When it comes to building design, U value calculations play a crucial role in optimising the thermal performance of new structures by evaluating how well different materials retain or lose heat.
By offering insights into which building elements are better insulators, U values enable project stakeholders to design buildings that consume less energy – resulting in lower heating and cooling costs and reduced carbon emissions during use.
In this guide, we cover everything you need to know about U values, including their importance, calculation methods, and relevance to UK Building Regulations.
Understanding U Values: What Is U Value?
U values, also known as thermal transmittance, measure the rate at which heat transfers through a structure (whether a single material or composite).
They are typically expressed in watts per metre squared kelvin (W/m²K) and quantify the insulation performance of the materials being measured. A lower U value indicates less heat loss and thus better insulation.
This is different to a G value, which is a coefficient used to measure the transmittance of solar gain through glazing – that is, how much heat is transmitted through a window from the sun’s rays. The value of G is a scale between 0-1, whereby a high G value of 1 represents the full transmittance of full solar energy.
Why Are U Values Important in Building Design?
U values are crucial in building design because they directly measure how well a building element, such as a wall, roof, or window, insulates – indicating how much heat is lost through that element.
This plays a key role in determining a building's overall thermal efficiency, energy consumption, and occupant comfort levels. In short, a lower U value signifies better insulation and greater energy efficiency for the overall building.
Building Regs U Values: Building Regulations Part L
Building Regulations Part L (Conservation of Fuel and Power: Approved Document L) is statutory guidance setting standards for the energy performance of new and existing buildings in the UK.
The Government’s approved methods for calculating the energy efficiency of buildings include SAP (“Standard Assessment Procedure” for domestic dwellings) and SBEM (“Simplified Building Energy Model” for non-domestic buildings).
Both methods use U values to assess building fabric performance, which is one part of the overall energy efficiency calculation. Measurements are set against notional buildings, which presents the worst case permittable. Project stakeholders should aim for higher U values to ensure compliance with Part L.
How to Calculate the U Value of a Building Element
To calculate the U value of a building element, you will first need to find the sum of the thermal resistances of each material layer within the element, including the internal and external surface resistances.
These figures are known as R values and are calculated by dividing the thickness of the material layer by its thermal conductivity (lambda value). You will then need to take the reciprocal of that sum.
The U value calculator equation is:
U value = 1 / (sum of all R values)
To comply with U value Building Regulations, U values should be calculated following the BR443 2019 “Conventions for U-Value Calculations” document published by the Building Research Establishment (BRE).
How Do U Values Differ from R Values?
While a U value measures the rate of heat transfer through a building component, like a wall, window, or roof, R values measure the thermal resistance of a single material layer, signifying its ability to resist heat flow.
In short, a low U value and a high R value indicate greater insulation. In this way, R values are an inherent part of the overall U value calculation.
U Value & Thermal Performance Calculations from Stroma
Stroma Built Environment’s experienced team of energy assessors and consultants can advise on all aspects of compliance with Building Regulations Part L and Part O, including the calculation of U values to assess the thermal performance of elements in building design stages.
We can also measure the U values of existing buildings using HTC (Heat Transfer Coefficient) testing, which compares the temperature difference between inside and out. Individual elements of existing buildings can also be measured using heat flux plates and thermal imaging. These methods are especially valuable for informing retrofitting decisions.
For more information about Part O and Part L compliance, please contact the Stroma Built Environment team.