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 Homestar Technical Clarification Rulings

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Technical Question Title Tool Version Date Released Credit Name Tool Category TC Link
TQ217 HC4 Slab Edge Exemption v5 HC4 - Moisture Control HC Link
Approved Ruling:
6 and 7 Homestar homes are exempt from the HC4 mandatory minimum fRsi requirements stated in the Homestar v5 Technical Manual. Thermally broken window frames with a minimum R-value of 0.25 are still required for all Homestar v5 projects This ruling is in response to uncertainty regarding the real life moisture accumulation risk of certain junction thermal bridges in New Zealand climates. The fRsi guidance provided in the Homestar v5 Technical Manual is based on static modelling with the threshold of eliminating any moisture occurring at the junctions. The approach of eliminating moisture risk by fully insulating the junctions remains prudent however static modelling does not consider other factors such as the ventilation rate of the conditioned space and the thermal mass of the envelope components. More importantly, static modelling provides no detail regarding the length of time that moisture is likely to be present in any location or if the building envelope is more or less prone to either accumulating moisture or drying out over time. Additionally, there remains several market concerns about the available options for insulating the edges of concrete slabs and concrete mid-floors. These concerns include cost and durability. NZGBC has commissioned dynamic hygrothermal modelling research to re-assess the risk of moisture accumulation over time at specific junctions within typical building envelopes. While this research is being completed and the market continues to respond to any demand for slab edge insulation, NZGBC has made the decision to allow 6 & 7 Homestar v5 projects to proceed without specific fRsi requirements. While slab edge insulation will not be explicitly required for the duration of this Technical Clarification, insulation in this location may be necessary for achieving the requirements of EF4: Energy Use and HC1: Winter Comfort.