Help for home heat loss model


This is a simple model for home heating which gives you an idea of what proportion of heat loss is through the different elements (walls, windows, roof etc.) and how much you might save if you added insulation or draught proofing. Reducing your heat loss saves you money on your heating bills. To run the model, select the shape closest to your house and what sort of house it is. Then select the features you have now under 'What it is now', and what you think you might like to try under 'With changes'. Run the model and see how much of a reduction those changes would make.

This model does not give you an absolute figure for your heat loss - you probably don't even know the exact size of your house. It will tell you what proportion of the heat loss is probably going in the various directions. Also it does not take into account common features such as attic rooms and so on - these don't usually make a huge difference but they complicate the model a lot. I have selected only features which make a significant difference.

Your house may have mixed construction, for example you might have an old house with solid walls but an extension with cavity walls; the model allows for two types of for walls and two types of windows. So if you have an extension you can specify different constructions for walls-1 and walls-2 and say roughly how much you have of each.

Note: this simple model does not take into account the difference in temperature between different parts of your house. It is likely that you are warmer upstairs than downstairs. A difference of 1C could mean 10% difference in heat losss so you may be losing a little more from your roof and a little less through your floor. Also this model ignores hot water usage and boiler losses - you may be able to reduce your heating needs by taking fewer baths, shorter showers and upgrading your boiler.

This model is much simpler than the UK government standard SAP methodology but I have taken data from that standard for many of the numbers in the model. The source document is Standard Assessment Procudure (SAP) 2005 edition version 9.9.

What does W/C/m² mean?

This is Watts/degree C/square metre total floor area (including upper floors).

Heat loss is measured in Watts. The average kettle is 1-2kW which is 1000-2000 watts. The heat loss depends on the difference in temperature between inside and outside: the bigger the temperature difference the faster the house loses heat. By adjusting for floor area, we allow for different sizes of house. For example a house might have a total floor area of 100 m² and the heat loss might be 3 W/C/m². Then if it is 15C warmer inside than outisde the overall loss would be 3 x 15 x 100 = 4,500 W or 4.5 kW (so more than 2 kettle's worth!)

Why do you want to know the shape of the house?

The shape of the house is very important because it affects how much wall area you have compared to floor/roof area and how much wall area compared to volume (which is important for the ventilation). The least wall area house would be a narrow terraced house. The worst would be the L-shape detached house. If you haven't got a lot of wall area then insulating it may not make a huge difference to your bills.

Why haven't you asked how much insulation for the walls?

If you have insulated your walls already it is probably recent and building regulations require that renovation is done to high standard (to U-value 0.3). It may be that you have put in less insulation than this for lack of room, but if you have even half the insulation that has much more than half the impact - as with roof insulation there are diminishing returns.

What is PassivHaus?

This is a standard set by the Passiv Haus Institute in Germany, designed to achieve a comfortable temperature throughout the year almost entirely from the heat of your body and appliances that you run in the house. It also uses MVHR. It is very difficult to modify an existing house to achieve PassivHaus standard.

What is MVHR?

You must have fresh air, and that means air must be going out, which normally takes a lot of heat with it. With mechanical ventilation with heat recovery (MVHR), air coming in and out goes through a heat exchanger so the air going out warms the air coming in.

What kind of floors have I got?

Solid floors are in direct contact with the ground (albeit with a damp barrier). They are usually concrete. They don't bounce when you jump up and down.

Suspended floors are usually wooden floorboards with an air space below. You can put insulation below and between the floorboards but you must ensure there is ventilation to the boards from below.

What is thermal bridging?

This is where heat crosses your walls not through the bricks and insulation (if you have any) but taking an easier route such as along a metal service pipe or through a metal wall tie (which keeps the 2 sides of your cosily insulated cavity wall together). Some thermal bridging is necessary for the building structure but clever construction required under recent building codes (only since about 2003) should minimise this. It is conventional to assume a factor depending on the building age which is what I have done.

How does the model work?

Ventilation: is calculated as ACH (air changes per hour) times the occupied volume of the house times 0.33 (which allows for the heat capacity of air).

Walls, windows etc. are calculated as U-value times the area of the element e.g. U-value for wall times area of wall. The U-values are taken from SAP except for solid walls where I have used 1.5 instead of 2.1. This is consistent with preliminary results from a survey for the Energy Saving Trust.

For calculating the areas and volumes I have assumed

  • Height is 2.5m per floor.
  • The gross wall area and volume are taken from the house dimensions.
  • Average windows area is 0.13 x total floor area area + 6 (m²). This formula is from SAP.
    • If lots of windows, add 25%.
    • If less well lit subtract 25%.
  • Doors are 1.85m² each
  • Net wall area is gross walls minus windows and doors.
  • Floor and roof area are taken from the house dimensions.
  • Total floor area is the area of the floor times the number of floors.
  • For thermal bridging, we calculate the total house envelope gross walls + floor + roof

    Your house may vary from these values - if you know the actual values then it is easy enough to redo the calculations with those values.