 1.00 Draughty


These examples are only guidelines. If more detailed information about airtightness is known, the value closest to the true value should be selected irrespective of building age and design.

An appropriately ventilated home has a local air leakage rate of between 0.35 - 0.50 ac/h. A value below this is overly airtight and will require additional ventilation. A value greater then this may result in excessive heat being lost in the air leaving the home.

Open fires

Each fire will add 40 m3/h to the air leakage rate.

Metal-flued heaters

Each heater with a flue restrictor will add 20 m3/h to the air leakage rate.

Retrofit air tightening

If the house is draughty (typically built before 1960) but it has been retrofitted with sheet lining and is therefore more airtight, 0.1 ac/h are subtracted from the air leakage rate.

If the house is draughty but the windows were replaced with airtight aluminium windows or the air-leakiness of old timber windows was reduced, 0.2 ac/h are subtracted from the air leakage rate.

Kitchen and bathroom vents

These are used to calculate the contribution of targeted mechanical ventilation to the air leakage rate.

Air leakage rate

The air leakage rate describes how airtight the building is. It is confined to leaks through cracks which the occupant has no control over. This is the lowest possible ventilation rate. It does not take account of local wind exposure and climatic conditions. Buildings on Wellington hilltops and sitting between Kerikeri shelter belts can have the same air leakage rate.

Local air leakage rate

When the air leakage rate is adjusted for site exposure (wind exposure) and the air leakage zone, it becomes the local air leakage rate.

Site exposure reflects the extent of wind shielding by local terrain and buildings. One of four wind exposure classes can be assigned to a house, from sheltered to exposed. If one side of the house is more exposed, choose the exposure class that relates to the prevailing wind.

The air leakage zone takes account of the windiness of the climate. ALF automatically selects the air leakage zone when the building location is entered on the Climate and heating page. New Zealand has four air leakage zones reflecting regional variations in average wind speed, weighted to reflect the extent that they drive infiltration.

For the local air leakage rate, ALF applies the larger of 0.5 ac/h or the estimated infiltration rate for heat loss calculations. A lower rate would make the building feel 'stuffy' and occupants would open windows to increase the air flow.

The local air leakage rate is multiplied by the house volume and the specific heat capacity of air. The result is the specific infiltration heat loss. Multiplying the specific infiltration heat loss by the ALF value results in the infiltration heat loss in kWh.