Detailed Design Considerations
Internal Dry Linings
Building Regulations
Typical U-values for new build external walls are shown
below:
| |
U-value (W/m2K) |
|
| England, Wales & NI |
0.27 |
|
| Scotland |
0.25 |
|
| Eire |
0.27 |
|
When substantially replacing external walls or their external
render, cladding or internal surface finishes as part of a material
alteration or change of use, the Regulations say “a reasonable
thickness of insulation” should be provided.
Thermal Insulation
Internal insulation can be used for:
- lining new solid walls which have an external cladding or
render
- upgrading existing solid walls to improve their thermal
insulation
Placing insulation on the inside of an external wall improves
the thermal response time of the building and is more suited to an
intermittent heating regime.
The following design issues need to be addressed when applying
internal insulation:
- thermal bridging – where insulation is interrupted at the
junctions with partitions, separating walls and openings
- air infiltration – preventing air passing from the cavity
behind the insulation into the building
- condensation on masonry walls – incorporate a vapour control
layer on the warm side of the insulation
- dampness – some existing solid walls may be temporarily damp,
this should be cured before proceeding
Avoiding Thermal Bridging
When a wall is insulated with an insulated dry lining board, there
is no thermal bridging through the board, but there are potential
thermal bridges at the junctions with internal partitions,
separating walls and floors and at window and door openings.
In new construction, the thermal
bridge at the junction with internal walls can be minimised if the
blockwork backing has a low thermal conductivity.
When dry lining existing walls, the dry lining can be returned a
short distance along the internal wall to reduce the thermal
bridge, where this is practicable.
At window and door openings, the insulation should always be
returned into the reveal and the soffit, otherwise there is a high
risk of condensation occurring on the uninsulated reveals.
Where an independent steel framework is used as the lining, there
should always be a continuous layer of insulation on one side of
the steel frame to avoid severe thermal bridging through the steel
members.
Avoiding Air Infiltration
Air infiltration behind an insulated lining and through gaps and
cracks in the external wall can significantly reduce the thermal
performance of the wall. The key design points are:
- seal the joints at the junctions between the plasterboard
surface and surrounding walls and floors
- seal around service penetrations, such as waste pipes
- where an insulated dry lining is used, apply continuous ribbons
of adhesive at the perimeter of each area of wall, at window and
door openings and around service penetrations such as electrical
socket outlets
Avoiding Condensation
The main way of preventing condensation forming is to place a
vapour control membrane on the warm side of the insulation.In
addition, the following detailing points will help to reduce the
amount of warm room air getting behind the insulation:
- seal the joints at the junctions between the dry lining and
surrounding walls and floors
- keep the number of service penetrations to a minimum
- seal around penetrations, such as waste pipes, with an
expanding foam
Dampness in Existing Walls
Dry lining should not be used to isolate a dampness problem. Any
dampness in an existing wall should be cured before internal
insulation is installed. Existing solid walls may be temporarily
suffering from damp following treatment for rising damp or after
curing rain penetration problems or rectifying leaking gutters or
downpipes.
Where dampness has not had time to dry out after remedial measures
have been taken, use an insulated independent lining to provide a
break in the moisture transmission path.
Where doubts remain about the resistance of the wall to rain
penetration, despite repointing or rendering, use an insulated
independent lining. The independent lining should be at least 25mm
clear of the existing masonry and the resulting cavity should be
ventilated to the outside at high and low level. Particular care is
needed to seal the insulated lining at the perimeter, at all
service penetrations and around openings.
Detailed Design Considerations
External Wall Insulation
External Wall Insulation
External insulation of solid masonry walls has the benefit of
keeping the structure warm. The thermal mass of the masonry
external walls helps to keep temperatures stable – storing heat in
the winter and reducing solar gains in the summer.
External wall insulation is equally suitable for new and
refurbishment projects.
Many properties, particularly those built in the 60s and 70s, are
suffering from the deterioration of external wall fabric, rain
penetration, condensation and excessive heat loss due to the lack
of adequate insulation.
These problems can be overcome with external wall insulation
systems incorporating insulation behind a new render finish or
cladding.
In refurbishment projects, the addition of external insulation is
often combined with the replacement of windows, which provides a
good opportunity to return the insulation into the reveals to avoid
thermal bridging.
Render Finishes
The wide variety of colour and texture available in renders offers
architects great scope in creating attractive yet practical
designs.
A number of proprietary renders are available as alternatives to
traditional sand/cement renders. Specialist manufacturers should be
consulted regarding the specific requirements for their
products.
Render on insulation is subject to greater temperature fluctuations
than render applied direct to masonry.
The following precautions should be adopted to minimise
cracking:
- reinforce the render with a mesh
- provide movement joints at maximum 5m centres
- use a light coloured finish to minimise the effects of solar
gain
- use a render incorporating a polymer and/or reinforcement
fibres
Galvanised or stainless steel beads should be provided at
structural joints, at angle changes and around structural openings,
as well as at movement joints. In refurbishment projects, sill and
head drips are likely to need extending or reforming.
The insulation should be returned
into reveals and soffits at openings to avoid thermal
bridging.
Fire
Where a combustible insulation is used behind render, the
reinforcement mesh must be securely fixed through the insulation to
the substrate. In addition, non-combustible cavity barriers must be
provided at each storey to comply with the Building Regulations -
further guidance is given in BR135, published by the Building
Research Establishment.
Detailed Design Considerations
External Cladding
Rainscreen Cladding
Rainscreen cladding systems comprise a carrier framework fixed to
the building structure, which supports the cladding panel, a
ventilated cavity, and thermal insulation fixed directly to the
building structure.
Rainscreen cladding is ventilated, giving the advantage that the
wall element as a whole can breathe, thus reducing the risk of
condensation forming. The ventilated cavity allows air pressure
equalisation to occur, thus precipitation is not inclined to be
driven into the cavity.
The ventilated cavity could allow smoke and fire gases to spread,
cavity barriers must therefore be incorporated at appropriate
intervals in the construction.
The wide variety of cladding panels now available offers architects
scope for creating attractive yet practical designs for the
external fabric of new buildings.
Curtain Walling Systems
Insulated curtain walling systems offer designers a wide range of
options and visual effects using external wall cladding panels
faced with a variety of materials.
Curtain walls differ from rainscreen cladding systems in that
curtain walls are sealed. The air pressure outside the wall may
differ from that inside.
There are three principle types of curtain walling system:
- Stick curtain walling panels, which incorporate mineral wool
insulation slabs, are fitted inside a carrier framework fixed to
the building structure. The carrier framework remains visible.
- Front seal modular panel curtain wall panels, which incorporate
mineral wool insulation slabs, are attached to a carrier framework
fixed to the building structure. Only the gap between panels is
visible and this is usually sealed with a silicone gasket.
- Unitised curtain walls are large, integral factory assembled
units, sometimes one storey high, incorporating mineral wool
insulation, windows, ventilators, doors and opaque facings. They
are taken to site for fast track fixing directly onto the building
structure.
Insulated curtain wall systems are suitable for use on new
buildings and can also be used to refurbish existing constructions.
They are generally quicker to construct than rainscreen cladding
systems because the units are
assembled off-site and delivered to site ready for direct
fixing.
For very fast-track schemes, unitised curtain walls are ideal as
they enable external facing, insulation and internal lining to be
fixed on site in one operation with no need for a separate
framework. They can be faced in a variety of materials including
aluminium, steel, natural stone or glass.
Typical Specification Clauses
1) Internal Dry Lining
Polyfoam Linerboard of insulation thickness ......mm with integral
9.5mm plasterboard to be fixed directly to the external walls by
adhesive*/plaster adhesive dabs*/mechanical fixing* using the
method and materials recommended in Knauf Insulation literature.
(*delete as required)
When the adhesive has set, secure with two ......
(manufacturer/reference) mechanical fixings along the top edge of
each board (for adhesive/dabs).
Alternatively, refer to NBS clause: K10/185
2) Independent Internal Lining
Construct a timber*/steel* stud framework ...... mm away from the
internal face of the blockwork.
a) Infill the gap between the steel frame and the block
wall with Crown DriTherm Cavity Slab ......mm thick, laid in broken
bond. Crown DriTherm Cavity Slab ......mm thick to be cut as
necessary and friction fitted between the steel studs.
b) The independent lining to be insulated with Crown
Universal Slab CS24 ......mm thick. Insulation to be friction
fitted between the studs. The wall panel to be lined internally
with Polyfoam Linerboard of insulation thickness ......mm with
integral 9.5mm vapourcheck plasterboard screwed to the steel studs
at 150mm centres.
c) The independent lining to be insulated with Crown
Universal Slab CS24 ......mm thick. Insulation to be cut as
necessary and friction fitted between the timber studs.
Alternatively, refer to NBS clause: K10/145 or K10/155
3) External wall insulation
Fix Polyfoam Cavityboard ...... mm thick to the substrate with
temporary fixings, with the rebate orientated to shed water
outwards.
Expanded metal lath …... (material/ reference) to be fixed through
the insulation into the wall using ...... fixings. Provide stop and
drip beads around openings, at corners, at base of insulation and
at DPC level. Form expansion joints using …... beads at minimum ….m
horizontal and …...m vertical centres. Horizontal joints to align
with …...(eg window heads). Apply two coat render as specification
clause .…
4) Rainscreen cladding
Rocksilk Rainscreen Slab ......mm thick to be fixed independently
against the building substructure using proprietary insulation
fasteners in accordance with the design specification.
The insulation should be close butted and fitted around all
adjacent parts of the rainscreen support cleats to minimise any
thermal bridging. Once the insulation is firmly in place the
application of the over cladding can proceed
5) Curtain Walling
Rocksilk Universal Slab RS45*/RS60*/ Crown Universal Slab CS24*
......mm thick to be housed within the opaque modular panels as
part of the proprietary curtain wall system specified in clause
…... of the specification. (*delete as required)