Introduction to Rainscreen Systems - Knauf Insulation

About Knauf Insulation

The Knauf group was established in Germany in 1932 and today remains one of the world's leading building materials producers, with over 100 manufacturing locations. Knauf insulation has over 30 manufacturing sites worldwide producing glass and rock mineral wool, extruded polystyrene and extruded polyethylene insulation. In the UK the four manufacturing plants can be found at Saint Helens and Cwmbran, where they produce glass mineral wool, Hartlepool where they produce polyfoam, extruded polystyrene and extruded polyethylene and, Queensferry where rock silk rock mineral wool is produced. These four plants ensure that Knauf Insulation remains today the UKs principle insulation producer. 

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Technical Support

At Knauf Insulation we recognise that possessing a thorough knowledge of insulation products is essential to enable us to reduce energy consumption. Out technical advisory centre TAC continues to provide personal higher level Technical Support including U value calculations, condensation analysis and assistance with CAD drawings and NBS clauses.

Support is also available on our website. There's also a selection of sector guides, product data sheets, COSHH data and the new industry leading Knauf insulation solutions for buildings.

Ecose Technology

Knauf insulation ECOSE technology is a revolutionary new formaldehyde free binder technology, based on rapidly renewable materials instead of petrol based chemicals. It reduces imported energy and delivers superior environmental sustainability. ECOSE technology was developed for glass and rock mineral wool insulation but offers the same potential benefits to other products where resin substitution would be an advantage.

Products made with ECOSE technology are more sustainable as, they have the added benefit of using a binder that is up to 70% less energy intensive and traditional binders. ECOSE technology uses rapidly renewable bio based materials to replace traditional chemicals.

The manufacturing emissions are also reduced and since there is no formaldehyde or fennel used with, it contributes to improving the indoor air quality of buildings. The distinctive new look to Knauf insulation mineral wool products is all thanks to ECOSE technology and creates the next generation of insulation products. This binding process leads to a natural shade of earthy brown free from die or colourants. This breakthrough is a result of five years of intensive research and development as part of our commitment to being at the forefront of sustainability.

With ECOSE technology rapidly renewable bio based materials replaced the traditional chemicals to create more advanced and sustainable products. In addition, ECOSE technology could help to reduce our carbon footprint in a world where we all need to tread a little more light. 

Objectives

This module introduces rain screen systems we will see how the choice of insulation thanks design as well as thermal and fire performance. After completing this module, you will: understand the principles of rain screen systems; understand what regulations apply to them; be aware of the different types of insulation and their performance characteristics; understand the importance of insulation fire resistance in rain screen systems.

Programme Structure

This module contains four main sections: 

1. The first section gives a basic overview of ring screen systems, their benefits and applications.
2. The second sections looks of the standard and regulations that apply to the systems.
3. The third sections look at the system in more detail at construction and performance criteria.
4. The fourth section looks at what is important in the selection of insolence.

DIfferent Systems

This is section one, in this section we will look at the main principles and applications for rain screen systems, whatever your building project be it, new build or refurbishment, low rise or high rise residential or commercial, traditional or contemporary. It's likely that there is a rainscreen system that meets your needs.

Ventilated Cavity Systems

The main principle behind rain screens is the provision of a ventilated cavity between the exterior insulation and the cladding.

Rainscreen Cladding

Panel joints are opened so any water that penetrates behind the cladding can be removed by either running down the inside face of the panels and out of the base, removed by the natural stack effect of the ventilated cavity or by the evaporation process.

A rainscreen system is pressure equalised which means the pressure inside the cavity is the same to pressure outside. This is because the cavity and the panel joints are open to the outside air because of pressure equalization water is less likely to be driven into the cavity through the panel joints. Condensation forming on the external wall is also prevented water can condense on the back of the architectural panel, drained down the cavity and evaporated or removed from the cavity by ventilation.

In all instances there is no adverse effect on the structure of the building. Thermal bridging is prevented because the insulation layer isolates the building structure from the external environment by forming a continuous unbroken layer in closing the structural walls. Temperature variation in the load bearing structure is also less likely to occur.

The biggest variation of temperature will actually occur inside the insulation material. In this way the internal structure is protected from high thermal stresses and so the risk of structural movement can surface cracking is reduced.

Rain screen systems help to keep the building cool in hot weather, a very large portion of the solar radiation energy is dispersed before it even reaches the thermal insulation material. Depending on the type and surface of the rainscreen panels some radiation will be reflected. The panel itself absorbs some of the incoming energy and the air in the air gap is heated up, creating a chimney stack effect that conveys continuously fresh outside air into the cavity cooling down the whole construction.

The opposite is also true in cold and inclement weather conditions the rain screen will provide initial protection from direct cold wind, rain and snow and a cavity will provide a dry and ventilated secondary line of defence before the final insulation layer. 

Benefits of Rainscreen Systems

The many benefits of using rainscreen insulation systems include; less risk of condensation forming on external walls, some types of buildings substructure act as a heat store, increasing thermal efficiency; they can prevent cold bridging; there are a huge variety of decorative external envelopes for both traditional and contemporary building styles; they provide a stabilised structural fabric that prevents movement due to thermal shock; they contribute towards sound reduction; and rain screen systems are lightweight when compared to brick and masonry.

Benefits for New Build Applications

For new build applications rain screen cladding systems offer the designer and building many advantages. It's easy to meet the thermal performance requirements of the current building regulations and allow for future changes. It requires less skilled trades than traditional construction. There's a wide range of construction options and design flexibility and it's very cost effective construction times. The rainscreen systems also have huge advantages for refurbishment applications such as upgrades to run down buildings, extending building life through weather proofing and thermal upgrade of properties, remodelling old or redundant buildings for new uses, and because of the inherent flexibility of the systems it has a relatively small impact on the building users, its inhabitants and the interior spaces.

Rainscreen Cladding on a Masonry Substrate

Rain screen cladding is a lightweight alternative to masonry cladding that is fast to erect and available in a very wide range of decorative finishes. We will also be looking briefly at light steel frame systems. Light steel frame system that is fast to erect, has the potential to reduce size and landfill waste and is often used in conjunction with rain screen systems.

Building Regulations and Standards for Rainscreen Systems

This is section 2 in this section we will look at the standards and regulations that apply to rain screen systems, how fire can spread within a system and how the regulations can affect the choice of insulant.

Where rain screen systems are concerned it's necessary to consider several sections of the building regulations.

All of the following approved documents will be relevant to some extent. For the purposes of this CPD module we will be looking at the thermal and fire performance of insulation.

Fire Spread Through Cavities

In some circumstances fire can spread through cavities within external cladding systems, these cavities can be part of the system design such as rain screen cladding or be formed by the delamination or differential movement caused by the fire itself. As flames into these cavities they can become elongated leading to increased flame length of between 5 to 10 times the original length reaching far into the cavity space.

Unchecked fire can spread rapidly through the buildings reaching other flaws via window openings and repeating the process. 

Approved document B

Approved document B fire safety gives guidance for external thermal insulation it states, the external envelope of a building should not provide a medium for fire spread, if it is likely to be a risk to health and safety.

The use of combustible materials in the cladding system and extensive cavities may present such a risk. Approved document B requires that buildings over 18 metres are required to have class 0 national class, or class BS3 D2 or better European class.

Max Dimensions of Cavities in Non-domestic Buildings

In addition, cavity barriers should be used to subdivide any cavity that exceeds certain distances, approved document B gives a table for these. The table however does not apply to all cavities for example, the cavity for a masonry wall, it would however apply to a rainscreen system. In addition, every cavity barrier should be constructed to provide at least 30 minutes fire resistance if it meets the provisions for cavity barriers in appendix A table A1.

BRE Report BR 135 - Part 1

Approved document B makes reference to the BRE report, BR 135- part 1 fire performance of external insulation for walls of multi storey buildings.

BRE Report BR 135 - Part 2

The report describes full scale test data on cladding systems from BS 8414-1:2002 or BS 8414-2 2005. The report divides insulating materials typically external cladding systems into three groups, non-combustible materials, and materials of limited combustibility. Generally, rock and glass mineral wool products which are formed into slabs.

Thermosetting products such as polyurethane foam PUR, polyisocyanurate foam PIR and phenolic foam are typically provided as boards. These products are often faced with materials such as glass fibre or aluminium foil. Thermoplastic expanded polystyrene EPS and extruded polystyrene X PS can be supplied in both the fire retardant and non-fire retardant form, again this material is generally supplied as boards.

The relative fire performance of the three insulation groups are summarised in the BR135.

Fire Barriers

A number of cavity barriers are on the market which employ the use of intumescent strips or coatings in order to seal cavities. Typically fire barriers a mineral fibre slab MFS or mineral fibre lamella MFL which are manufactured from rock mineral wool, the same material used to insulate the building. The key design elements for fire barriers are:

• The fire barrier should form a continuous band through the insulation layer at each floor level,
• Any abutting of material should ensure that no cavity exists for fire to track or pass through,
• The non-combustible material should be bonded and tide back to the wall.

Cavity barriers are made from rock mineral wool a building insulated with rock mineral wool does not require fire barriers.

System Details

This is Section 3 in this section we will look at the systems in more detail looking at construction and performance criteria. 

Rainscreen Cladding

Typical construction consists of brackets, are fixed to the building supporting structure through thermal brake pads and layer of insulation is fixed independently against the building substructure using proprietary insulation fasteners.

Rock mineral wool rainscreen slab is recommended for this application as it is lightweight but rigid enough to resist the compression forces generated when installing the insulation slabs to the building substrate. Rails are then fixed to the brackets which provide a supporting framework for the outer cladding panels to minimise thermal bridging, the brackets should be of sufficient depth to allow the panel support rails to be located clear of the face of the insulation.

A ventilated cavity remains between the insulation and the external cladding. The advantages are slabs knit together preventing cold bridging at joints, lightweight slabs, easy to cut and fit, non-combustible fire safe products, friction fit behind and between cladding rails, water repellent slab, can use rock mineral wool slabs which is a Zero - ODP & Zero - GWP insulation product.

The manufacture of rock mineral wool has a very low impact on the environment. The U value of a proprietary rainscreen cladding system is in part determined by the degree of thermal bridging in the system, typically 110mm-120mm of rock mineral wool rainscreen slabs will achieve a U value of 0.35 but it is advisable to consult individual rainscreen cladding manufacturers for U values appropriate for their system. Rock mineral rainscreen slab is classified as euro class A1 to BS EN ISO 13501-1.

Rainscreen Panels

Rainscreen cladding are available in a wide range of materials textures and colours giving the designer a huge variety of design choices.

Joint details between panels may vary according to the type of building and site conditions. Joints may be left open or baffled as shown. For low rise and small constructions, it is common to leave joints open baffled joints are more likely to be used in high rise buildings and where high wind conditions are expected. Baffle joints can also provide a black joint finish between panels as a desired aesthetic.

Fixings

There are two main types of fixing systems for rainscreen cladding, visible fixing systems and invisible fixing systems. Surface fixing have been widely used in the past and are still common today. They are normally fixed aluminium T sections using rivets or to timber battens using screws. In more recent times invisible fixing systems have become increasingly common and, are divided between mechanical fixings, adhesive fixings, and edge retention fixings.

Light Steel Frame Walls with Rainscreen Cladding

Light steel frame wall systems are often used in conjunction with rainscreen facades typical construction consists of: 

• An inner leaf comprising a double layer of plasterboard;
• A vapour control layer;
• Glass mineral wool roll or rock mineral wool slabs between steel studs;
• Cement fibreboard is fixed to the outside of the frame;
• Rock mineral wool rain screen slabs are close butted and fixed independently against the building;
• Substrate using proprietary insulation fasteners;
• Support brackets are fixed back to the steel studs with a thermal brake pad incorporated between the brackets and the cementitious board;
• Green screen support rails affixed to the brackets to support the final cladding layer;
• A ventilated cavity remains between the insulation and the external cladding.

The advantages are:

glass mineral wool roll or rock mineral wool slabs are manufactured to fit tightly between standard stud widths;

the insulation is compressible and can be cut slightly oversized to friction fit preventing cold air penetrating through joints;

• Glass mineral wool or rock mineral wool products and non-combustible with a euro class A1 classification;
• Improves acoustic performance of walls;
• Easy to install and cut around brackets and penetrations;
• Designed to adapt to a building's minor imperfections making installation an easier process.

Several insulation types can be used with this system glass mineral wool wrong has a thermal conductivity of 0.040. Rock mineral wool slab has a thermal conductivity of 0.035 or 0.037. Rock mineral wool rainscreen slab has a thermal conductivity of 0.035.

This is Section 4 in this section we will look at the selection process for choosing an insolent. When choosing an insolent type many factors must be considered.

Summary

So in summary we've seen how rainscreen cladding systems can be suitable for most building types, both new build and refurbishment; how it can benefit building performance what regulations are important to thermal and fire performance; the different properties of various insulation types in both thermal and fire resistance performance; the importance of cavity barriers in preventing fire spread; and the advantages of using rock mineral wool.