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The Annual Gain Factor value is measured in kWh/m2. The AGF-values describe the amount of solar gains which are available through windows in a specific climate. The gains are dependent on the orientation of the windows, thus for each climate location there are nine AGF values provided, one for each of the major compass orientations and one AGF-value for horizontal windows (skylights). A different set of nine AGF-values exists for each of the four heating schedules because the time of day of the gains has to match the time when heating is required, including some acceptable delay to take account of thermal storage effects. An acceptable delay of five hours between gains and heat requirements is implicitly included in the AGF-values.


The Annual Loss Factor value is measured in hour x °C x 1000. It is used to calculate the thermal heat losses of the building, as well as the Warm-up Energy. An ALF-value is a single factor which describes the severity of the climate and the energy demand for heating. One ALF-value exists for each climate, heating schedule and heating level. In simplified terms the ALF value is the temperature difference between the interior temperature setting and the outside ambient temperature accumulated over the heating season. The larger the ALF value the colder the climate is. Longer daily Heating Schedules (i.e. Heating Schedule 3 or 4) and higher Heating Levels also increase the ALF-value. The ALF-value implicitly takes account of ambient temperatures, the length of the winter season and the effect of solar radiation on heat loss. The energy lost through a particular building component is calculated by multiplying the Specific Heat Loss with the ALF-value.

Air Leakage Rate

The Air Leakage Rate is defined in Air Changes per Hour (ac/h) or how often the total volume of air in the building is changed per hour. The number of air changes per hour are dependent on how airtight or leaky the building is. The Local Air Leakage Rate includes the environmental factors of Air Leakage Zone and Site Exposure. Common Air Leakage Rates range between 0.3 and 1.5 ac/h.

Air Leakage Zone Factor

A factor taking account of the average wind speeds in the selected climate zone. It is automatically selected by selecting the location of the building.

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The Building Performance Index is an index describing the heating energy efficiency of a building. The smaller the BPI, the smaller the heating requirement is. As of October 2007 there are separate BPI calculations for Climate Zones 1 & 2 and Climate Zone 3. More information about BPI is available from the help section.

Building Thermal Envelope

The roof, wall, glazing, and the floor construction between unconditioned external spaces and conditioned spaces, enclosing all habitable spaces, bathrooms, kitchens and other rooms in the building likely to require heating for occupant comfort.

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Climate Zone

NZS 4218:1996 'Energy Efficiency - Housing and Small Building Envelope' separates New Zealand into three climate zones according to the severity of the climate. The allocation is approximately: Zone 1 for the north of the North Island, Zone 2 for the rest of the North Island excluding the Central Plateau and Zone 3 for the central plateau and the whole of the South Island.

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Degree Days

[DD] Measure of the severity of the climate. The larger the number of degree days the cooler the climate. The number of the degree days is defined in relation to a base temperature. The base temperature is commonly either 15°C or 18°C. The number of heating degree days is calculated as the sum of the daily differences between the average ambient temperature and the base temperature for all the days on which the average ambient temperature is smaller than the base temperature. Example: The heating degree days (base 15°C) of three days with ambient temperatures 11°C, 9°C and 16°C is: (15-11) + (15-9) + 0 = 10DD15

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Effective Thermal Mass

Thermal mass which contributes to heat storage processes. The effectiveness of how much heat is absorbed and later released by the Thermal Mass depends on the heating schedule and on the total amount of the Thermal Mass. If for example a building has an exposed slab-on-ground floor only the top 200 mm actually take part in daily heat flow processes. The Effective Thermal Mass is defined in a way to allow the Warm-up Energy to be calculated by multiplying the Effective Thermal Mass with the ALF-value.

Effective Thermal Mass Density

Effective Thermal Mass divided by the total Floor Area.

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Gain Load Ratio

Ratio between the Total Heat Load and the Total Heat Gain.

Ground Floor Area

The area of the floor on the ground or footprint of the building. It does not include the area of internal floors between different storeys of the building.

Ground R-value

The R-value of the ground underneath the floor including the concrete slab itself. The total R-value of a slab floor is a combination of the R-value of the slab and ground underneath the slab, the insulation and any floor coverings. The R-value of the ground underneath the slab is generally much higher than the R-value of the slab itself, so they are combined and defined as the Ground R-value. The Ground R-value depends on the complexity of the floor shape (Area to Perimeter Ratio), the soil conductivity and the thickness of external walls.

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Heater Efficiency

The heater efficiency is not explicitly taken account of in the ALF calculations. ALF calculates the amount of heat which is necessary to achieve the specified heating schedule and level. The amount of purchased energy can be calculated by dividing the heating energy by the heater efficiency. If the house is for example heated with a heat pump with an efficiency of 200% then the purchased energy is half the heating energy.

Heating Level

The heating setpoint to which the house is heated during the periods defined in the Heating Schedules. During the other times no heating is applied and the temperatures are floating. ALF allows three different heating setpoints: 16°C, 18°C and 20°C. Other Heating Levels can be interpolated if required. Most New Zealand houses are heated to comparatively low levels, i.e. 16°C or 18°C.

Heating Schedule

The heating schedule describes during what times of day the house is heated to the selected Heating Level. ALF allows four different heating schedules: i.e.

  • Evening only (5:00 pm - 11:00 pm),
  • Morning and evening (7:00 am - 9:00 am & 5:00 pm - 11:00 pm),
  • Morning to evening (7:00 am - 11:00 pm) and
  • Continuous (24 hours).

During the other times of day the house is not heated and the temperatures are floating.

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Internal Gain Factor

The internal gain factor is used to calculate the internal gains for the heating season for each of the four heating schedules. The Total Internal Heat (in Watts) describes the average power released through the internal gain contributors (people, appliances and hot water). The Internal Gains (energy) depends on how long this power was released, i.e. it depends on the number of hours each day (Heating Schedule) and on the length of the heating season. Both are taken account of by the Internal Gain Factor.

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Kilowatt Hour; 1000 Watt Hours (Wh): Unit of energy. Examples:

1. A 1000 Watt heater which runs for an hour consumes 1000 Watt Hours or 1 kWh.

2. A 200 Watt light bulb which is on for 10 hours consumes 200*10 = 2000 Watt Hours = 2 kWh.

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Local Air Leakage Rate

The Local Air Leakage Rate is calculated by multiplying the Air Leakage Rate, related to a building design, by the environmental factors for Air Leakage Zone and Site Exposure.

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Optimisation Target Cell

The cell on the 'Design' pages for which the optimum value is determined. This is the cell which is selected at the time when the 'Optimise Current Cell…' menu item is clicked.

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Perimeter Wall

Wall around the perimeter of the ground floor in houses with suspended floors. The Perimeter Wall is generally constructed of concrete or weatherboard. Average heights are between 300 and 600mm.

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Measure of the thermal resistance of a building component. The R-value is measured in m2 °C / W. Example: If a wall has an R-value of 2.0 m2 °C / W and a temperature difference of 5°C exists between opposite sides of the wall (20°C inside, 15°C outside). In one hour energy of 5/2 Wh = 2.5 Wh = 0.0025 kWh flows through each m2 of the wall.

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The Solar Heat Gain Coefficient is an index describing the proportion of light which is let through a window. The larger the SHGC the more light can enter through the window. SHGCs range from approximately 0.70 for single clear glass to 0.31 for low emissivity glass (toned or coated glass).


The shading defines how much light is entering through a window in respect to an unshaded window. A window which is shaded to 20% by trees or other obstacles has a shading of 20%. Keep in mind to consider the sun path primarily during the winter heating months. The window orientation is automatically taken account of by the Annual Gain Factors. An unshaded South facing window thus has 0% shading.

Shading Coefficient

The shading coefficient describes what percentage of solar radiation is allowed to pass through a window. The HIGHER the Shading Coefficient the MORE solar radiation can enter through the window. The Shading Coefficient is defined in respect to a single clear 3mm glass: Shading Coefficient Type X = SHGC Type X /SHGC 3mm clear single. Example: A single glazed 3mm clear glass window has a Shading Coefficient of 0.70/0.70 = 1.0, a double glazed tinted window has a Shading Coefficient of 0.36/0.70 = 0.51.

Site Exposure

The site exposure is a factor related to the wind shelter of the building. It is a local environmental factor used to calculate the Local Air Leakage.

Slab Floor

Concrete slab on ground floor type.

Specific Heat

The ability of materials to store heat. The Specific Heat is measured in Wh/m2 °C and describes how much energy is stored in one m2 of the particular material for each °C it is warmed. The Specific Heat is used to calculate the Thermal Mass of the building.

Specific Heat Loss

Specific heat loss is measured in W/°C and describes the thermal performance of a complete component whereas the U-value and R-value describe the thermal performance of one m2 of the component. The specific heat loss is generally calculated using R-values and component areas: Specific Heat Loss = Area / R-value. The Specific Heat Loss is independent of the climate and describes solely the building performance. In order to calculate the heat loss (energy) the specific heat loss is multiplied with the ALF-value.

Specific Heat Loss Density

Specific Heat Loss divided by the Total Floor Area.

Suspended Floor

Usually a timber floor which is suspended above the floor sitting on posts and floor joists. Suspended floors may have perimeter walls sheltering the crawl-space from wind.

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Total Heat Gain

The sum of internal gains from metabolic heat, hot water heat and appliance and lighting heat gains and the solar heat gains.

Total Heat Load

The sum of heat losses from conductance through walls, floors, roofs and windows, air leakage losses and the Warm-up Energy.

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Measure of the thermal conductance. The U-value is the inverse of the R-value: U = 1/R. The U-value is commonly used to describe the thermal performance of windows.

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Watt. Measure of the heat flow rate and power. The power multiplied with the period for which the power was used is the energy (see kWh).

Warm-up Energy

Energy required to warm-up the house after being allowed to cool. This energy is only required for intermittent heating schedules (Schedule 1, 2 or 3). The energy depends on the amount of thermal mass present in the building, the climate and the heating schedules and levels.

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