Non Domestic Building - Retrofit
Technical Toolkit
Building Fabric
Non Domestic Buildings
2.2.9
Retrofit
Building Fabric
NZEB Building Fabric
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Minimise heat loss and therefore heat demand
Minimise thermal bridging
o Ensure continuous layer of insulation around the building fabric envelope
Maximum permitted U-values:
o Foundation/Ground floor = 0.13 W/m2k
o Walls = 0.12 W/m2k
o Flat roof & Pitched roof = 0.11 W/m2k
o Windows = 1.5 W/m2k
Maximum permitted Primary Energy Demand = varies for different building types
The building fabric relates to the elements and materials used to construct a building. The building envelope consists of
the elements which separate the external environment from the internal environment, or in other words it is the ‘skin’
of the building. The aim of the building fabric is to create an acceptable level of thermal comfort for the particular use of
the building. CIBSE Guide A, Table 1.5 gives criteria for thermal comfort levels for a large variety of building types, such
as airports, banks, schools, sports halls and hotels.
This section outlines the physical building elements in terms of current guidelines and regulations.
Building Regulations Part L2B covers major renovations in buildings other than dwellings, and outlines the design
characteristics that should be adhered to in order to minimise energy consumption. When carrying out major
renovations, the building regulations should be adhered to in similar manner to a new build.
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The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.
Non Domestic Building - Retrofit
To reach NZEB standards, maximum permitted U-values for various building fabric elements have been suggested.
These are discussed in more detail for each building fabric element, and in the insulation section of this toolkit. The
maximum Primary Energy demand for non-domestic buildings for various building types has been discussed in Section
2.1.8: Practical Aspects.
Heat Loss
The building envelope must be durable, water tight, structurally sound, secure, aesthetically pleasing and economic.
However, it must also be built in such a way as to reduce its energy requirements, by minimising heat loss in order to
reduce the amount of energy required to heat the building. The building fabric, therefore, must strike a balance
between the different requirements in terms of thermal comfort – including ventilation and daylight, whilst also
providing protection from thermal and moisture elements. The fabric design is a major factor in determining the
amount of energy a building will use during its lifetime.
When renovating a building, the building fabric can be upgraded to reduce heat loss through the building fabric
elements by addition of insulation layers. This is harder to do than during construction of a new building, as the main
structure and building fabric elements are already in place. Therefore, methods of insulation will depend on the existing
structure type. Each structure element is discussed in more detail throughout this chapter.
The following are regulations and Accredited Construction Details (ACD’s) for building construction elements:
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Energy Performance Building Directive (EPBD, 2010)
Department Of Community & Local Government - Part L Accredited Construction Details
The heat loss (or gain) to a building through the building fabric is found by adding the fabric heat losses to the
infiltration heat losses.
Qtotal = ΣQfabric + Qinfiltration
Qfabric = U.A.Δt (W)
Qinfiltration = ρ.Cp.V.Δt
Where:
A = area of the envelope (m2)
Δt = difference in temperature between inside and outside ambient (0C)
ρ = density of air (kg/m3)
Cp = specific heat capacity of air (J/kgK)
V = air volume flow rate due to external wind pressure (m3/s)
The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.
Non Domestic Building - Retrofit
V=
Where:
Vol x N
3600
Vol = volume of the room (m3)
N = air change rate assumed due to external wind pressure (air changes per hour)
The equation for heat loss due to infiltration can be simplified to:
Qinfiltration =
N . Vol. Δt
Typical air change rates vary depending on the nature of construction and the exposure of the site. Values can be
obtained from CIBSE Guide A, Tables 4.13 - 4.21
Fabric Thermal Properties
The properties of the buildings materials will determine how that building will respond to the surrounding environment,
both internally and externally. There are two classes of construction fabric:
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Opaque – walls, roofs, floors etc.
Glazed – windows, rooflights etc.
The fundamental properties of each building fabric element which affect the thermal properties of the material are
described in detail in Chartered Institution of Building Services Engineers Guide A and Chartered Institution of Building
Services Engineers Guide F, and include:
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Density – mass per unit volume.
Specific heat capacity
Thermal conductivity and resistance – ability to conduct heat through its surface
Emissivity
Solar transmittance
Solar absorptance & reflectance
Thermal transmittance (U-values)
Thermal admittance (Y-values)
The thermal performance of the building fabric plays a vital role in whether a home is compliant with the building
regulations, in terms of its contribution to attaining the overall energy and emissions target by limiting heat loss. The
amount of heat lost through each element of the building fabric is shown.
The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.
Non Domestic Building - Retrofit
Fig1. Heat Loss From Building (Source: Carbon Trust)
Thermal Comfort
Sections 1.2.2 and section 1.3 of Chartered Institution of Building Services Engineers Guide A discusses thermal
comforts, its components and what factors affect thermal comfort in a building. Although perception of thermal
comfort differs from person to person, there are guidelines that should be taken into account when designing
improvements to a home in terms of the main factors of thermal comfort, which are:
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Temperature
Humidity
Air quality (velocity and freshness)
Lighting levels
Noise levels
Table 1.5 of Chartered Institution of Building Services Engineers Guide A outlines the parameters for thermal comfort
for varying building types, in terms of recommended acceptable temperatures, humidity, air changes, lighting levels and
noise levels.
U-Value
The thermal transmittance, or U-value is a measure of the rate of heat flow across or through the element, and is
measured in w/m2K. The higher the U-value, the greater the rate of heat flow through the material. It is based on the
conductive capabilities of materials as well as the thickness of the layer within the envelope.
The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.
Non Domestic Building - Retrofit
Chartered Institution of Building Services Engineers Guide A, Chapter 3 contains Tables with thermal conductivity and
thermal resistances that can be used to calculate U-values. U-values for individual layers are then used to calculate an
overall U-value for bridged layers.
Fig2. Thermal Transmittance through multilayer element (Source: Unknown)
Y-Value
The thermal admittance, or Y-Value is the measure of the rate of heat flow between the internal surface of the building
and the air temperature of the room, and is measured in w/m2K. The thermal admittance of the material creates a time
lag between the heat transfer, which is determined by the materials nearest to the internal surface.
The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.
Non Domestic Building - Retrofit
Fig3. Thermal Admittance Of Multi Layer Element (Source: Unknown)
Thermal Mass
The thermal mass of the building is the amount of heat storage capacity the building fabric has. The heat stored is used
when required to heat or cool a building. Thermal mass storage is best suited to buildings in a cold climate that are used
continuously, as it helps maintain a steady internal temperature. There are 2 elements that quantify thermal mass:
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Specific Mass – Total Mass of building / Floor are of building (kg/m2)
Response Factor – related to the thermal admittance
Thermal Bridging
Thermal bridging occurs at any junction between building fabric elements, and can cause a reduction in internal
temperature and an increase in heat loss due to a break in insulation. There are two types of thermal bridging:
1. Repeating thermal bridges – bridges that occur at regular patterns within the building element, by items such as
cavity wall ties, mullions, noggins, joists etc. This type of thermal bridge is included when calculating the U-value
of the building element
2. Non-repeating thermal bridges –bridges that occur at junctions between elements such as walls, roofs and
floors, around doors and windows etc. The heat loss at these bridges are calculated separately
The images shown below show the importance of closing the thermal bridge. The orange colour around the window
(highlighted in green) shows the amount of heat escaping through the building envelope, due to incorrect insulation
around the window lintel, which has left a thermal bridge.
The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.
Non Domestic Building - Retrofit
Fig4. Thermal Bridging (Source: MIT Field Intelligence Laboratory)
The Sustainable Energy Authority Ireland has developed a thermal bridging spreadsheet, allowing the thermal bridging
factor to be calculated:
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SEAI Thermal Bridging Factor Application
SEAI Thermal Factor Spreadsheet Tool
There are a number of computer simulation programmes that can be used during the design stage of a building, in order
that different materials, thicknesses and layers can be tested to ensure the required design specification are met:
 IES VE (Virtual Environment)
 SBEM
 DEAP
Further Information:
Low Energy Architectural Research Unit - Information On Building Fabric – CLEAR Project, (Comfortable Low Energy
Architecture)
Low Energy Architectural Research Unit - Basics Of Thermal Comfort
European Union Law - Energy Performance Of Buildings Directive
Department Of Community & Local Government - Part L Accredited Construction Details
Carbon Trust - Building Fabric Manual
Leeds University – Low Carbon Housing Learning Zone - Information On Building Fabric
The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.
Non Domestic Building - Retrofit
Leeds University – Low carbon Housing Learning Zone - Information On Thermal Bridging
Sustainable Energy Authority Ireland - Thermal Bridging Calculation Tool
Video links:
Energy Quarter - Thermal Bridging
Energy Quarter - Air Tightness
The sole responsibility for the content of this toolkit lies with the authors. It does not
necessarily reflect the opinion of the European Union. Neither the EACI nor the
European Commission are responsible for any use that may be made of the
information contained therein.