Passive House window thermal comfort requirements for NZ climate zones

The Passive House Institute’s certification criteria require that the thermal comfort criteria be achieved for all windows and doors except where they are located where they will not impact thermal comfort (clearstory or entrance door), they are small (less than 1 sqm) and /or isolated so that in combination with surrounding surfaces thermal comfort would likely be met. This criteria is intended to ensure winter thermal comfort.

Note that thermal comfort criteria do not apply to the PHI Low Energy Building Certification. The moisture protection requirements (fRSI), however, do apply to the PHI Low Energy Building Certification.

The PHPP software uses a window external thermal comfort temperature to check the thermal comfort criteria. The window thermal comfort temperatures are the minimum 12-hour rolling average for a ‘typical’ year. In general, for NZ we’ve used the NIWA EPW (Energy Plus) typical or reference years for determining this. The table below is current as of 9Oct2019 when PHI agreed to revise the Wellington & Queenstown window thermal comfort temperatures.

Remember in PHPP that the temperatures are adjusted for altitude. To clarify, this means if your weather station is at 2m altitude and your building site is at 102m of altitude, you would use a temperature 0.6˚C lower. For Dunedin it would reduce from -1.4˚C to -2.0˚C if you went 100m above the weather station altitude.

Note that for most of NZ the Climate Zone is Warm-temperate. Queenstown, Lauder and Invercargill are Cool-temperate and Kaitaia, Auckland New Plymouth, Tauranga, and Napier are Warm climate (at the weather station altitude). This is important because each climate zone warm, warm-temperate and cool-temperate has different moisture protection requirements (fRSI) which must be met.

Thermal comfort criteria for windows in PHPP is dependent only on the installation PSI value, U-value for the frame and the frame width, glazing edge PSI value and the U-value for the glass. These are all shown below for clarity.

As windows become large, the U-value for the glass dominates, but for smaller windows, installation and frame values are very important. The rightmost column in the graph for the ‘Vertical Window U-value minimum’ shows the lowest value for each NZ climate zone at the weather station altitude.

The biggest implication is that it is possible, with very good frames and installation details, to use double glazing in all of the NZ climate zones from a thermal comfort perspective. This requires a large building in a colder climate to meet the heating demand or load requirement but opens up interesting possibilities for a hotel in Queenstown or an apartment building!

—22 May 2020

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