PH certification criteria requires that a minimum surface temperature be achieved.
1. Determine the exterior temperature to use – call this the Hygiene External Temperature. This temperature can be any value lower than the internal temperature. I typically use 0 °C as the Hygiene External Temperature and 20 °C as the interior temperature but this is only a matter of convenience as the temperature difference is divided out in step 5 following. PHI typically uses -10 °C outside and 20 °C inside.
2. Calculate your thermal bridge as you typically would per ISO10211 “Thermal bridges in building construction — Heat flows and surface temperatures — Detailed calculations” and using the PHI recommended boundary conditions. Remember it doesn’t matter what temperatures you use for your calculations as long as you are consistent as they are used as ratio’s.
3. To be pedantic you need to redo the PSI calculation per ISO13788 “Hygrothermal performance of building components and building elements — Internal surface temperature to avoid critical surface humidity and interstitial condensation — Calculation methods” but to simplify things PHI recommends: A. Change your thermal bridge model to use the Hygiene External Temperature as the exterior temperature, and B. Change the internal surface resistance to 0.25 m2K/W.
4. Calculate the temperatures throughout your cross section detail and find the minimum temperature that is exposed to interior conditions.
5. Now calculate the Minimum Temperature Factor from:
Prior to 5Oct2017 this article recommended using the following table below which provides the Hygiene External Temperature for each climate region. It noted that this temperature is different than the Thermal Comfort temperature provided in PHPP version 9.6 and newer. I was unable to find a proper reference for the table below but believe it came out of one of the older PHI Passive House Certification Standards. New Zealand ranges from the Warm climate zone in Auckland to Warm-temperate through most of the country to Cool-temperate with only isolated high altitude regions (ski resort cafe) considered Cold climate. In practice it doesn’t matter which temperature is used as the Minimum Temperature Factor is unaffected by this.
Prior to 17Sept16 this article incorrectly stated the exterior temperature to use for the hygiene analysis was the minimum 12 hour annual average. This is incorrect for us in certification to the Passive House Institute standard.