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.

Roofs and Skylights


On this page you can enter information about the dimensions and construction types of the roofs. The page also contains the links to the 'Window' Pages on which you enter skylight specifications.

The top of the page shows a check box which will automatically set all the roofs to the construction type and insulation level of the first roof.

Calculating Specific Roof Losses

Depending on the placement of the insulation in the roof, different areas have to be used for the heat loss calculation. If the insulation is installed at ceiling level find the area of each ceiling type from the house plan. Do NOT include any interior ceilings, i.e. ceilings between conditioned floor levels, but use only ceilings facing to the outside or unconditioned spaces. Note that the ceiling area will usually be the same as the Ground floor area except where any portion of the upper floor area extends beyond the line of the thermal envelope of the lower floor. There may be a need to treat the overhanging portion of the floor as a suspended timber floor, which will form part of the thermal envelope). If the insulation is installed in the roof plane (generally called 'skillion roofs') then take this roof area. This area will generally be slightly larger than the ceiling area because of the slope of the roof. To calculate the roof area of a sloped roof multiply the ceiling area with one of the given factors depending on the average roof slope.

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Same Roof Construction Check Box

If you check this box all the roofs will automatically be set to the construction type of the first roof. Also the insulation levels of all roofs will be set to the one of the first roof.

If most of the roofs are of the same construction type it is recommended to check this box. You can then enter this construction type and insulation R-value for the first roof (Roof 'A'). If you enter the dimensions of other roofs the construction and insulation is automatically inserted by the program. When you have entered all the roof information the Same Roof Construction Check Box can be switched off and the roofs with different construction types can be modified.

Be careful not to click the box once you have entered different constructions and insulation R-values. They will be overwritten if you click the check box.

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Roof Length

Enter the roof length in this box. The roof area is calculated by multiplying the length by the width of the roof. Do not include any areas of overhangs because they are not relevant for heat loss calculations. The area through which heat escapes is only the area above any rooms.

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Roof Width

Enter the roof width here. The roof area is calculated by multiplying the length by the width of the roof.

Do not include the area of overhangs (soffits) in the ceiling and roof area calculation because it is usually not part of the thermal envelope.

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Roof Net Area

The roof area, which is shown in the box, is the total roof area of this roof excluding the skylight area in this roof. It is calculated by multiplying the length and width of the roof and subtracting the area of all the skylights in the roof.

Roof heat losses can generally be assumed to occur through flat ceilings, with area equal to the floor. This is a reasonable approximation for all but very steep roofs of the kind which enclose a living space rather than ceiling space. In this case, the area of the roof shall be calculated explicitly instead of set equal to the floor area.

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Roof Construction

The drop-down menu contains abbreviations of the most common roof construction types. Clicking on one of the abbreviations opens a page with a picture of the selected construction type ('Construction' Page). In this page the construction and insulation details can be selected, as well as the R-value of the insulation material. The R-value of the construction (i.e. the selected construction with the specified insulation material and R-value) is calculated automatically by the program as soon as the construction and the insulation are selected.

If the INSULATION R-value that you have chosen significantly exceeds the highest tabled value for this type of construction then the resultant CONSTRUCTION R-value can only be estimated and is therefore unreliable.

In this case you may first check that it is physically possible to install the selected/proposed insulation into the construction type being considered.

If this is the case then the actual CONSTRUCTION R-value should be calculated using recognised methods/first principles i.e. NZS 4214:1977 or it should be based on values derived directly from thermal resistance test data that may be available from manufacturers on a specific system.

Where such a value is to be used it can be entered directly by selecting the construction type 'Custom' (the first construction type in the list).

If the Same Roof Construction Check Box is checked then all except the first roof construction drop down lists are disabled. In order to enable them you have to un-check the Same Roof Construction Check Box.

In case you want to bring up the 'Construction page' of the currently selected construction again you have to click once more on the same construction type in the drop-down list.

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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.