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|
|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.