Financial Calculations

Heat Loss Through Roof

The table below shows a breakdown of the heat lost through the various surfaces of a 1970's Wimpey house. It shows

  • Heat loss without insulation. (As it was when the house was first built).
  • Heat loss with insulation installed.
  • The heat loss expressed as a percentage of the total.
  • The annual cost of the heat loss.
With respect to the roof the addition of loft insulation was calculated to
  • Reduce the heat loss by 97%
  • Reduce costs by an estimated £381 per annum

Heat Loss without insulation.


Approximate Annual Cost.


Heat Loss with Insulation.


Approximate annual cost.


Annual Saving.


Roof297 (45%)39510 (5%)14381
Walls177 (27%)23769 (34%)93144
Doors and Windows133 (20%)17686 (42%)11561
Floor52 (8%)7040 (20%)5515

The calculations for heat loss per degree difference between inside and outside temperature gave a figure of 10.44W. Using this figure the calculated heat loss for a cold winters day would be as follows

Heat loss through ceiling with insulation = 10.44W/K
Outside temperature = -1C
Indoor temperature = 18C
Temperature difference = 19C
Heat loss = 10.44x19 = 198W

This is 1% of the boilers output.

For comparison, if there were no insulation, the heat loss would be

Heat loss through ceiling without insulation = 296.84W/K

Heat loss = 296.84 x 19 = 5640W

This is 35% of the boilers output.

And finally, the heat lost through an uninsulated loft hatch will be

Heat loss = 2.46 x 19 = 47W