Building heat loss is measured by the sum total of the heat loss for each external surface of a structure. The heat loss of these individual surfaces is determined using the formula H = A x U, where A is the area of the surface and U is the "thermal transmittance coefficient"; this is basically a measure of how well a material conducts heat. The value of U differs from material to material.
Measure the length of the first outer surface of the building, such as an outside wall or the roof, using the tape measure. Measure the height of the surface using the tape measure. Measure the length and width of any doors, windows or other gaps in the wall, as they must be calculated separately. Note the measurements.
Calculate the area of the surface by multiplying the length by the width. Calculate the area of any doors, windows or other gaps and subtract these from the area of the surface. Multiply the area of the surface by the U value of the material it is made from to find the heat loss. Repeat for the doors, windows and other gaps.
Note down the heat loss for the first surface. Measure the lengths and widths of the other surfaces in the structure, taking into account any other doors, windows or gaps. Measure the temperatures inside and outside of the structure and note the difference on your paper. Multiply each heat loss value by the temperature difference.
Add together the heat loss values for each room, such as two walls, a floor and a roof, to produce a room-by-room set of heat losses. Multiply each result by the number of "air changes" for each room in the structure depending on the room's typical use. Add the results together to find the total heat loss for the structure.
Heat moves from hot areas to cold ones, so if there is no temperature difference between one side of a wall and the other, there is no heat loss through that wall. As a result, the majority of walls inside a structure can be discounted because the temperature will be the same on both sides.
Windows and doors must have their heat loss values calculated separately from the walls they are in because they are made of separate materials. Do not make the mistake of ignoring doors and windows when calculating areas, or averaging out U values for a surface because it makes calculations easier, as this renders the heat loss value highly inaccurate.