Air leakage rate

The air leakage rate shows how often the total volume of air in the building is changed each hour. It is measured in air changes per hour (ac/h). The number of air changes per hour depends on how airtight or air-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 that takes account of the average wind speeds in the chosen climate zone. ALF automatically selects this when the location of the building is selected.

AGF-value

Annual gain factor values (AGF-values) are the solar gains available through windows in a specific climate. They are measured in kWh/m. The gains depend on window orientation. For each climate location there are nine AGF values – one for each major compass orientation (north, northeast, etc.) and one for skylights. There is a different set of nine AGF-values for each of the four heating schedules because the time of day of the gains has to match the time when heating is required. AGF-values include a delay of five hours between gains and heat requirements to account for thermal storage effects.

ALF-value

The annual loss factor value describes climate severity and heating energy demand. The larger the ALF value, the colder the climate. The ALF-value calculates thermal heat losses and the warm-up energy required. ALF-value is measured in hours x °C x 1000. One ALF-value exists for each climate, heating schedule and heating level. Longer daily heating schedules and higher heating levels increase the ALF-value. The energy lost through a building component is calculated by multiplying the specific heat loss with the ALF-value.

Read more about how the ALF-value is calculated

​BPI

An index describing the heating energy efficiency of a building. The smaller the BPI, the smaller the heating requirement. Building Code clause H1 Energy efficiency requires houses to be constructed with a BPI that does not exceed 1.55. The Verification Method H1/VM1 states that BRANZ ALF calculates the BPI for detached dwellings. There are separate BPI calculations for Climate Zones 1 and 2 and Climate Zone 3.

Read more about how the BPI is calculated

Building thermal envelope

The roof, walls, glazing and floor between unconditioned external spaces and conditioned spaces. The building thermal envelope encloses all habitable spaces likely to require heating for occupant comfort, including bedrooms, living rooms, bathrooms and kitchens.

Climate zone

NZS 4218:2009 Thermal insulation – Housing and small buildings separates New Zealand into three climate zones according to the severity of the climate. The division is approximately:

• Zone 1 – Thames Coromandel District, Franklin District, and all districts north
• Zone 2 – The rest of the North Island excluding the Central Plateau
• Zone 3 – The Central Plateau, the South Island and Stewart Island.

Construction R-value

The combination of the R-values of the different components of a building element less the effect of any thermal bridging in the framing. In many cases the construction R-value may be lower than the R-value of the insulation.

Degree days

Degree days measures climate severity. The more degree days, the cooler the climate. The number of degree days is defined against a base temperature (commonly 15°C or 18°C). The number of heating degree days is the sum of the daily differences between the average ambient temperature and the base temperature for each day the average ambient temperature is smaller than the base temperature.

For 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

Effective thermal mass

The thermal mass that contributes to heat storage. This depends on the heating schedule and the total amount of thermal mass. When the effective thermal mass is multiplied by the ALF value, the warm-up energy is found.

Effective thermal mass density

Effective thermal mass divided by the total floor area.

Gain-load ratio

Ratio between the total heat gain and the total heat load.

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 under the concrete slab and the concrete slab itself. The R-value of the ground is generally higher than the R-value of the slab. 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. (The total R-value of a slab floor is a combination of the ground R-value together with the insulation and any floor coverings.)

Heater efficiency

ALF calculations do not account for the efficiency of heaters in a house. ALF calculates the amount of heat required to achieve the specified heating schedule and level. Dividing the heating energy by the heater efficiency gives the amount of purchased energy. For example, if the house is heated with a heat pump with an efficiency of 200%, then the purchased energy is half the heating energy.

Heating degrees

In relation to a location and a heating month, means the degrees obtained by subtracting from a base 14°C the mean (calculated using the approved temperature data) of the outdoor temperatures at that location during that month. (Defined term taken from H1 - Energy Efficiency, 4th edition, Amendment 3, Ministry of Business, Innovation and Employment).

Heating level

The heating setpoint in °C which the house is heated to during the time periods defined in the heating schedules. ALF gives a choice of two heating setpoints: 18°C and 20°C. Intermediate heating levels can be interpolated if required. Previous versions of ALF had a heating option of 16°C but this is now considered to be an unhealthy heating level.

Heating schedule

The times of day when the house is heated to the selected heating level. ALF has four heating schedule options:

• evening only (5–11 pm)
• morning and evening (7–9 am and 5–11 pm)
• morning to evening (7 am–11 pm)
• continuous (24 hours).

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

Internal gain factor

This shows the internal gains for the heating season for each of the four heating schedules. The total internal heat (in watts) describes the average energy from internal gain contributors (people, appliances and hot water). The internal gains (energy) depends on the number of hours of heating each day (the heating schedule) and the length of the heating season.

kWh

A unit of energy equal to 1000 watt hours (Wh). Examples:

• A 1000 watt heater running for an hour consumes 1000 watt hours or 1 kWh.
• Two 60 watt light bulbs lit for 10 hours consume 60 W x 2 x 10 = 1200 watt hours = 1.2 kWh.

Local air leakage rate

This rate is calculated by multiplying a house’s air leakage rate by the environmental factors for air leakage zone and site exposure.

Perimeter wall

A foundation wall that encloses flooring that is part of the thermal envelope.

R-value

Measure of the thermal resistance of a building material or element. The higher the R-value, the more resistant the component is to heat passing through it. The R-value is measured in m² °C / W.

For example, assume a wall has an R-value of 2.0 m² °C / W and there is a temperature difference of 5°C 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.

Shading

Shading defines how much light is blocked from entering a window because of eaves, plants etc. Shading of ‘a little’, ‘some' and 'a lot' means 30%, 50% and 70% of available sunlight is blocked from entering the window respectively. Keep in mind the sun path primarily during the winter heating months. Window orientation is automatically taken account of by the annual gain factors. An unshaded south-facing window thus has 0% shading.

See the information page for solar gains for more information about shading.

Shading coefficient

The amount of solar radiation that can pass through a window relative to a standard single sheet of clear 3mm glass. The higher the shading coefficient, the more solar radiation can enter through the window. For 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 relates to the wind shelter of the building. It is a local environmental factor used to calculate the local air leakage.

Slab floor

A concrete slab-on-ground floor.

SHGC

This figure describes the proportion of light that is let through a window. The larger the SHGC, the more light can enter through the window. SHGC's range from approximately 0.70 for single clear glass to 0.31 for low-emissivity glass (coated glass).

Specific heat

Otherwise known as specific heat capacity. The ability of a material to store heat – specifically, how much energy is stored in one m² of a material for each °C it is warmed. The measurement is Wh/m²°C. The specific heat is used to calculate the thermal mass of the building.

Specific heat loss

This measure describes the thermal performance of a complete component. It is measured in W/°C. (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. This figure describes only the building performance and is independent of climate. To calculate the heat loss (energy), multiply the specific heat loss by the ALF-value.

Specific heat loss density

Specific heat loss divided by the total floor area.

Suspended floor

Usually a timber floor that is suspended above the ground on piles, bearers and joists. Suspended floors typically have a foundation wall sheltering the crawl-space from wind.

Thermal mass

Heavyweight building materials that absorb the sun’s heat during the day (even if not directly sunlit) and release it slowly at night. In an insulated house they moderate daily temperature swings. Exposed concrete floors are a common type of thermal mass in houses.

Total heat gain

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

Total heat load

The sum of heat losses from conductance through walls, floors, roofs and windows, air leakage losses and warm-up energy.

U-value

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

Warm-up energy

The energy required to warm-up a house after it has cooled. This energy is only required for intermittent heating schedules (Schedule 1, 2 or 3). The amount of warm-up energy needed depends on the amount of thermal mass present in the building, the climate and the selected heating schedule and level.

W

A measure of heat flow rate and power. The power multiplied by the period for which the power was used is the energy (see kWh).