U-values: definition and calculation

U-Values

provided by John Brennan: Senior Lecturer, Edinburgh School of Architecture and Landscape Architecture.

What, Why, When, How, Extras

 

What is a U value?

A U value is a measure of heat loss in a building element such as a wall, floor or roof. It can also be referred to as an ‘overall heat transfer co-efficient’ and measures how well parts of a building transfer heat. This means that the higher the U value the worse the thermal performance of the building envelope. A low U value usually indicates high levels of insulation. They are useful as it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials.

Why use U values?

U values are important because they form the basis of any energy or carbon reduction standard. In practice, nearly every external building element has to comply with thermal standards that are expressed as a maximum U value. Knowledge of how to simply calculate U values at an early stage in the design process, avoids expensive re-working later on in a project. It allows the designer to test the feasibility of their project at an early stage to ensure it is fit for purpose and will comply with regulatory frameworks.

When to use U-values

U values are calculated at stages D onwards in the design process. A critical milestone in any building project is obtaining building regulation approval. For this a SAP calculation for housing or an SBEM procedure for non domestic work is obligatory. As part of this process, the build up of any external construction element must be specified and from this its U value can be derived.

How to use U values

Key points:

  • To calculate the U value of a building element such as a wall, floor or roof, you need to know the build up of that element. Each building material should be positioned properly in sequence. The thickness of each building material also is required.
  • The other key property you need to obtain is the conductivity of each building material. This is a measure of its inherent ability to facilitate the passage of heat. It is normally referred to as a ‘k value’ and values for materials can be found in publications such as the New Metric Handbook and the Architects’ Pocket Guide
  • The properties of the internal and external faces of the constructional element under scrutiny need to be allowed for. These are called external resistances and are fixed values.
  • The U value is defined as being reciprocal of all the resistances of the materials found in the building element.
  • The resistance of a building material is derived by the following formula:
    R = (1/k) x d
  • where k is the conductivity of the building material and d is the material thickness.
  • The formula for the calculation of a U value is
    U(element) = 1 / (Rso + Rsi + R1 + R2 ...)
  • where Rso is the fixed external resistance
  • where Rsi is the fixed internal resistance
  • and R1… is the sum of all the resistances of the building materials in the constructional element.

 

Calculations

U value calculations

 

Design Procedure:

  • Most U value calculations are now undertaken with bespoke software such as the BRE’s U value calculator, or packages produced by companies such as BuildDesk.
    BuildeskScreenshot


  • Ensure you have drawn out your wall, roof or floor element and know the thickness and location of each constituent material.
  • Do not bother trying to work out the U value of a window, element by element. This sometime is required in PassivHaus calculations but manufacturers and reference books tend to give fixed values for various glazing types.
  • If you have a frame construction, then the conduction of heat through the frame element will be very different than through any intermediate insulating element. The key example is timber frame. Here, you need to take sepearate U values through the frame item and the insulating element and average them. Software packages usually have a module to cope with this.
  • Remember to include internal and external resistances. Typically, the internal resistance for a wall is 0.13 m2K/W and the external resistance is 0.04 m2K/W . Again, information about this can be found in the New Metric Handbook or the Architects’ Pocket Book.
  • You can now input data into a software package to derive the U value for the building element under investigation.
  • Current limiting U values [area weighted] in England and Wales are 0.35 W/m2K for walls and 0.25 W/m2K for roofs.

 

Related strategies

Current legislation and targets

 

Take this further

Further Reading:

  • Anderson. 2006 “Conventions for U value calculations : BRE 443” BRE Scotland (for further reading and full explanation of calculation methodologies)
  • Baden-Powell 2001 “Architect’s Pocket Book” Architectural Press (for sample K values and internal + external resistances)
  • Bougdah & Sharples. 2010 “Environment, Technology and Sustainability” Taylor & Francis (places concepts such as the U value in a wider perspective)

Web resources:

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