Issue 34 – December 2011
BRANZ has received a question about using the NZS 4218:2009 calculator (available on the BRANZ website) where skylights are used but the skylight doesn't show up in the summary sheet for the reference building.
The rules in NZS 4218:2009 when using the calculation method require that the reference building roof calculation is roof area plus skylight area. This gives a total roof area, which is divided by the required construction R-value (R2.9 in Zones 1 and 2 and R3.3 in Zone 3) to obtain a reference building heat loss for the total roof area.
Because the reference building calculates the whole roof area (including the skylight area) at the same R-value, the skylight area is not separated out. In the proposed building, the calculations are separated into the roof area divided by the proposed construction R-value of the designer's choice to obtain a heat loss, then the skylight area is divided by the skylight R-value to give the heat loss for the skylight. The combined total of the heat losses of the proposed roof and skylight are added together to give the heat loss of the total roof area in the proposed building. The proposed building must not have a greater heat loss than the reference building.
To compensate for the additional heat loss through the skylight, which will always be lower than the R-value required for the roof, the roof, walls or floor R-value will need to be increased to compensate for the lower R-value of the skylight.
There is no maximum area for skylights in the calculation method. In the schedule method, there is a maximum of 1.5 m2 or 1.5% of the roof area (whichever is the greater) and a minimum R-value set.
Summarised from BRANZ Guideline November 2011.
- NZS 4218:2009 Thermal insulation – Housing and small buildings