Windows and Doors
What do Uw and Ug mean when choosing windows?
Understand the difference between Uw and Ug and why the frame, glazing and installation must be considered together.
Window specifications often include values such as Uw and Ug. They describe heat transfer, but the key is knowing which part of the window each value represents.
Uw versus Ug
Ug describes heat transfer through the glazing unit. Uw refers to the complete window under stated assessment conditions. Lower values generally indicate less heat transfer.
- Do not compare one supplier’s Ug with another supplier’s Uw.
- Ask which window size and configuration the Uw value refers to.
- Check that figures match the proposed system.
Glazing is not the only factor
Frames, glazing spacers and installation methods all influence overall performance.
- Ask about the glazing unit and spacer type.
- Consider solar exposure and shading needs.
- Pay attention to sealing around the frame.
Thermal comfort in daily life
Airtightness, ventilation and correct use matter alongside energy figures.
- Ventilate appropriately for room conditions.
- Watch for condensation and sources of indoor moisture.
- Choose opening sashes where ventilation is practical.
Compare complete-window data together with the installation specification. One isolated number does not guarantee a good result.
