Financial Calculations

Draughty Windows

It may seem surprising but double glazed windows can become draughty as the seals lose their resilience and the hinges wear.

Typical Heat Loss for average sized house = 0.33kW 

Equivalent to 

  • 1 Radiator 500mm x 400mm 
  • 5.5 x 60W bulbs
  • 4.47p/hour (electricity) 
  • 3.9p/hour (gas)

How this is calculated

The perimeter of all the opening windows in a typical 3 bedroom house is approximately 31m. If the gap around the perimeter was on average 1mm then this would be equivalent to a hole 20cm(8in) in diameter. That’s quite a big hole!

In order to calculate how much energy is being lost, assume that it is half this size at 10cm(4in) in diameter (because the wind only blows on one half of the house), raining outside, just above freezing, 18C inside and the draught is a fast walk at 1m/s.

Taking all this into account then 1/3kW of heating would be required which is equivalent to approximately ⅓ of the output of a 600 x 900 radiator or the heat generated by 3 active people.

With the current gas price of 3.9p/kWh then the cost is 1.66p/hour or £28 for the heating season.

If the fuel was electricity then the cost is 4.47p/hour or £75 for the heating season.

Further Information

Research Paper is a very interesting and informative paper on the effectiveness of a range of measures to reduce heat loss from a typical single glazed window.