Basics of U-Values
What are U-values?
Written by David Fraser, Rainscreen System - Business Unit Manager in SFS Group Fastening Technology Ltd.Ā
U-values express the heat loss, or thermal transmittance, through building fabric elements ā 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 elementās 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 buildingās heat loss (around 35% of the total).
ļ»æThe simplified method for calculating U-values is called āthe combined methodā. 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.
ļ»æThe 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 ā 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 howĀ U-value calculations are adjusted accordingly. SFSās Project Builder tool provides tailored solutions for rainscreen facade projects, helping specifiers to find the optimal thermal solution to meet a projectās U-value requirements.
However, the nature of most insulation materials ā which rely on air ffor their performance ā 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.
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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.Ā
U-values express the heat loss, or thermal transmittance, through building fabric elements ā 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 elementās 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 buildingās heat loss (around 35% of the total).
ļ»æThe simplified method for calculating U-values is called āthe combined methodā. 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.
ļ»æThe 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 ā 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 howĀ U-value calculations are adjusted accordingly. SFSās Project Builder tool provides tailored solutions for rainscreen facade projects, helping specifiers to find the optimal thermal solution to meet a projectās U-value requirements.
However, the nature of most insulation materials ā which rely on air ffor their performance ā 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.