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External Walls - Contact & Ventilated Facades

Many older buildings require an ‘energy efficiency’ upgrade of existing external walls to reduce heating bills and increase comfort.

In Central and Eastern Europe, the upgrade of insulation in the external solid walls of high rise flats can increase thermal performance to reduce overall consumption of energy and also increase the building aesthetics.

Design Considerations: Upgrading Insulation in an Existing Masonry Wall

Building with solid masonry walls has been a traditional method of construction for many centuries. However, solid masonry walls offer little thermal insulation. The upgrade of a solid wall requires an insulation layer to be installed either on the inner or outer face of the wall. These options will alter either the internal dimensions or the exterior appearance of the building. Positioning the insulation on the outside of the wall means that the mass of the wall will be heated creating a thermal store in the walls. This is desirable if the property is occupied 24 hours a day, such as a hospital or nursing home. However, a building in occasional occupation will require a quick warm up period which is best achieved by placing the insulation on the inside of the wall – click here for Knauf Insulation internal solutions. Placing insulation on the inside of an existing wall will reduce floor area leading to many designers considering external insulation such as contact and ventilated facades.

Another important design consideration is the prevention moisture ingress through solid walls by using a suitable new cladding or render. Depending on the building type and architectural style, these traditionally include render coats, tile hanging, timber cladding and more recently, ventilated facades and rainscreen cladding. Increasingly, decorative facades which are ventilated are being specified by building managers and architects .
The benefits of a new 'Ventilated Facade' in an existing building:
  • A total solution combining a thermal, aesthetic and waterproofing upgrade
  • Relatively low surface preparation of the existing external walls – brackets can be easily fixed
  • Fast installation without the need for scaffolding
  • Excellent acoustics
  • No 'wet' trades involved
  • Easy maintenance
  • Lower cost

Knauf Insulation Products

Knauf Insulation Cladding Slab is a semi-rigid –slab, non-combustible, unfaced glasswool.
Rocksilk RainScreen Slab is a rock wool slab specially developed for rainscreen cladding applications.

Detailed Design Considerations

This section explains the design considerations for the following external wall systems for new or existing low and high rise buildings:

 

  • External Wall Insulation – Applied Render on Insulation
  • External Cladding – Ventilated Façade and Curtain Walling
  • Internal Insulation

1. External Wall Insulation – Applied Render on 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 older 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 5 metre 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.

2. External Cladding – Ventilated Facades & Curtain Walling

Ventilated FacadeSystems

This modern cladding system consists of a metal framework fixed to the building structure, which supports a decorative cladding panel outside of a ventilated cavity with thermal insulation.
Ventilated facade cladding has 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 wide variety of cladding panels now available offers architects scope for creating attractive yet practical designs for the external fabric of new buildings.
Ventilated façade systems have the following advantages over applied render contact façade systems:
  • Less surface preparation of the existing walls for faster installation
  • More cost effective insulation from lower density glasswool (24kg/m3) compared to stonewool (110kg/m3)
  • Better thermal performance from glasswool
  • More options for external finishes such as pre-coloured aluminium panels, fibre cement or polymer panels
  • Faster installation as ventilated façades are factory made
  • No scaffolding

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 also fast to construct than external insulation and render 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.

3. Internal Dry Linings

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.

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