Listed in the table below are the standard NFRC winter and summer environmental conditions:
| Environmental condition | NFRC winter | NFRC summer |
|---|---|---|
| Outdoor air temperature | -18 °C | 32 °C |
| Outdoor wind speed (convection) | 5.5 m/s (26 W/m2K) | 2.75 m/s (15 W/m2K) |
| Outdoor sky temperature | -18 °C | 32 °C |
| Outdoor sky emissivity | 1 | 1 |
| Indoor air temperature | 21 °C | 24 °C |
| Indoor convection | ASHRAE/NFRC inside model | ASHRAE/NFRC inside model |
| Indoor room temperature | 21 °C | 24 °C |
| Indoor room emissivity | 1 | 1 |
These are the standard environmental conditions defined in the NFRC standards and widely used by the industry. It is possible to define local environmental conditions and use them in a specific region.
In the US, the NFRC winter U-value is used (called U-factor and with the imperial unit).
In Singapore, the NFRC summer U-value is typically used, and, in our test reports, both the winter and summer U-values are reported.
For most glasses, the summer condition U-value is smaller than the winter condition Value. The main reason is that the outdoor wind speed in the summer conditions is lower and it results in a lower outdoor side convective heat transfer rate.