We were asked why the U-value of a glass is different when the glass is installed horizontally.
There are 3 heat transfer modes: conduction, convection, and radiation. The convection part is dependent on the glass tilt and it affects the glass U-value.
By default, we evaluate the U-value of a glass with the vertical tilt, which is the most common position of glasses. For a horizontally tilted glass, the U-value is significantly greater than the U-value of the same glass with the vertical tilt.
Besides the dependency on tilt, the U-value is also dependent on the glass height. Other thermal properties (e.g. SHGC) are dependent on the tilt too.
However, it does not mean that the glass U-value shall be evaluated with different tilts. There are primarily two applications:
- Glass performance rating
- Fenestration performance rating
For glass performance rating, it is sufficient to evaluate the glass U-value with the vertical tilt only. With this standardized tilt, fair comparisons can be performed conveniently.
For fenestration performance rating, the glass tilt is considered in the evaluation by default.