Heat Loss Calculator

Find out how much heat is lost from your home through the walls, roof, floors and windows, and how much you can save with different kinds of insultion.

Enter information you know off the top of your head such as the number of rooms, whether they are small, medium or large and if your windows are single or double glazed. You may need to check how thick the insulation is in your attic, and if you have solid or cavity walls. The calculator will tell you exactly how much heat is lost through each area of your home, how much this costs you, what the CO2 emissions are, and the best forms of insulation to use.

We hope you enjoy using the calculator, and find it useful. Good luck!

Example values are in place for a typical three bedroom semi-detached house.

Particulars of the house
Name of House (for your reference)
Today's Date
Type of House
Number of Rooms
Rough Size of Rooms
Number of Floors
Floor area - interior m2
Ceiling area - interior m2
Floor area - exterior m2
Roof area - interior m2
Roof area - exterior m2
Wall height m
Wall length m
Exterior Wall Area - Gross m2
Window and Door area (% of walls) m2
Exterior Wall Area - Net (less windows) m2
Volume of house m3
Rough Internal Temperature (select)
OR enter Specific Internal Temperature degrees C
Internal Temperature used  degrees C
Rough External Temperature (select)
OR enter Specific External Temperature degrees C
External Temperature used  degrees C
Construction & Materials
Exterior Walls
Exterior wall lining
Annual cost @ p/kWh £
Annual cost with insulated dry lining £
Roof Insulation
Annual cost @ p/kWh £
Annual cost with 300mm insulation £
Annual cost @ p/kWh £
Annual cost with low-E double glaze £
Ground Floor Type
Annual cost @ p/kWh £
Annual cost with suspended floor £
Air changes
Annual cost @ p/kWh £
Annual cost with low draught house £

The annual heat loss from your house is kWh

What does this mean? kWh (kilo Watt hours) are units of energy. So this is the amount of energy lost through the walls and windows of your home every year.

Insulation Level Cost CO2 emissions
Current £ kg
Good £ kg
Saving £ kg

By applying good insulation you can cut heat loss, fuel bills and CO2 emissions by



The following costs and emissions are based on typical fuel figures.

Heat loss for house with existing insulation

  Heat Loss (kWh) Percentage Cost @p/kWh CO2 (kg)
Walls % £
Roof % £
Windows/Doors % £
Ground Floor % £
Draughts % £
Total £

Heat loss for same house with best insulation installed in each area

  Heat Loss (kWh) Percentage Cost @p/kWh CO2 (kg) Saving
Walls % £ %
Roof % £ %
Windows/Doors % £ %
Ground Floor % £ %
Draughts % £ %
Total £ %

Mukti Kumar Mitchell, North Devon, February 2009

What is the purpose behind the HEAC?

It is now widely recognised that CO2 emissions are causing climate change, which threatens nature and civilisation. The UK Government is committed to a 20% reduction of national CO2 emissions by 2020. Domestic energy use causes 25% of national emissions, and heating uses 90% of energy in the home. If we could all reduce our house heat-loss by half, that would cut national emissions by 10%. As you will see from the calculator, it is not that difficult to cut the heat loss from the average house by half, as most old houses leak a lot of energy! New houses are built to more stringent requirements and waste less energy. But most houses in Britain need to be improved. This calculator helps you see what changes can be made to your house to reduce its heat loss.

Reducing heat loss also saves money. The cost of energy is rising steadily as we turn to the smaller remaining oilfields. Conserving heat reduces the need for new energy sources such as windmills or nuclear power stations, keeping the country a nicer, safer place.

Sources include:

Mukti Mitchell

Mukti Mitchell is a sailor, carpenter, environmental designer and pioneer of low carbon lifestyles who lives by the sea in North Devon. He designed and built the revolutionary zero-emission microyacht Explorer, nominated "Innovative Boat of the Year" 2005 at the IPC Marine Awards, and set up Mitchell Yachts to manufacture these. In 2007 he sailed around Britain to promote low carbon lifestyles, supported by HRH The Prince of Wales, James Lovelock, Jonathon Porritt, Zac Goldsmith, Tim Smit, Tony Juniper, Satish Kumar, Caroline Lucas, Stephen Tindale and Jonathan Dimbleby.

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