U-value and heat loss

What is a U-value?

Make yourself a cup of tea/coffee before getting stuck into this!


When we talk about the 'U-value'of a particular component of a building such as a wall, roof or window, we’re describing how well or how badly that component transmits heat from one side to the other. On a cold day in the UK when we’re warm and cosy on the inside of the building, we will be happier the lower the U-value is – because it means that our wall or roof or window is quite good at holding-up the heat getting to the outside. 

The U-value (properly known as ‘thermal transmittance’) is a useful rating number by which the construction industry can describe how good or bad a wall/roof/window/door/ground-floor is at letting heat pass through it. 


A relatively ‘low’ u-value indicates that the wall or roof etc. is poor at transmitting heat and therefore good at helping to retain heat within the building. 



All individual materials like glass, concrete, clay bricks etc., have a U-value. However, the usual way we encounter U-values is where several material elements are compounded to form part of the building envelope. So for example, the outside wall of a house made from blocks, insulation and bricks will have an overall U-value just as a roof comprising tiles, insulation, underlay and plasterboard.

The overall U-value is related to the u-value and thickness of each of the individual elements of the wall, roof or floor. 

In designing a wall, roof or ground-floor, an architect will juggle the types of materials and their thicknesses until the best possible overall u-value for the job is reached. 



How we calculate a U-value




The U-value of a building component like a wall, roof or window, measures the amount of heat (measured in Watts) lost through a square metre (m2) of that material for every degree ( measured in degrees Kelvin, K) difference in temperature between the inside and the outside. So:


U = 1/R in W/m2K or Watts per square metre per degree Kelvin


Where R is the Thermal Resistance of the material - the higher the value, the greater the resistance to the the flow of heat.


R = t/ λ


Where ‘t’ is the thickness of the material in metres and λ is the Thermal Conductivity (sometimes known as the ‘k-value)


The lambda (λ) value, or the Thermal Conductivity of a material, is a value that indicates how well a material conducts heat. The lower the λ value, the better the insulation property of the material.


Examples of Thermal Conductivity

Wood fibre insulation has Thermal Conductivity of 0.038 W/mK

Glass fibre insulation has Thermal Conductivity of 0.044 W/mK

And the Thermal Conductivity of dense concrete is around 1.5 W/mK

In comparison, the Thermal Conductivity of copper is a whopping 401 W/mK – which is why some of your kitchen pans might have copper bottoms.


Some typical U-values


Excellent: <0.10 

Good:  < 0.15

Building Regs: 0.18

Bad : Traditional Cavity Wall without insulation: 1.50

Bad: Traditional solid brick wall: 2.00


Windows ('overall': glass and frames combined)

Good: Usually Triple-glazed <0.85

Building Regs: Usually Double-glazed 1.40

Bad: Double-glazed: > 3.70

Bad: Usually Single-glazed 4.80


Pitched roofs:

Good:  < 0.10

Building Regs: 0.13

Bad: Traditional uninsulated pitched roof: No effective resistance