The revisions to the
1995 version of Approved Document L Conservation of
Fuel and Power introduced in
April 2002 have been introduced as part of the government's programme to reduce
the effects of global warming by limiting greenhouse gas emissions wherever
possible. In construction this means reducing fuel consumption by energy
efficient design of the complete building fabric, limiting heat loss through
thermal bridging in the structure and limiting air leakage through the building
envelope. The details below have been set out to clarify the requirements for
Part L2 ie non-domestic buildings.
Details of methods for showing compliance are available in the Approved Document
itself, the MCRMA Technical Paper 14. This can normally be obtained from their
website
www.mcrma.co.uk
Two factors requiring
consideration in order to establish compliance with Approved Document (AD) L2
are the U-value requirements of . 25
W/m²_k and the
f-factor required to prevent condensation risk on the internal
surfaces of the building.
You are required to consider the two
elements of the building envelope: plane elements and linear details. Plane
elements require U-value and f-factor calculations. Details require f-factor and
Psi-value figures which will be dealt with seperately.
Various systems can be modelled in accordance with the Approved Document L2
using Trisco, a three-dimensional sofware package which is fully compatible with
BN EN ISO 10211-1/10211-2.
The heat flows must be modelled in this manner for all standing seam systems
which contain repeating point thermal bridges ie clips or halters in the plane
element. The effect of insulation tucked under bar and bracket systems is
covered in the model.
The U-value is then calculated from these results.
The point thermal
bridges caused by brackets (ie Ashgrid) or fasteners in plane elements do not
lower the f-value below 0.9 which mean they would not impose a risk of internal
surface condensation with Humidity Classes 1 to 5.
In the case of air conditioned buildings in which the internal humidity is
controlled independently of the external environment the set values of the
temperature and humidity should be used to calculate the internal moisture load.
Following research commissioned be the MCRMA, the BRE report of 2000 concluded
that " metal liners provide an effective vapour control layer in twin skinned
metal roofing construction. So long as the cladding is installed to a reasonable
standard with a well sealed liner, it is not necessary to use a separate plastic
VCL or breather membrane in most applications."
With more complex build-ups in very high temperature/humidity buildings, extra
precautions are still needed in both built-up and composite systems.
Constructions using structural decks usually require a separate VCL.
The f-factors and Psi-values
performances are required to limit heat loss through linear thermal bridges.
This is heat loss through the thermal bridge over and above the heat loss
through adjoining plane elements. The Psi-value is used to establish the heat
loss through linear thermal bridges such as ridge details, valley gutters etc.
The total Psi-value should not exceed 10% of that through the plane areas as
specified in IP17/01.
If this figure is exceeded then it is necessary to modify individual details to
reduce the loss through the bridges.
Standard details of "robust" details are also given in MCRMA Technical Paper
14.
In order to determine the total heat loss through the Plane Area , the square
meterage of each plane element is multiplied by its U-value to give the total
loss (AU) and the AU values added to achieve the total.
The total of the Psi-values
for the thermal bridges is then determined by multiplying the length of each
bridge by its Psi-value. The figures are then added together to give the new
total linear Psi-value.
It is much better to remove the need for a survey by demonstrating to a client that the cladding has been properly designed and installed than to run the risk of potential delays and contractual difficulties due to subjective interpretation and limiting factors of a practical nature. Please refer to MCRMA Technical Paper 14.
Air Permeability
It will be necessary to
demonstrate compliance with a report from a "competent person" that appropriate
design details and construction techniques have been used or for buildings over
1000 M² floor area, to carry out an air leakage teat. The acceptable limit is
10M²/Hour/M² at 50 pascals. This comes into force fully as of
30th September 2003.
It has been shown in tests carried out by the BRE that correctly installed
built-up systems, incorporating sealed profiled metal liners, to form an
effective vapour control layer in twin-skinned metal roof constructions, can be
expected to achieve an air permeability well within the limit. An example
calculation is given in MCRMA TP 14 giving a result as low as 0.23m³/h/m².
Project specific advice should be sought where separate VCL's are required or
if there is any doubt as to requirements. We recommend that side and end
laps/joints and all perimeter joints should be effectively sealed, not only to
reduce air leakage but to provide vapour control. Attention must be paid to
leakage through all other associated elements within the envelope. It is also
important to remember that all other parts of the building need to comply.
However well designed a system is, it
will fail and require expensive retesting, possibly delaying completion of the
contract by many weeks, if it has not been well installed. It is therefore
essential installation is carried out by experienced contractors, is well
supervised and follows the relevant guidance.
Some buildings covered by L2 but employ "domestic style" details such as
schools or small office buildings may need to employ the "robust construction
details" for domestic type constructions given in a guide published in
association with the Approved Documents although the guide is mainly applicable
to part L1.