The insulation is normally a mineral wool quilt. Built-up
cladding systems are assembled on site. The design and components
used are usually part of a proprietary system. There are three main
positions for the insulation:
It should
be noted that the Proportional Area Method of calculating U-values
in
BS EN ISO 6946 does
not adequately take account of the effects of metal spacers within
insulation, because the heat flow is non-linear. Therefore
alternative methods must be used. Finite element analysis is
required to carry out accurate thermal modelling, but some
simplified methods have been developed:
Alternatively, the Knauf Insulation Technical Advisory Centre
can calculate the thickness of insulation needed to achieve
specific U-values, including the effect of thermal bridging by rail
and bracket and Zed spacer systems only.
Vapour Resistance and Condensation
Risk
The risk
of condensation within the construction varies with the conditions
inside the building. These conditions are dependent on a number of
factors and it is important that the designer assesses the risk at
an early stage so that appropriate systems and materials are
used.
BS EN
ISO 13788:2001
contains a method for calculating the necessary thermal quality of
building envelopes to avoid either condensation or mould growth.
The British Standard categorises buildings into a number of
‘climate classes’ depending on their internal environment, as shown
in the table below.
The f values in the table are the
temperature factors necessary to avoid condensation. They can be
used to calculate the minimum internal surface temperature to avoid
condensation. The methodology is given in the British
Standard.
Minimising the
Condensation Risk
There are
two main ways to minimise the risk of condensation in this type of
construction:
- Providing adequate ventilation to replace humid air with drier
air as close as possible to the water vapour source, using fan
assisted air extraction if necessary.
- Stopping relatively warm, humid air reaching colder surfaces,
by including a vapour control layer on the warm side of the
insulation and providing some ventilation to disperse any water
vapour which does get through.
Vapour control layer
A vapour control layer is required in the roofs of all grades of
building. This is to restrict the amount of water vapour, from
inside the building, which enters the construction by diffusion and
air leakage. The vapour control layer can be formed by ensuring
that the laps in the metal liner are well sealed. Alternatively a
separate membrane can be used. In both cases, the vapour control
layer should be continuous.
The vapour control layer should be integrated with and be
sealed to other building elements such as, roof to wall
connections, adjoining masonry, upstands and roof penetrations such
as rooflights. Joints in the vapour control layer should be
minimised and suitably sealed. Penetrations should be avoided
wherever possible, but where these are necessary, they should be
suitably framed with upstands or curbs to permit the installation
of vapour seals.
In Humidity class 5 buildings, sealed fasteners, ferrules etc.
should also be used to further improve the effectiveness of the
vapour control layer. Failure to suitably seal penetrations or
connect the vapour control layer to other elements will seriously
downgrade the performance of the vapour control layer. In all
classes of buildings, the rib voids in the outer profiled sheet
should be ventilated.
Mineral wool has negligible resistance to water vapour. The
small quantity of water vapour that diffuses through the vapour
control layer will readily pass through the insulation and be
vented away.
Internal humidity classes and the minimum
temperature factor necessary to prevent
condensation
| Humidity Class |
Building type |
Minimum f Value |
| 1 |
Storage areas |
0.30 |
| 2 |
Offices, shops |
0.50 |
| 3 |
Dwellings with low occupancy |
0.65 |
| 4 |
Dwellings with high occupancy, sports halls, kitchens, canteens
and buildings heated with un-flued gas heaters |
0.80 |
| 5 |
Special buildings, eg laundry, brewery, swimming pool |
0.90 |
Breather membranes
Even where an effective vapour control layer is provided,
condensation can still form on the inner face of the outer sheet.
Under clear night skies, supercooling of the roof sheets below
ambient temperatures can cause water vapour in the roof voids to
form as condensation on the outer sheet.
The provision of a vapour permeable underlay allows water
vapour to pass through it. However, any condensate on the outer
sheet will either fall onto the vapour permeable underlay and drain
to the gutter or, if conditions allow, will re-evaporate and be
carried away by ventilation.
The vapour permeable underlay also prevents air from moving
through, under and over the insulation to generate cold spots and
reduce energy efficiency. Where a vapour permeable underlay
intersects with a penetration or upstand, it should be detailed to
provide a waterproof escape for leakage and or condensation.
Ensure adequate movement of air through rib voids above the
underlay from eaves to ridge. Ventilated openings should be
resistant to ingress of rain, birds and large insects and not prone
to blockage by dust or debris. The vapour permeable underlay should
be certified Class ‘O’ or Class 1 fire performance, otherwise it
may affect the ability to ventilate the profile voids (see Fire
below).
Linear transmission values (psi
values)
In addition to heat loss through the
plane elements, heat loss through junctions and details now has to
be taken into account when establishing the overall heat loss from
the building.
Three dimensional numerical modelling is used to establish the
psi value for each individual junction and detail using complex
calculation programs. The total amount of heat loss through the
linear details must be no more than 10% of the total heat loss
through the plane elements of the construction.
Air Tightness
The Building Regulations now require
that buildings should be reasonably airtight to avoid unnecessary
space heating and cooling demand, and to enable the effective
performance of ventilation systems.
Air permeability standards are: 10.00
m3/h/m2 at a pressure difference of 50 Pa
Several measures can be adopted to achieve satisfactory
airtightness including the provision of a continuous air barrier in
contact with the insulation, sealing gaps around penetrations and
draughtproofing external doors and windows.
Buildings with a floor area in excess of 1000m
2
have to be tested, whilst smaller buildings can show compliance by
confirming that appropriate design details and building techniques
have been used. Further information and guidance can be obtained
from Building Research Establishment Information Paper IP 17/01 and
MCRMA
Technical Paper No 14.
Fire Protection
It is a
requirement of the Building Regulations that external walls shall
resist the spread of fire over the walls and also from one building
to another. The degree of fire resistance which the external wall
must provide will depend upon the size and use of the building and
its distance from any boundary. It varies between 30 minutes and 4
hours.
For more details refer to the
relevant Building Regulations. The fire resistance of a
non-loadbearing external system is determined by
BS 476: Part 22: 1987 (previously Part
8), and the performance is measured against only two criteria,
integrity and insulation.
Under Building Regulations, two
specific provisions are considered in relation to external walls
and the fire resistance of elements. These are for:
- walls within one metre of any boundary
- walls one metre or more from any boundary.
The insulation performance of walls within one metre of any
boundary must be maintained for the full period of fire resistance
required, and both sides of the wall in turn must be exposed to the
fire during testing.
The provision for walls one metre or more from a boundary is
designed to control a fire from inside a building. The wall
specimen under test must meet the required period of fire
resistance for that type of building (integrity of 30 minutes to 4
hours), and it must generally provide insulation for 15 minutes
except in a small number of specific cases where an emergency
escape route is against the side of a building, in which case the
requirement is for 30 minutes of insulation.
Knauf Insulation Crown Loft Roll has been tested or assessed as an
approved insulation material to give 15 minutes insulation time in
the fire wall designs of most of the major cladding system
manufacturers.
A typical cladding system that has been tested to achieve 15
minutes insulation is shown on page 16. Fire walls are tested and
certified as complete systems, comprising cladding sheet, liner
sheet, rails, spacers and fixings, and therefore the system
manufacturer’s design and specification should be followed in
detail.
Noise Control
Profiled metal clad buildings may need to incorporate noise
control measures in order to meet the requirements of various
legislation such as Health and Safety and Environmental Health
regulations. Noise control measures can be grouped into two
categories namely sound insulation and sound absorption.
Sound insulation
This is concerned with the sound being transmitted through an
element of a building. In the case of profiled metal roofs, this
would normally relate to insulation against airborne sound. The
sound reduction index or transmission loss of a building element
are terms which describe the resistance of that element to airborne
sound transmission.
Lightweight roof structures such as profiled metal decks
transmit a significant amount of sound. The acoustic properties of
mineral wool, combined with the relatively high density of the
inner and outer sheets, can reduce sound transmission to acceptable
levels.
This may be particularly important for buildings in or around
airports where aircraft noise could be intrusive. Factories with
high internal noise levels may require low sound transmission
levels to prevent annoyance to occupiers of neighbouring
buildings.
Sound absorption
This refers to absorbing the sound within the same area as the
sound source.This usually involves adding a sound absorbing lining
to all or part of the roof, ceiling or walls. Some manufacturers of
profiled metal cladding systems can supply perforated metal liner
sheets for use in buildings where increased sound absorption is
required.
These systems typically incorporate a sound absorbing layer
behind the liner perforations, followed by a vapour barrier then
thermal insulation quilt and outer cladding sheet. Mineral wool
slabs have excellent sound absorption characteristics, and can be
supplied as standard slabs, or can be fabricated to suit deck
profile, for those liner sheet designs which incorporate deep
perforated profiles.
Cladsafe
Cladsafe is a Latent Defects insurance scheme that has been
developed by the
MCRMA
for metal roofing and cladding of new and refurbishment projects.
Up to 12 years insurance is available to cover design, workmanship
and materials.
The scheme covers an independent technical audit of each
project to review design considerations and suitability of
materials, as well as monitoring of workmanship on site. A claim
can be made without the need to prove fault or resort to
litigation. Insurance should be arranged before work commences on
site.
This is to enable sufficient time for the risk to be assessed
and for the insurer’s technical advisors to carry out an
audit.
Specification Details
1&2) Wall rail and bracket
Crown FactoryClad Roll 40*/37*/35*
......mm thick, to be positioned over the inner lining sheet and
under*/ between* the spacer system prior to positioning of outer
cladding sheet. Insulation to be installed according to system
manufacturer’s instructions. (* delete as required)
3) Wall liner trays
Liner trays fixed horizontally to the
vertical steel members. Crown FactoryClad 40*/37*/35* ......mm
thick, placed in liner trays. Insulation should be cut to
accommodate the tray dimensions and positioned in tray prior to
fixing the outer weather cladding. Crown FactoryClad to be
installed according to system manufacturer’s instructions. (*
delete as required)
4) Underpurlin
Vapour check plasterboard or fibre
cement lining board to be supported in an industrial metal tee bar
system. Crown FactoryClad 40*/37*/35* ......mm thick, to be laid
over lining boards prior to positioning of outer cladding.
Insulation to be installed according to system manufacturer’s
instructions. (* delete as required)
5) Thermal overpurlin
Liner panels to be positioned over purlins and a metal spacer
system secured to the liner and purlin to ensure the full thickness
of insulation is maintained between the liner and cladding sheets.
Crown FactoryClad 40*/37*/35* ......mm thick, to be laid over the
lining sheets and installed according to system manufacturer’s
instructions, with all joints closely butted. Cladding sheets to be
securely fixed in position. (* delete as required)
6) Acoustic overpurlin
Perforated metal lining sheets to be positioned over purlins and
support brackets fixed to the purlins. Rocksilk Universal Slab
RS100, ......mm thick, to be laid over the lining sheets and
overlaid with a vapour control layer. Secure a metal spacer system
through the vapour control layer to the support bracket to ensure
the full thickness of thermal insulation is maintained between the
vapour control layer and cladding sheets. Crown FactoryClad
40*/37*/35* ......mm thick, to be laid over the vapour control
layer and tucked under the metal spacer as each tier is completed
with all joints closely butted. Cladding sheets to be securely
fixed in position. (* delete as required) Insulation to be
installed according to system manufacturer’s instructions.