External Wall Insulation (EWI) cladding requirements, design considerations: weathertightness, resistance to wind driven rain penetration, long-term durability, thermal insulation, interstitial condensation, impact resistance, wind loading resistance, fire performance, sound insulation, aesthetic appearance, resistance to mould and algal growth, Building Defects Investigations.
The main requirement of any external wall insulation (EWI) system is to provide a weathertight skin around the building which keeps the weather (wind, rain, cold) out within which the climate can be controlled in a cost-effective manner and applies to all new build, refurbishment and re-modelling of existing buildings.
Fig 1. EWI Semi- Detached Housing
Fig 2. EWI 4-Storey Block of Flats
The basic choice for the external skin is between a ventilated, rain screen cladding system (not covered here), or an insulated render cladding system (generally known as external wall insulation EWI) fixed directly to the external surface of the structure without a continuous cavity.
The ability of the external cladding system to be weathertight depends not only on the nature of the various materials and components which make up the system but also its design. The long-term performance of the external wall insulation (EWI) system will depend on the careful selection, combination and durability of each of the materials and components used in the system, and will also depend on the overall systems’ design and ability to withstand the combined effects of heat, wind driven rain, cold and freezing conditions.
The external wall insulation (EWI) system will also have to withstand thermal (heating and cooling) and moisture (water absorption expansion and drying shrinkage) movements as well as the mechanical effects of wind pressure and suction, and impact loads.
1. Long term durability
An external wall insulation system (EWI) should be resistant to the effects of the natural weathering conditions likely to be encountered in the United Kingdom including the combined effects of heat, wind driven rain, cold and freezing cyclic conditions.
External wall insulation (EWI) cladding systems should have satisfactorily completed tests in accordance with MOAT No 22: 1988, UEAtc Directive for the Assessment of External Insulation Systems for Walls and be the subject of a current valid independent third party UKAS accredited Certification and have been assessed to have a minimum life expectancy of at least 30 years, provided any damage to the surface finish is repaired immediately, and regular maintenance is undertaken.
Fig 3. EWI Render with Spar Dash Finish
Fig 4. EWI with Plain Finish Render
Long term performance in use of external wall insulation (EWI) cladding systems with cement render finishes installed strictly in accordance with independent third party UKAS accredited Certification technical specification and design requirements and the manufacturer’s instructions indicate that the systems can perform satisfactorily in use in the United Kingdom and can improve the thermal performance and appearance of residential domestic two-storey dwellings and on low, medium and high-rise multi-storey housing blocks.
1. Resistance to wind driven rain penetration
External wall insulation (EWI) systems should provide protection against the ingress of wind driven rain penetration.
It is the responsibility of the designer of the external wall insulation (EWI) system to ensure that the system is designed strictly in accordance with the requirements of independent third party UKAS accredited Certification technical specification and design requirements and the manufacturer’s instructions and incorporate design details to prevent water ingress and to shed water clear of the system.
2. Thermal insulation
In general, an external wall insulation (EWI) system should improve the thermal performance of external walls and can contribute to meeting the requirements for thermal insulation in the national Building Regulations when designed and installed strictly in accordance with normal good practice and current valid independent third party UKAS accredited Certification provided detailing techniques are carried out to a high standard to prevent the ingress of rainwater into the insulation.
3. Interstitial condensation
Improving the thermal insulation of a buildings may increase the risk of condensation within the wall structure. It is therefore essential that designers follow the guidance in BS 5250: 2011 Code of practice for control of condensation in buildings, on safeguarding the health of the occupants by considering likely sources of moisture, how to avoid the build-up of excessive moisture, and prevention of mould growth and condensation.
It is the responsibility of the designer of the external wall insulation (EWI) system to ensure that an appropriate condensation risk analysis of all parts of the construction has been carried out to minimise the risk of condensation.
4. Impact resistance
External wall insulation (EWI) systems have to withstand different levels of impact depending on where they are located on the building façade and whether they will be subject to special circumstances, for example, high levels of vandalism, etc.
Hard body tests are required to be carried out on external wall insulation (EWI) systems in accordance with ETAG 004: 2011, Guideline for European Technical Approval of External Thermal Insulation Composite Systems with Rendering, to determine the category of suitability for use.
In areas prone to vandalism or abnormally rough use, it may be necessary for the designer to increase the gauge of the reinforcing mesh within the render or provide additional protection such as crowd barriers, etc. Any change to the reinforcing mesh must be the subject of a current valid independent third party UKAS accredited Certification as it will affect the performance of the render.
5. Wind loading resistance
External wall insulation (EWI) systems also have to withstand wind loadings. For further details click on Wind Loading Resistance.
6. Fire performance
For guidance on the fire requirements for external wall insulation (EWI) systems with render finishes refer to the relevant appropriate independent third party UKAS accredited certification requirements, Building Regulations (fire safety, and materials and workmanship) and any other regulations that relate to the installation of the measure.
7. Sound insulation
There is an increasing awareness by building designers of noise pollution problems and of the need to provide external wall insulation (EWI) systems solutions which will reduce sound transmission both entering and exiting buildings. Typical buildings which require sound insulating walls include, hotels in busy city locations, airport commercial buildings, office buildings, industrial buildings, and leisure and entertainment buildings. Despite this growing demand, we can find no readily available published data on the sound transmission properties of typical wall constructions with external wall insulation (EWI) cladding.
8. Aesthetic appearance and resistance to mould and algal growth
External wall insulation (EWI) systems with render finishes, in addition, to providing good sound technical performance are also required to provide a good long-term visual appearance at least equal to that of traditional render finishes.
Traditional render finishes and polymer-modified cement-based renderings containing Portland cement may be subject to lime bloom. BS 5262:1991, Code of practice for external renderings (no longer current but referred to in Building Regulations guidance) clause 34.1 states:
“The incidence of lime bloom depends mainly on the weather conditions at the time of finishing the surface and the subsequent drying. It may be exacerbated by rendering when the background is very wet.”
Conversely, the occurrence of lime bloom may be reduced by avoiding application of the renders in adverse weather conditions.
External wall insulation (EWI) cladding systems should be proven by test to have satisfactory performance to the effects of mould growth and lichens.
In common with traditional renders, the appearance can generally be restored. However, in all cases the manufacturer’s advice should be sought regarding suitability of remedial treatments and types of coatings to ensure compliance of the fire performance requirements of the system is unaffected.
Building Defects Investigations
If you require assistance in carrying out inspections of existing external wall insulation systems with render or brick slip masonry finishes on multi-storey buildings please click here to contact us.