Basics of U-Values
What are U-values?
Written by David Fraser, Rainscreen System - Business Unit Manager in SFS Group Fastening Technology Ltd.Â
U-values express the heat loss, or thermal transmittance, through building fabric elements – including floors, walls and roofs. They are given in the units W/m2K, meaning the amount of heat energy in Watts (W) that moves through each square metre (m2) of the building fabric, per degree of temperature difference either side of the build-up (in degrees Kelvin, K).
The U-values calculated for individual building elements can be used as part of whole building calculations that establish compliance with the energy efficiency requirements of national building regulations. As such, U-values tend to be the starting point for anybody specifying building fabric, because of the relative importance of thermal performance.
SFS are NOT thermal engineers, however we can make recommendations based on the point loss (chi value) of our subframe bracket systems.
How are U-values calculated?
There are both simplified and detailed methods for calculating U-values. The scope of simplified calculations is more limited, but the method can be applied to most common constructions.
Whichever calculation method is adopted, the output is the same: the heat loss through one square metre of the building element in question. The U-value is then multiplied by the element’s total area to give its overall heat loss.
External walls typically account for the largest surface area of the building fabric, and therefore the greatest proportion of a building’s heat loss (around 35% of the total).
The simplified method for calculating U-values is called ‘the combined method’. It is set out in the international standard BS EN ISO 6946:2017, which describes how to assess constructions as a series of clearly defined layers, where each layer has consistent thickness and thermal properties.
The standard also includes how to deal with layers that are bridged by other materials, as well as corrections that improve overall accuracy. Although it is a simplified form of calculation, the combined method (and corrections to it) have been verified using the thermal modelling techniques that constitute the detailed/complex calculation method.
The importance of thermal insulation to U-values
Building fabric elements must serve multiple functions, like providing shelter and security, alongside contributing to thermal comfort. While each function is equally important, energy efficiency has come to the fore due to the frequent updating of standards and regulatory requirements – especially as we move towards the built environment becoming net zero.
To meet even the minimum standards set out in national building regulations, it is impossible to avoid the specification of thermal insulation as part of building fabric elements. Insulating materials are considerably more thermally efficient than other common building materials, so their use avoids building elements being many times thicker than they already are.
Learn more about what rainscreen subframes are, and read about how thermal bridging affects the performance of rainscreen wall build-ups, and how U-value calculations are adjusted accordingly. SFS’s Project Builder tool provides tailored solutions for rainscreen facade projects, helping specifiers to find the optimal thermal solution to meet a project’s U-value requirements.
However, the nature of most insulation materials – which rely on air ffor their performance – means they are unable to serve a structural function. Components such as fixings and brackets often have to penetrate through insulation layers, providing a path for heat loss that should be accounted for in U-value calculations.
The combined method includes corrections for relatively simple types of fixings. Where fixings are more complex, such as the brackets that form part of rainscreen subframe systems, detailed calculation must be used to work out their effect on insulation layers. The outputs of the detailed calculations can then form part of the combined method calculations.
Frequently Asked Questions...
What is a U-value in building construction?
A U-value is a measurement of thermal transmittance, showing how much heat energy passes through a building element such as a wall, roof, or floor per square metre for each degree of temperature difference across it. It is expressed in W/m²K and helps specify the thermal performance of materials and assemblies.
Why are U-values important for energy-efficient buildings?
U-values are crucial in building design because they quantify how well a building’s fabric retains heat. Lower U-values indicate better insulation and reduced heat loss, helping buildings meet energy efficiency targets, improve comfort, and comply with national building regulations.
How are U-values calculated for building elements?
U-values are calculated by summing the thermal resistances of each layer in a building element and taking the inverse. Methods such as BS EN ISO 6946:2017 consider material thickness and thermal conductivity to determine overall heat loss.
What does a lower U-value mean for a building envelope?
A lower U-value means reduced heat loss through the building envelope, reflecting better insulation. Buildings with lower U-values consume less energy for heating or cooling and better support sustainability targets.
How does insulation affect U-values?
Insulation materials significantly reduce U-values by resisting heat flow more effectively than standard construction materials. They are essential for achieving regulatory compliance without excessive build-up thickness.
What factors besides material layers can influence a U-value?
Thermal bridging from fixings, structural elements and penetrations can increase the effective U-value. Accurate calculations must consider these influences for realistic building performance assessments.
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Basics of U-Values
What are U-values?
Written by David Fraser, Rainscreen System - Business Unit Manager in SFS Group Fastening Technology Ltd.Â
U-values express the heat loss, or thermal transmittance, through building fabric elements – including floors, walls and roofs. They are given in the units W/m2K, meaning the amount of heat energy in Watts (W) that moves through each square metre (m2) of the building fabric, per degree of temperature difference either side of the build-up (in degrees Kelvin, K).
The U-values calculated for individual building elements can be used as part of whole building calculations that establish compliance with the energy efficiency requirements of national building regulations. As such, U-values tend to be the starting point for anybody specifying building fabric, because of the relative importance of thermal performance.
SFS is not a thermal engineering consultant; however, we can provide product-specific point loss (chi value) data for our subframe bracket systems to support project calculations.
How are U-values calculated?
There are both simplified and detailed methods for calculating U-values. The scope of simplified calculations is more limited, but the method can be applied to most common constructions.
Whichever calculation method is adopted, the output is the same: the heat loss through one square metre of the building element in question. The U-value is then multiplied by the element’s total area to give its overall heat loss.
External walls typically account for the largest surface area of the building fabric, and therefore the greatest proportion of a building’s heat loss (around 35% of the total).
The simplified method for calculating U-values is called ‘the combined method’. It is set out in the international standard BS EN ISO 6946:2017, which describes how to assess constructions as a series of clearly defined layers, where each layer has consistent thickness and thermal properties.
The standard also includes how to deal with layers that are bridged by other materials, as well as corrections that improve overall accuracy. Although it is a simplified form of calculation, the combined method (and corrections to it) have been verified using the thermal modelling techniques that constitute the detailed/complex calculation method.
The importance of thermal insulation to U-values
Building fabric elements must serve multiple functions, like providing shelter and security, alongside contributing to thermal comfort. While each function is equally important, energy efficiency has come to the fore due to the frequent updating of standards and regulatory requirements – especially as we move towards the built environment becoming net zero.
To meet even the minimum standards set out in national building regulations, it is impossible to avoid the specification of thermal insulation as part of building fabric elements. Insulating materials are considerably more thermally efficient than other common building materials, so their use avoids building elements being many times thicker than they already are.
Learn more about what rainscreen subframes are, and read about how thermal bridging affects the performance of rainscreen wall build-ups, and how U-value calculations are adjusted accordingly. SFS’s Project Builder tool provides project-specific outputs for rainscreen façade projects, helping specifiers review thermal options against a project’s U-value requirements.
However, the nature of most insulation materials – which rely on air for their performance – means they are unable to serve a structural function. Components such as fixings and brackets often have to penetrate through insulation layers, providing a path for heat loss that should be accounted for in U-value calculations.
The combined method includes corrections for relatively simple types of fixings. Where fixings are more complex, such as the brackets that form part of rainscreen subframe systems, detailed calculation must be used to work out their effect on insulation layers. The outputs of the detailed calculations can then form part of the combined method calculations.
Frequently Asked Questions...
A U-value is a measurement of thermal transmittance, showing how much heat energy passes through a building element such as a wall, roof, or floor per square metre for each degree of temperature difference across it. It is expressed in W/m²K and helps specify the thermal performance of materials and assemblies.
U-values are crucial in building design because they quantify how well a building’s fabric retains heat. Lower U-values indicate better insulation and reduced heat loss, helping buildings meet energy efficiency targets, improve occupant comfort, and comply with national building regulations.
U-values are calculated by summing the thermal resistances of each layer within a building element and then taking the inverse. Simplified calculation methods, such as the combined method defined in BS EN ISO 6946:2017, consider each material’s thickness and thermal properties to determine overall heat loss.
A lower U-value means reduced heat loss through the building envelope, reflecting improved insulation performance. Lower U-values can support reduced heat loss through the building fabric and contribute to whole-building energy assessments, depending on the full design and use of the building.
Thermal insulation materials significantly reduce U-values because they resist heat flow more effectively than most structural materials. Insulation is essential for meeting minimum regulatory U-value targets without requiring excessively thick construction build-ups.
Thermal bridging through fixings, structural elements, and penetrations in roofs or walls can increase the effective U-value of a building element. Accurate U-value calculations must account for these factors to ensure reliable building performance predictions.