The introduction of Building Regulations, Approved Documents
L1A, L1B, L2A and L2B Conservation of fuel and power has been in force since 1st April
2006. These updated and revised regulations have been introduced in an effort to further reduce
the Nation’s carbon dioxide emissions from buildings.
This guide defines basic requirements of the Building Regulation
and details the methods of compliance. For simplicity the guide demonstrates
calculations for fenestration products by the Elemental method. Requirements
for overhead glazing and centre pane U-values, etc are considered in separate
sections. Conservatories that are separated from the host building are exempt,
but note that the interconnecting doors and windows must comply with the requirements
of AD L1 and L2, as if they were external elements.
Introduction to the Requirements
The Building Regulations for England and Wales - Parts L1A, L1B, L2A
Conservation of Fuel & Power. 2006
New Build and Replacement
The current document has been effective since 1 April 2006 and is in four
parts; Part L1A for new Dwellings (domestic applications includes both private
and public housing) and Part L2A for new non dwellings or non-domestic applications.
The required overall U-value for aluminium roof glazing is 2.2 W/m2K. There are different
U-value requirements in the Approved Documents for wood and PVC-U frames (2.0W/m2K)
than for metal-framed glazing (2.2W/m2K). This difference is to allow for
the improved solar gain factors related to slender metal frames provided by
increased glass areas.
Part L1B covers existing Dwellings (domestic applications includes both private
and public housing) and Part L2B covers existing non dwellings or non-domestic applications.
Replacement rooflights may be assessed solely by the centre
pane U-value of the glazing, which shall be a maximum of 1.2 W/m2K. Compliance is achieved by the normal application route
to Local Authority Building Control Departments. Alternatively for domestic
replacement work, by self-certification once an installation company has been
accepted as meeting the requirements of FENSA Ltd *.
FENSA Ltd is a Government-approved private company.
1.Guide to Patent Glazing requirements
of Building Regulation Approved Document L1A & L2A: 2006
The U-value is measured in Watts per square metre per degree
of temperature difference (W/m²K). It is a measurement of the rate at
which heat is lost through a material. The lower the U-value the lower the
heat loss. A whole glazed area U-value is determined from the area weighted
average values of three components, the frame, the centre pane and the insulated
glass unit edge effect.
The U-value requirements for roof and vertical patent glazing
are given in the table below for the Elemental Method: -
For Dwellings the maximum areas for windows, doors and rooflights
should not exceed 25% of the total floor area.
For non-dwellings see the figures below: -
Methods of compliance
There are three methods to establish acceptable thermal performance
needs for an installation. Failure to achieve the desired result by one method
of calculation does not imply that the glazing is unsuitable and by considering
an alternative calculation method the same areas of glazing may comply. The
methods differ for dwellings and non-dwellings, they are: -
Dwellings to Approved Document L1
1. Elemental Method in which the area weighted average U-value
of all the fenestration elements must not exceed the specified values.
There are three methods of establishing the product U-values:
a) Use the Indicative value tables A2 and A3 in the Document
b) By calculation in accordance with the Standards specified in the Approved
Documents and the BRE Guide BR 443 ‘Conventions for U-value calculations’.
c) By measurement (hotbox) on sample windows of size and configuration given
in GGF Data Sheet 2.2.
Note: These tests are generally time consuming and expensive, particularly
on non-standard sizes, compared with calculation methods.
2. Target U-value Method, this is under the control of the building designer
and takes into account insulation levels of individual elements of the building
envelope and the efficiency of the heating system.
3. Carbon Index Method, as with 2 this is under the control
of the building designer, developed from the SAP method of the previous Regulations
Non-dwellings to Approved Document L2
1. Elemental, this is similar to the Elemental Method procedure
2. The Whole Building Method is under the control of the
building designer and is based on achieving a maximum emission or energy consumption.
This permits flexibility with “trade off” between different areas
of the building (e.g. floors / walls / roofs / windows).
3. Carbon Emissions Calculation Method - as with 2 this is
under the control of the building designer. The method allows designs that
have no greater carbon emissions than a notional building of the same size
and shape designed to comply with the Elemental Method.
Note for simplicity only the Elemental method is consider
in this publication.
Test and calculation assessment dimensions –
Windows & Doors
To demonstrate conformity the standard window dimensions
of 1.23 m wide by 1.48 m high are specified in the CEN Window and external
pedestrian door product standard pr EN 14351, defined in BRE Guide BR 443
‘Conventions for U-value calculations’, also the Glass and Glazing
Federation data sheet 2.2 as illustrated below. Alternatively, the size and
configuration of the test sample should be representative of those to be installed
in the building.
Indicative window U-values
2. Routes to compliance
2.1. Indicative U-values. The indicative whole
window U-values given in Tables A2 and A3 of the Regulation and reconstructed
on page 7 of this publication, provide conservative values for guidance on
typical window products. Compliance with the specified U-values should require
no more than a cross section detail of the window framing with dimension of
the thermal barrier separation width, the gap between panes and a statement
of glazing type, with its emissivity.
Variations in framing dimensions and more precise
values make the detailed calculation procedures a preferred alternative.
2.2. Measuring thermal performance by test
2.2.1 BS EN ISO 12567-1 Thermal performance of
windows, doors and shutters-Determination of thermal transmittance by hot
box method: Part 1: Windows and doors
This method tests a window assembly to the assessment
dimensions as stated previously. The limitation of this type of test is that
the assessment sample may not be representative of those to be installed onto
2.2.2 prEN 12412-2 Windows doors and shutters
– Determination of thermal transmittance by hot box method – Part
This method tests the window frame profile with
an insulation panel with thermal conductance of 0.035 W/mK; Frame
U-values determined by this test method should not be used in condensation
2.3 Simplified calculation frame and whole window U-values
BS EN ISO 10077-1 Thermal performance of windows
doors and shutters – Calculation of thermal transmittance Part 1: Simplified
This Standard provides a simplified method of calculating frame and whole
window U-values and requires only basic dimensions of the window framing and
thermal barrier and data from tables to be entered into the equation.
This simplified calculation procedure was used
for the preparation of the table of indicative window U-values (Tables A1
and A2 of the Building Regulation).
A typical calculation to this standard is set
out in Annex A of this publication and the BRE U-value Calculator (http://projects.bre.co.uk/uvalues)
includes window U-values to this standard.
Note. This simplified method is not suitable
for curtain wall framing.
2.4. Numerical calculation
prEN ISO 10077-2 Thermal performance of windows
doors and shutters – Calculation of thermal transmittance Part 2: Numerical
method for frames.
This standard provides a more detailed numerical
approach for assessing frame and whole window U-value, by computer simulation
software. The procedure in the standard, known as the ‘linear’
method, assesses the frame U-value with an insulation panel replacing the
glazing. With the ‘linear’ method of the standard two conditions
have to be simulated to obtain the results, one to calculate the U-value of
the frame, and a second simulation to calculate the linear thermal transmittance
?, of each glazing condition.
A typical calculation to this standard is set
out in Annex B of this publication.
ISO DIS 15099 Thermal performance of windows,
doors and shading devices – Detailed calculations.
The BRE Conventions includes the ‘linear’
method of prEN ISO 10077-2 and ISO DIS 15099, which includes the ‘linear’
method but allows for the alternative ‘edge’ method. It is also
suitable for structural sealant glazing systems (SSG) and condensation prediction
assessments. Both methods produce essentially the same whole window U-values.
A typical calculation to this standard is set
out in Annex B of this publication.
Software, such as Bisco, FRAME, WINDOW 5 and
THERM 5 for the simulations is inexpensive, with WINDOW 5 and THERM 5 being
free issue, including comprehensive instruction manuals, from the Lawrence
Berkeley National Laboratory http://windows.lbl.gov
2.5. Comparison of whole window U-value calculation results by ‘linear’
and ‘edge’ methods.
In calculations based on example D.4 of prEN
ISO 10077-2 set out in Annex B, it will be seen that there is no variation
in the calculated U-value, ie 2.17W/m2K. Any variations in calculated values
will therefore be minimal and well within experimental uncertainty.
Further evaluation confirms this result. An exercise
assessing forty-four window combinations, with aluminium, aluminium wood clad,
PVC-U, steel and wood framed windows with three to five glazing options, by
both ‘linear’ and ‘edge’ methods, the variations between
the methods are insignificant.
A comparison of calculation example results to the three standards is given
in Annex B of this publication.
2.6. Presentation of results
The presentation of results for test or calculation
is specified in the appropriate standard and include typical, cross section
frame details with principle dimensions, material thermal conductances and
Results should be rounded to one decimal place. When the value is less than
1.0, it should be rounded to two decimal places. In accordance with normal
conventions 2.24 becomes 2.2 and 2.25 becomes 2.3 (see also BRE Conventions
for U-value calculations, BR 443 Section 6.)
2.7. Simulators and self-certification
In the future it may become necessary to consult
an Independent Authority who may carry out the assessment or approval of simulators.
In this assessment the simulator will be expected
to carry out the ten example calculations, D.1 to D.10 given in Annex D of
prEN ISO 10077-2: Thermal performance of windows, doors and shutters –
Calculation of thermal transmittance – Part 2: Numerical method for
frames and submit results for approval.
3. Curtain wall/coupled windows
The Building Regulations Approved Document L2
does not make a positive statement concerning composite/coupled windows and
curtain walling other than to refer to Trade documentation, produced by the
CAB and the CWCT. In view of this it is highly recommended that the building
designer consults the curtain walling designers/suppliers at an early stage
of a project in order that a proper and constructive assessment of the building
performance can be made.
Curtain Wall being a cladding fenestration system,
larger than a composite window, usually provides an enclosure to the building
facade. These systems are generally constructed of vertical and horizontal
members, assembled and connected together on site known as ‘stick’
systems or assembled and glazed off site, in panel form, known as ‘unitized’
systems. Assemblies pass in front of and are attached to the supporting structure
of the building (floors etc) providing a grid for the glazing and infill panels
to create a continuous facade to selected areas of the building. The basic
requirement in ADL 2 for such facades is 2.2 W/m²K for glazed or vision
areas, with 0.35 W/m²K for spandrel areas considered as walls. Any internal
walls or structure behind the panel area will improve this U-value.
Considering the Elemental Method of calculation,
the weighted average of the installation (large composite/coupled windows
and curtain walling) can be considered as a “glazed area” with
a U-value of 2.2 W/m²K.
Calculations are to be carried out in similar manners to the CAB document
'Setting the Standard No 4', or the detailed calculations described in section
The percentage of the curtain wall façade glazed vision areas (ADL2,
Table 2) should relate to the entire external wall not only to the areas of
curtain wall facade. Buildings with large areas of glazed curtain wall should
be designed by the 'Whole Building' method or the 'Carbon Emissions' method.
It is recommended that the performance of these large areas be assessed at
the design and planning stage so that any necessary adjustments may be made
to the overall building performance concept.
An alternative approach is proposed in a quartet
of CWCT publications, on the subject but generally relate specifically to
the 40% maximum allowable glazed vision area related to the curtain wall façade
area. i.e. a module with 40% vision area and 60% spandrel area with a weighted
average U-value of 1.2 W/m²K for the whole vision and spandrel area,
allowing for benefits from other building works. This U-value is not applicable
where the 40/60% proportion area arrangement differs. In such cases the area
weighted average U-value has to be calculated.
Pr EN13947 – Thermal performance of curtain
wall – Calculation for thermal transmittance – Simplified method
has been published for public comment but cannot yet be considered or assessment
of standard size curtain wall assemblies.
4. Rooflights and overhead glazing
In accordance with document L, any part of a
roof or inclined glazing having a pitch greater than or equal to 70° can
be considered as a wall.
The procedures for measuring and calculating
the U-values of roof windows and overhead glazing installations are based
on assessing the product in the vertical plane. For sloped glazing an allowance
is made in ADL1 for dwellings, tables A1 and A3. However for non-domestic
buildings, ADL2, no adjustment has to be made. Therefore the compliance of
glazed roof windows, patent glazing and other forms of sloped glazing to ADL1
and ADL2, should be based on products assessed in the vertical plane.
Rooflights and overhead glazing are in a similar
situation to curtain wall facades, insomuch as the CEN standard relating to
rooflights, prEN 12567-2 (Thermal performance of windows and doors -Determination
of thermal transmittance by hot box method - Part 2: Roof windows and other
projecting windows), has been published for public comment but is not yet
The test specimen size is slightly larger than
the vertical window size of 1.23 x 1.48 m as the roof or projected window
has to fit over, rather than into the test rig aperture. The window is tested
in the vertical plane.
Calculation of U-values for roof and overhead
glazing by the “edge method” of ISO DIS 15099 provides suitable
results for the overhanging glazing conditions which are not fully framed.
Figure 3 illustrates this and shows a typical un-framed condition in the vertical
position as required for test purposes.
6. Shop fronts and entrances
"Display windows are glazed areas to display
goods on sale within the building - these are not required to meet the 1 April
2002 disciplines of AD L2.
Vehicle showrooms are classified as display areas up to 3m high adjacent to
ground or footpath level; above this level the glazing should follow the requirements
of AD L2.
Shop entrance doors, vehicle access and other large heavy duty doors need
not be considered in the thermal performance assessment.
Similarly such doors which can be considered for public access when fitted
with mechanical control devices and those likely to be left open for operational
reasons are not required to meet the regulations."
7. Conservatories and Atria
Assemblies of vertical and overhead glazing do
not need to comply if a wall and door(s) separate them from the host building.
Such a wall and door(s) must be in accordance with the requirements of ADL1
and 2 and any heating should be separately controlled. Otherwise the conservatories
and atria should be in accordance with the regulations.
8. Comparison of thermal assessments calculation methods
All methods and standards referred to in this
publication strive to produce an overall window or façade area U-value
but differ in their approach and for this reason it is not possible to directly
compare the separate elements of, for example frame and glazing edge derived
by different methods, see also Annex B3. Whichever method is used it should
be noted that obtaining better values than those from physical test should
not be possible.
In the case of condensation prediction U-frame
values derived by BS EN ISO10077-1 or prEN ISO 10077-2 where the appropriate
glazing is replaced by an insulating panel it will be found that much lower
frame U-values result than are found in the as- built construction. The CAB
publications- Setting the Standard No. 3 and 4 provide useful information
on assessing the surface condensation risk of aluminum fenestration framing.