ALF help

Design help

On the Design pages you enter information about the particular house design and location to perform an ALF analysis on it.

Ventilation

Background

On this page you can enter information which is used to determine the air leakage heat loss of the building. The method follows the approach to take a Basic Airtightness based on age and complexity of the building and then to apply a number of correction factors depending on different design features of the building. The Air Leakage Rate is then adjusted taking account of site exposure and windiness of the climate. The result is the Local Air Leakage Rate.

Air tightness and air leakages are measured in air changes per hour (ac/h).

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Air tightness & classification

You can select one of the four values for the Basic Airtightness:

 

Basic Airtightness

Typical Examples

airtight:

0.25 ac/h

post 1960, simple small rectangular design, airtight joinery, all windows with gaskets

average:

0.50 ac/h

post 1960, larger than 120 m2

leaky:

0.75 ac/h

post 1960, complex shape, some feature strip lining material, generally larger than 200m2

draughty:

1.00 ac/h

pre-1960, strip lining, strip flooring, often high stud

The typical examples in the table are only guidelines. If more detailed information about the house airtightness is known then the value closest to the true value should be selected irrespective of the age and design of the building.

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No. of Open Fires

Enter the number of open fires or any other large openings in the roof. Each fire will add 40 m3/h to the Air Leakage Rate.

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Metal Flued Heaters

Enter the number of heaters with flue restrictors. Each heater with a flue restrictor will add 20 m3/h to the Air Leakage Rate.

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Metal Flued Heaters

Enter the number of heaters with flue restrictors. Each heater with a flue restrictor will add 20 m3/h to the Air Leakage Rate.

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Window losses

Buildings have an associated ventilation loss through windows based on the number of passive vents and windows cracked open.

If only some of the rooms have passive vents then you should only check this box if the vented rooms are generally heated.

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Retrofit air tightening

If the house is draughty i.e. built before 1960, then a correction can be made if it has been internally relined. It is assumed that this will have airtightened the building and 0.1 ac/h are subtracted from the Air Leakage Rate.

If the house is draughty i.e. built before 1960, but the windows were replaced with airtight aluminium windows or old timber windows were airtightened then 0.2 ac/h are subtracted from the Air Leakage Rate.

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Building wind exposure

The micro-climatic house location in relation to nearby terrain features and other buildings is used to determine the local wind exposure of the house.

The Building Wind Exposure Factor reflects the extent of shielding by local terrain and buildings etc. This is a graphical description of the range of wind exposure classes that can be assigned to a house. If one side of the house is more exposed than the others, choose the exposure class that relates to the prevailing wind.

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Kitchen & Bathroom vents

Used to calculate the targeted mechanical ventilation's contribution to the air leakage rate.

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Air Leakage Rate

The Air Leakage Rate describes how airtight the building is. It does not take account of the local and climatic wind exposure and conditions. A building has for example the same Air Leakage Rate whether it is on the hill tops of Wellington or sheltered between trees and houses in Kerikeri.

Once the environmental factors are considered the leakage rate is then called the Local Air Leakage Rate.

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Local Air Leakage Rate

The Local Air Leakage Rate is the Air Leakage Rate adjusted for the Site Exposure and the Air Leakage Zone. The Site Exposure takes account of local wind exposure whereas the Air Leakage Zone takes account of the windiness of the climate. The Air Leakage Zone is selected by selecting the location of the building on the 'Climate & Heating' page.

A minimum Local Air Leakage Rate of 0.5 ac/h is used. This is because lower Local Air Leakage Rates would lead to 'stuffiness' in the building and occupants would open windows to increase the air flow.

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Air Leakage Background

The air leakage component of the building heat loss has always been difficult to calculate because it depends on the air-tightness of the building and the occupant management of windows and other ventilation devices. All of these factors are unknown at the design stage so certain assumptions have to be made about air leakage and occupant provided ventilation.

In previous versions of ALF air change rates were calculated from a measure of the complexity of the building and local climate factors. The algorithms for this were developed from field studies showing that more architecturally complex buildings were less airtight than those of simple design. While this was a useful way of estimating the air-tightness of conventional timber framed residential buildings, the relationship – not proven for other types of construction (eg. concrete block and panel construction) – was frequently used by previous ALF users well outside its range of application. Here a simpler approach is offered.

Different base air change rates are specified for houses built before and after 1960 and for general construction features. Although 1960 is a somewhat arbitrary date, it does mark the replacement of strip flooring with particleboard and the earliest examples of aluminium window frames. New homes are allocated a base infiltration rate according to their floor area and complexity of design. There are four categories provided:

Base Air Change Rate

[Air changes per hour]

Typical example

Airtight

0.25 ac/h

simple, small rectangular design, airtight joinery, all windows with gaskets

Average

0.50 ac/h

larger than 120 m2

Leaky

0.75 ac/h

complex shape, some feature strip lining materials, generally larger than 200 m2

Draughty

1.00 ac/h

pre-1960, strip lining, strip flooring, often high stud

The base air change rate is modified according to design features like the number of open fires, number of solid fuel heaters with flue restrictors, number of passive vents and for pre-1960 houses, retro-fit air-tightening of old wooden windows and retro-fit air-tightening of leaky linings (plaster board linings in lieu of match lining).

The difference between air leakage and ventilation must be noted here. Air leakage is confined to air leaks through cracks over which the occupant has no control. This is the lowest possible ventilation rate.

Actual ventilation rates will be higher, as they include the effect of opening doors and windows. This will be particularly true in houses which are quite airtight and need extra ventilation to prevent stuffiness, condensation and mould growth.

This building design specific air leakage rate is finally multiplied with factors taking account of the Air Leakage Zone and the Site Exposure.

The Air Leakage Zone Factor is chosen according to the New Zealand climate. New Zealand is divided into four air leakage zones which reflects regional variations in average wind speed and which is weighted to reflect the extent that they drive infiltration. The data have been prepared from weather data measured in highly populated areas and are not as accurate in uninhabited or mountainous areas.

The Site Exposure Factor reflects the extent of shielding by local terrain and buildings etc. This is a graphical description of the range of wind exposure classes that can be assigned to a house. If one side of the house is more exposed than the others, choose the exposure class that relates to the prevailing wind.

The approach taken here is to use the larger of 0.5 ac/h (minimum building averaged ventilation rate required to comply with NZS 4303:1990 'Ventilation fro Acceptable Indoor Air Quality') and the estimated infiltration rate for heat loss calculations. In summary, when the Local Air Leakage Rate is less than 0.5 ac/h, this procedure assumes that the occupants of a house will open windows to provide an acceptable level of air quality, regardless of the indoor and outdoor temperatures.

The resulting Local Air Leakage Rate is multiplied by the house volume – the product of the floor area and the average ceiling height – 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 .

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 Create an ALF account

Register now to create your free ALF account.

Every user needs to create an ALF account. Creating an ALF account will ensure:

  • Confidentiality for your project.
  • Allow BRANZ to record usage of the ALF tool – BRANZ cannot access any individual projects. Your privacy is assured.

 Contact and Feedback

Contact us at Branz for further information about the ALF 3.2: Annual Loss Factor.