In the course of our day to day business we encounter plenty of customers, builders and trades who find U values a little confusing especially when it comes to understanding what the U value actually means and how it will affect or benefit the performance of a building, so we have compiled a brief ‘U-Value for dummies’ style explanation to help.
Are we assuming that if you are reading this you are a dummy? Certainly not……… but would you be happy explaining what a U value is, how it is used or how to calculate it to someone else? Probably not, (unless you are qualified to do so) yet U values crop up again and again in all sorts of places from the Building Regulations, to sales literature from manufacturers and magazine articles, so it’s worth a run through the basics to help understand what they are so next time you hear someone using the term when talking about building you will have a better grasp of whether or not they have a grasp of what a U value is and what it actually means.
The basics about U values?
U-values measure how effective a material is an insulator. The lower the U-value is, the better the material is as a heat insulator.
U-values are generally used to describe the thermal performance (heat loss) for a section of construction that involves several materials – such as a wall made up of timber, insulation and plasterboard. They are used as a general guide to the performance of a building element.
U-values (sometimes referred to as heat transfer coefficients) are used to measure how effective elements of a buildings fabric are as insulators. That is, how effective they are at preventing heat from transmitting between the inside and the outside of a building. Along with U values you often hear R-values, and R value is a measure of thermal resistance rather than thermal transmission, they are often described as being the reciprocal of U-values, however R-values do not include surface heat transfers – more on this later.
It is generally accepted that the lower the U-value of an element of a building’s fabric, the more slowly heat is able to transmit through it, and so the better it performs as an insulator. Very broadly, the better (i.e. lower) the U-value of a buildings fabric, the less energy is required to maintain comfortable conditions inside the building.
Thermal performance is measured in terms of heat loss, and is commonly expressed in the construction industry as a U-value or R-value. U-value calculations will invariably be required when establishing construction strategies. A number of the terms have subtly similar meanings, and conflicting interpretations can be found across the internet. The various terminologies, and how they relate to one another, are explained below.
What is a U-value?
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 the inside (usually) to the outside. 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 quiet good at holding-up the heat getting to the outside.
A ‘Component’ might be a homogeneous material (such as a concrete retaining wall) or a series of materials in contact (such as in a cavity wall).
The technical name for which we use the shorthand ‘U-Value’ is Thermal Transmittance.
The U-value of a building component like a wall, roof or window, measures the amount of energy (heat) lost through a square metre (m2) of that material for every degree (K) difference in temperature between the inside and the outside.
Before we start looking at what that means, let’s sort out the units we use to define it.
- Energy flows along in watts (which is a measure of energy in ‘joules’ flowing over a period of time in ‘seconds’ ).
- Temperature is measured in degrees Kelvin – which practically is degrees Celsius to the rest of us.
The actual equation involves a few more ‘values’ as you can see from the opening equation which when put together gives us the U-value of our wall or window. We’ll look at those in a moment, but the essential equation is this:
U = 1/R in W/m2K or Watts per square metre per degree Kelvin
Example of how U-values work:
The U-value of a single sheet of glass as found in a traditional window pane is 6.0W/m2K – which means that for every degree of temperature difference between the outside and the inside, a square metre of the glazing would lose 6 watts. So for example, if the temperature difference on a typical cold day was 15 degrees, then the amount of heat loss would be 15×6 = 90 watts per square metre. That’s a lot of heat!
By comparison, the U-value of a modern piece of triple-glazing can be as low as 0.7W/m2K – which is not very much heat at all.
‘R-value’ (reciprocal of U-value) means the Thermal Resistance or how much of a fight the material puts up against the heat passing through it, for a given thickness and area. The R-value is expressed as m2K/W
The heat flow through a building construction depends on the temperature difference across it, the conductivity of the materials used and the thickness of the materials. Of course the temperature difference is an external factor. The thickness and the conductivity are properties of the material. A greater thickness means less heat flow and so does a lower conductivity. Together these parameters form the thermal resistance of the construction.
If the component is a composite (consisting of several material elements), the overall resistance is the total of the resistances of each element.
A construction element with a high thermal resistance (e.g. rock wool), is a good insulator; one with a low thermal resistance (e.g. concrete) is a bad insulator.
Example of R-values:
100mm of wood fibre insulation board would have has an R value of 2.6 m2K/W whereas in comparison
100mm of glass fibre insulation batt would have has an R value of 2.2 m2K/W – which makes the wood fibre more resistant to heat loss.
The ‘R-Value’ too has its own equation that picks up on yet another ‘value’:
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’
The lambda (λ) value, or the Thermal conductivity, or ‘k-value’ of a material, is a value that indicates how well a material conducts heat. It indicates the quantity of heat (W), which is conducted through 1 m² wall, in a thickness of 1 m, when the difference in temperature between the opposite surfaces of this wall equals 1 K (or 1 ºC). In practice λ is a numerical value expressed in terms of W/(mK). 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.
That’s quite enough ‘values’ for now!
Calculating a building element’s U-value
Below is an example of how to calculate a rough U-value of a typical UK cavity wall, though with a 100mm cavity. More accurate calculations will involve extra data including loss through thermal bridging; thermal bypass as well as extra materials such as mortar joints.
Therefore the overall wall element U-value = 1 / R = 1 / 4.16 = 0.24 W/m2K
Completing the calculation
As calculation of U-values can be time consuming and complex (particularly where for example cold bridging needs to be accounted for), numerous online U-value calculators have been released. However, many of these are only available on subscription, and those that are free tend to be too simplistic. Another option is to request a calculation from for example an insulation manufacturer, whose product is being specified.
Building Regulations Approved Documents L1A, L2A, L1B and L2B in England and in Wales all refer to the publication BR443 Conventions for U-value calculations for approved calculation methodologies , while the companion document U-value conventions in practice. Worked examples using BR 443 provides useful guidance. The two main commercial U-value calculators are supplied by Build Desk (Windows only) and BRE (Windows only). The Build Desk calculator is about as comprehensive and user-friendly as they come, but at a hefty annual licence fee. Both applications come as Windows-only which is a pain to Mac users.
Two free handy Apps for IoS are: U-Value Calculator from Mark Stephens of TeachPassiv, which needs manual entries; and U-Value Insulation Calculator from Dorada App Software.
Calculations such as these are used to help confirm the predicted behaviour (and compliance) of a building element, but before you consider it job done a quick canter through why relying too heavily on U values alone can result in poor performance.
Is there a problem using U values alone to choose building materials?
The answer is yes. First is because the way heat is transferred in buildings is not simple and involves different mechanisms which are not accounted for in a single calculation and second how individual structures behave can completely negate any expected performance predicted solely on U values.
We need to begin with heat transfer; this is the process of thermal exchange between different systems. Generally the net heat transfer between two systems will be from the hotter system to the cooler system.
Heat transfer is particularly important in buildings for determining the design of the building fabric, and for designing the passive and active systems necessary to deliver the required thermal conditions for the minimum consumption of energy.
The thermal behaviour of a system is a function of the dynamic relationship between the principle mechanisms, conduction, convection & radiation. In the UK by far and away the biggest mechanisms for heat loss is conduction and convection caused by air movement i.e. leaky buildings, despite some manufacturers’ claims radiation losses form only a tiny part of a buildings potential heat loss in the UK climate.
Below is an illustration of how different build ups can have the same U Value but remarkably different ‘phase shift’ which is the ability of a building section to delay heat transfer. An important consideration when designing certain types of building such as rooms in the roof or for lightweight construction such as timber frame.
So how can individual structure’s performance completely negate any expected performance predicted solely on U values?
Take for example a cavity wall; this example is used because it is still the most common form of domestic construction in the UK where a typical U value is (with no insulation) 1.5 W/m²K and Building Regulations require a minimum of 0.18 W/m²K. For an external wall, so obviously some form of insulation is needed, but even when this is calculated in there are other factors that can wreak havoc with a predicted overall mean U value these are.
- External temperature
- Emissivity of materials can have an influence
- Wind speed
- Driving rain
- Permeability (leaking air)
We must remember what building regulations are there for, they are not a quality bible for builders they are minimum standards, taken in isolation they can appear meaningless and can help create inappropriate solutions or even encourage materials to be chosen on a single performance metric that may exclude other consequential benefits, or even worse, encourage a critical breakdown in later function as can be seen with a growing number of retrofitted insulation projects where the consequential benefits of alternative materials (probably dismissed on price) were sacrificed on the altar of achieving the lowest price U value compliance and the result is damp followed by structural damage.
U-value matters, but equally if not more importantly, so does air permeability. Please remember the performance of a wall is affected by other factors not addressed by U-value classification.
Although the U-value laboratory test captures the effects of convective loops within the insulation, it cannot measure the amount of air leakage through a real wall assembly once the insulation is installed. The rate of air permeability in a wall is affected by:
- the density and continuity of the insulation,
- the presence or absence of an air barrier in the wall assembly,
- the wind speed, and
- the pressure difference between outside and inside the wall.
Let’s return to our cavity wall, this now includes integrated insulation which is normally PIR or mineral wool. Cold bridges or thermal bridges are clearly an interruption to the continuity of insulation and thus an increase of the general U-value of the wall. But there is a less obvious type of cold bridge, (shown left), known as thermal looping: an air gap of more than 1mm between the insulation and the inner wall leaf allows air circulation, creating convective currents and leading to a significant reduction of the overall U-value. This was first presented by Jan Lecompte in a paper of 1990, named ‘Influence of natural convection in an insulated cavity on the thermal performance of a wall’. How many of us know about it, and take care of it in our details?
There is one more reason that can needs to be factored in; build quality. All calculations made using building software are made on a presumption of correctly and perfectly constructed elements, although most models allow the addition of tolerances (or error) poorly fitted or badly constructed buildings can not only negate the designed benefits but can cause failure and significantly worse performance than predicted. This doesn’t have to be cowboy building it can be inadvertent, most jobbing builders wouldn’t notice the extended gaps down the side of insulation installed between studs as visually it may appear tight, but as many real life examples show added together such failures can lead to a gap in performance of up to 100% in some cases. So builders need to consider the ease of use when specifying, they also should choose the best product for each element and where special skills or attention to detail is required then this should be part of the builders brief.
What do we conclude about U values?
The U-value is a very useful measurement, but just because you know a product’s U-value doesn’t mean you know everything necessary to predict the real heat flow through a wall, floor or roof. Single metrics like U values (or calorific values given of food for example) are part of the calculation and often only give a broad indication of performance to help direct your choice or meet regulatory minimum standards, for a superior thermal performance you need to be building well beyond Regulations.
To sum up when considering how a building element is constructed consider target U values as a place to start not to end, ensure that the other features of the components are accounted for and always remember that simple easy construction methods allow builder error to be minimised and performance maximised.
With thanks to
www.greenspec.co.uk for technical details, calculations and examples
Additional material from original publication 2010/08/u-and-g-values-unified-theory-of by Ignacio Fernández Solla