The difference between NFRC winter and summer U-values

Related services Glass optical & thermal properties

Listed in the table below are the standard NFRC winter and summer environmental conditions:

Environmental conditionNFRC winterNFRC summer
Outdoor air temperature-18 °C32 °C
Outdoor wind speed
5.5 m/s
(26 W/m2K)
2.75 m/s
(15 W/m2K)
Outdoor sky temperature-18 °C32 °C
Outdoor sky emissivity11
Indoor air temperature21 °C24 °C
Indoor convectionASHRAE/NFRC inside modelASHRAE/NFRC inside model
Indoor room temperature21 °C24 °C
Indoor room emissivity11

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.

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