Usually heat transmittance and U-values are thought of only in relation to building heat losses.
However, with the ever-increasing risk of damage by condensation, the subject of heat transmittance
should always be coupled with vapour transmittance, since this is at the heart of a proper
understanding of domestic heating and its effect on buildings, condensation and people.
If a wall is at the same temperature throughout and heat is applied to the inside surface, that
surface will get hotter. The heat will start to conduct through the wall and if the wall is a poor
conductor, i.e. has a high resistance to heat transmittance, it will conduct the heat away slowly;
this gives the heated surface a chance to reach a higher temperature. So the higher the resistance,
the greater the temperature difference between the heated side and the other.
This makes it possible to measure the thermal resistance of a structure by reference to the
temperature difference between one side and the other, caused by applying heat at a rate of 1
watt to each square metre. The notation for this is °C m2/W. If you look at the notation for
U-values, in Chapter 1, you will see that it is W/m2oC, which is the resistance notation reversed.
This is logical and indicates that U-values measure the heat transmittance of a structure, which is
the reverse of its resistance.
High resistance means low transmittance; when one is multiplied by the other, the result is
always unity (1). A material with a resistance (R) of 0.25°Cm2/W has a U-valueof 4 W/m2oC. To
find one from the other we just divide it into 1. This is known as a reciprocal relationship.
With all materials, except air, resistance is proportional to thickness and the first column of
Table 4.1 gives the thermal resistivity of various materials per millimetre of thickness. The
resistance of air depends on other factors and can affect a structure in two areas.
Air spaces within a structure
Heat is transmitted across air spaces by radiation, conduction and convection. As the gap
widens, conduction becomes more difficult but convection easier. After 25mm, as fast as the
resistance increases because of poorer conduction, it is reduced by greater convection. There is
therefore no advantage from a heat-loss point of view in having an air gap wider than 25mm.
This gives air space resistances and the resistances of boundary layers.
Air boundary layers
Because of friction between the air and any surface, a thin, almost stationary, layer of air is held
against the surface, which has a resistance to heat transfer. The resistance is less on outside
surfaces because of wind movement and is less in windy exposed sites than sheltered ones.
Calculation of U-values
Using Tables 4.1 and 4.2 we can work out the U-value of most structures; a worked example of
the method for a brick wall appears later.
Floors can have U-values worked out for them in the same way as walls and roofs but the floor
does not lose heat evenly over its area. The ground beneath the floor tends to reach a steady
temperature close to that of the house so that the main heat loss is from the edges. A large floor
has a smaller ratio of edges to area than a small one. Also regularly having a boiler service can
greatly reduce the heat loss from room to room.
In addition, not all houses are detached, with all four edges exposed. Table 4.3 gives U-values
for different floor areas and a correction factor according to the number of edges exposed.
If your house is built on one continuous concrete slab which it shares with others, should you
work to the overall U-value or the value for just the area you occupy? Generally it is better in the
interests of accuracy to work to the value for your bit only. If you occupy the end house in a
block of five, your floor will lose more heat than your neighbour’s, who has only two edges
Most people in the trade work to a single U-value for floors regardless of size, mostly because
they don’t know of the variations and because the subject isn’t mentioned in many books. The
few who know do not think it worth bothering about, but they realize that the heat loss from the
three walls of an end house is more than that from the two walls of a middle house. So why not
allow for the third outside edge of the floor?
You may wonder if it is worthwhile using separate values for each room in a house. Usually it is
not, because the areas are quite small and the rooms are interconnected so that temperatures
tend to even out.
Where the temperature on one side of a structure is different from that on the other, there is said
to be a temperature gradient and this varies in proportion to resistance. For instance, if one third
of the resistance occurs across one section of a structure then one third of the temperature difference
also takes place across that section.