Roofs and skylights

ALF calculations and roofs and skylights.

Calculating specific heat 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 from the house plan. Use only ceilings facing to the outside or unconditioned spaces – do not include interior ceilings between conditioned floor levels. 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 (i.e. a skillion roof) then use this roof area. This area will generally be larger than the floor area of the room because of the slope of the roof.

Roof construction

As soon as the construction type and insulation levels are selected, ALF automatically calculates the construction R-value.

If the selected insulation R-value significantly exceeds the highest tabled value for this type of construction, the resulting construction R-value can only be estimated and is therefore unreliable. In this case, first check that it is physically possible to install the selected insulation into the proposed construction type.

If this is the case then the actual construction R-value should be calculated using recognised methods/first principles such as NZS 4214:2006 Methods of determining the total thermal resistance of parts of buildings. Alternatively, it can be derived directly from thermal resistance test data that may be available from the manufacturers of a specific system.

Where using such a value, enter it directly by selecting Custom R-value in the drop-down list.