How to Calculate a U-value?

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How to Calculate a U-value?

Date Published

What is a U-value?

The U-value assesses the rate of heat loss / gain through all the thicknesses of the combined elements that make up a building component such as a wall, floor or roof. It is measured in units of W/m2.K (Watts per metre squared Kelvin). It is a way of measuring the insulating properties of the building element. The lower the U-value, the better insulated the building element is. So a wall with a low U-value should prevent heat loss / gain better than a wall with a high U-value.

Why do we use U-Values?

The use of U-values allows for the comparison of different build-ups for applications, so the insulating properties of a solid wall could be compared to that of a cavity wall for example, or allows the comparison of two different types of insulation at different thicknesses.

It is the most accurate way of measuring the insulating properties of the application or material and Building Regulations/Standards specifies U-values which have to be achieved when building or refurbishing a building.

Without knowing the U-value of a wall or a floor or roof, you won’t know how energy efficient the whole building will be.

Before you start any building work you should calculate the U-value as part of the design process, to make sure that meets or is lower than that specified by Building Regulations/Standards.

How do We Calculate a U-Value

To calculate a U-Value you have to start with the R-Value or resistivity of the elements that make up the building element you are trying to calculate the U-Value for. The R-Value refers to a material’s ability to resist heat transfer at a certain thickness – the higher the better when looking for an insulation material. It is measured in m2.K/W. The R-Value is calculated as R = l/λ where l = the thickness of the material in metres and λ (lambda) is the thermal conductivity of the material in W/m.K.

U-value formula

U Value is the reciprocal of all resistances of the materials found in the building element. To calculate the U-Value of the building element, the R-Value of all the different components that make up that element will be considered.

U-Value = 1/(Sum of all R-Value)

U-Value (of building element) = 1 / (Rso + Rsi + R1 + R2 …)

where Rso is the fixed external resistance, Rsi is the fixed internal resistance and R1, R2 etc. are resistivity of all elements within the application, including that of cavities within the construction.

This is the basic formula, and by using this you could work out the U-Value for a particular application. However, there are also other factors which need to be taken into account including thermal bridging factors of fixings or stud work.

How to Achieve these U-values?

Different insulation materials will require a different thickness based on their thermal conductivity. We've produced some example calculations for you on our product pages, so you can see what thickness you'd need to specify.

Are the U-values you're specifying the most efficient for the Middle East building stock?

The operational expenditure of a commercial building is a fundamental metric in understanding the valuation, and thus the investment potential, of real estate. The operational expenditure is deducted from the future income streams of a building, and therefore, a higher operational expenditure will reduce future income streams, and as such, give a lower valuation.

Governments across the Gulf are committing to cut carbon emissions and energy demand. With programmes such as Dubai's integrated energy strategy, Saudi Vision 2030, Abu Dhabi Vision 2030 and Qatar National Vision 2030, it is essential that we look at how buildings can help play a part.

So, with energy prices and carbon emissions rising, people are keen to seek out energy reducing solutions. We've found insulation can be pretty effective, however, understanding the Return on Investment (ROI) that can be generated from reduced energy charges by going over and above what is required by local regulations, is essential to insulation being considered as one of the big players.

We asked Mott MacDonald to research the energy usage, carbon emissions and Return on Investment (ROI) when specifying additional insulation solutions versus doing the minimum and complying with local regulations.

"All of the models in each of the study locations showed a reduction in cooling and overall energy consumption when improvements were made to the U-values of the building’s wall, roof and floor elements "— Mott MacDonald

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How to Calculate a U-value?