Can equally insulating windows warm a room differently?
On a sunny afternoon, one room can receive more solar heat through its windows than another, even if the glazing has the same Ug value. The figures describe different things: U-factor concerns heat flow through glazing as indoor and outdoor temperatures differ; g-value reports total solar transmittance. A similar insulation figure alone cannot tell you how much solar energy will pass indoors. [1] [2]
- U-factor measures the rate of heat flow through glazing products; a lower U-factor means less heat flows through the window. [1]
- Pilkington’s Table 5 lists insulating glass units with the same Ug value of 1.0 W/(m²K), but different g-values: 0.47 for Optifloat Clear and 0.27 for Optifloat Grey. [2]
- The same table gives light transmittance values of 0.65 and 0.32 for those units, separately from their g-values. [2]
- The US Department of Energy distinguishes solar heat transmitted from visible light transmitted; its SHGC and visible transmittance measures refer to different things. [1]
What does the U-value tell you?
The U-factor describes the rate of heat flow through glazing products. A lower value means less heat flows through the window. Check whether a quoted figure applies to the glazing unit or to a broader product before comparing it with another figure; the Pilkington table here reports Ug values for insulating glass units. [1] [2]
What does the g-value add?
The g-value adds information about solar energy passing through the glass. Pilkington labels its total transmittance column “g value” and says the listed performance data was determined under BS EN 410 and BS EN 673. The table reports g-value, light transmittance and Ug in separate columns, so none of those figures should be read as a substitute for another. [2]
Similar Ug values can have different g-values
Pilkington’s Table 5 shows why insulation and solar transmission should be compared separately. Two insulating glass units use a 6 mm Optitherm S1 Plus inner pane with a 16 mm, 90% argon-filled cavity. With a 6 mm Optifloat Clear outer pane, the table gives Ug 1.0 W/(m²K), g-value 0.47 and light transmittance 0.65. With a 6 mm Optifloat Grey outer pane, it gives the same Ug, but a g-value of 0.27 and light transmittance of 0.32. These are manufacturer’s glazing-unit figures, not a like-for-like whole-window comparison or a forecast of room temperature. [2]
Why might the room still feel different?
A glazing g-value describes solar transmittance through the glass; it does not by itself predict how warm a particular room will become. The Department of Energy notes that solar heat gain depends on the fenestration product and that shades and blinds can reduce unwanted solar heat gain. The table’s glazing-unit figures are therefore one part of understanding a room’s sunlight, rather than a complete account of its exposure or comfort. [1] [2]
What should you ask when comparing windows?
Ask for the Ug and g-values and check that both describe the same kind of product. Also ask for the light transmittance figure if daylight matters to you: the Pilkington examples show that it is reported separately from g-value. Then ask how the specified glass and any shade relate to the window’s actual exposure to sun. Those details make the figures more useful for the room where you plan to sit. [1] [2]