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.(W/K) Approximate Annual Cost.(£) Heat Loss with Insulation.(W/K) Approximate annual cost.(£) Annual Saving.(£) Roof 297 (45%) 395 10 (5%) 14 381 Walls 177 (27%) 237 69 (34%) 93 144 Doors and Windows 133 (20%) 176 86 (42%) 115 61 Floor 52 (8%) 70 40 (20%) 55 15 659 878 205 273 601

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