Before you specify insulation for a project, look for manufacturer data that specifies thermal conductivity to the AS/NZS 4859.1:2018 Standard. Manufacturers and importers are required to provide a thermal value summary report, which includes a statement of conformance with the Standard and the statistical calculation of the thermal resistance/conductivity including ageing. The thermal performance of rigid insulation is of particular note. These products can have gas fill in the little cells that have lower thermal conductivity than air. Their performance may drop over time (ageing) and/or at lower temperatures (a 20% reduction is not unusual for PIR insulation). PHI has released a set of derating factors that should be applied to PIR products specified in buildings targeting Passive House certification—keep reading for details.
AS/NZS 4859.1:2018 requires the thermal conductivity to be reported for 15°C for New Zealand and 23°C for Australia. Therefore the exact same product may have slightly different performance data in each country. The Standard requires the thermal conductivity to be measured for 10 samples and a statistical mean value λ50/90 for all insulation products provided (excluding vacuum insulated panels). This means one test report from 2017 is NOT adequate to sell the product in Australasia.
AS/NZS 4859.1:2018 requires these rigid foam insulation to have ageing calculated:
- Extruded Polystyrene (XPS),
- Phenolic foam (PF),
- Polyisocyanurate (PIR) and
- Polyurethane (PUR).
Expanded Polystyrene (EPS) is excluded from this requirement because the blowing agent (pentane) is replaced by air almost immediately.
Source: AS/NZS 4859.1:2018 Material R-Value Declaration Procedure, a Kingspan knowledge article. Published 2020.
These aged λ50/90 values can be accepted for Passive House certification—with the following caveat. There is complexity around polyisocyanurate (PIR) insulation in cold locations, where thermal performance can be significantly reduced. (This is unlike most other insulation types that generally perform better the colder it gets outside because of reduced conduction, convection and radiation at lower temperatures.)
Source: Rockwool Building Science Research Summary Issued: 2017-09-13. (Temperature units in F at the top of the graph have been deliberately blurred to prevent confusion.)
PHI requires either specific manufacturer test data to determine PIR performance at the coldest month average temperature; or that the designer uses a conservative reduction in thermal performance.
PHI has provided a rough estimate of a derating factor that can be used. Look for the coldest month on the climate sheet for the building location, then multiply the PIR thermal conductivity with the appropriate derating factor. This will be added to the PHPP manual and certification criteria in future. Until then, use the following numbers.
Table of derating factor for PIR insulation products (if no product-specific test data is available)
| Temperature during coldest month | Derating factor |
| >29°C | 1.0 |
| >16°C | 1.04 |
| >13°C | 1.08 |
| >0°C | 1.13 |
| >-9°C | 1.17 |
For example, in Wellington at sea level the coldest monthly temperature is 9.1°C so the thermal conductivity of the PIR (aged λ50/90 value) is multiplied by 1.13. This increases the amount of insulation required for the building to meet certification requirements.
If we take somewhere very cold by our standards, Cardrona is still above 0°C during the coldest month in a typical year (in the town at least). But the derating factor still calculates a 13% reduction in thermal performance. That’s not insignificant.
References:
AS/NZS 4859.1:2018 Material R-Value Declaration Procedure | Kingspan AU
Cold-Weather Performance of Polyisocyanurate by Martin Holladay, Green Building Advisor