U values And Double Glazing

To help you decide whether double-glazing is worthwhile or whether you might do better just to have insulated shutters or a draught sealed single-glazed window, here are some comparative U values, assuming a normal degree of exposure in all cases.

Glazing W/m2deg C

  • single 5.6
  • double 6mm gap 3.4
  • 12mm gap 3.0
  • 20mm or more gap 2.9

Shutters (closed) W/m2deg C


  • 12.5mm shutter 1.14
  • 25mm 0.82
  • 50mm 0.53


  • 12.5mm 0.96
  • 25mm 0.72
  • 50mm 0.49

Average U values for shuttered windows are shown below. The first table shows the average value if the shutters are open for eight hours a day and the second assumes they are open for twelve hours. In all cases the windows are assumed to be draught sealed.

Open eight hours, closed sixteen hours


  • 12.5mm shutter 2.6
  • 25mm 2.4
  • 50mm 2.2


  • 12.5mm 1.6
  • 25mm 1.4
  • 50mm 1.3
  • 100mm 1.2

Open twelve hours, shut twelve hours


  • 12.5mm shutter 3.4
  • 25mm 3.2
  • 50mm 3.1


  • 12.5mm 1.9
  • 25mm 1.8
  • 50mm 1.7

The average values over twenty-four hours may be slightly different than shown in these figures because it is colder at night than in the daytime and these averages make no allowance for this.

The final choice will depend on you and how much money you want to spend. The important thing is not to have double-glazing just because it is the thing to do, as it may not be the best thing to do if you are really trying to save energy. The costs you will have to work out for yourself.

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