CPD 11 2022: Rainscreen insulation and cladding retrofit

This CPD sponsored by Rockwool explains how rainscreen cladding can be a cost-effective and environmentally friendly way to upgrade a building, and the specification factors to consider

Insulation being fitted to the external wall and then cladding panels fixed on top

A brief history of rainscreen cladding

Situated on a promontory in the Sognefjord on the west coast of Norway, the 12th-century Urnes Stave church is one of the earliest examples of rainscreen cladding still standing.

The technique of using timber cladding, adopting closed joints, and having openings at both the top and bottom to allow for drainage and evaporation of any penetrating rainwater was developed over centuries of trial and error. The Norwegians called this method the “open-jointed barn technique”, due to it originally being used in the construction of barns.

It was not until the 1940s that research was undertaken into the principles behind rainscreen, and it was quickly established that the technique was vastly superior to any other method in use. In 1953, one of the first tall buildings to use rainscreen cladding was constructed. Designed by Harrison & Abramovitz, the Alcoa Building in Pittsburgh, Pennsylvania, placed 50in x 55in double-glazed, heat-absorbing windows within a predominantly solid aluminium skin.

The aluminium and low-density polyethylene plastic composite panel manufacturing technique, originally developed to meet the growing needs of the transportation industry, was verified in 1965. Researchers at Swiss Aluminium proposed the idea of combining aluminium alloy sheets with various other materials, and then carried out a large number of tests. But it was not until the 1980s that the benefits of rainscreen systems and ACM panels became widely understood and installations of rainscreen cladding systems began to rise.

At the same time, high-rise cladding fires resulted in the Department of the Environment identifying increased risk associated with the use of combustible cladding systems. The Grenfell Inquiry that began in 2017 led to further inquiries and a combustible ban in 2018, which meant only non-combustible cladding could be used on external walls of certain buildings over 18m. In Scotland, a ban on its use in certain buildings over 11m was introduced in 2022.

Learning objectives

This CPD will explain:

  • The benefits of thermal retrofitting
  • The types of systems used to upgrade multiple‑occupancy dwellings
  • The structure and advantages of insulated rainscreen cladding
  • Key considerations when designing and planning.
Insulation can be fitted directly to the external wall before fixing cladding panels on top, or alternatively directly rendering over the insulation

The benefits of thermal retrofitting

Sustainability: Every year in the UK, around 50,000 buildings are demolished, creating 126 million tonnes of waste, which accounts for two-thirds of the UK’s total annual waste.

Despite these new buildings being more energy efficient, their creation has a big environmental cost. There is a trade-off between operational carbon and embodied carbon. Embodied carbon emissions are locked into the building and arise from everything to do with the building’s construction, which typically involves carbon intensive processes.

For a new office building, 35% of its lifetime CO2 emissions have already been emitted upon completion. For a typical housing block, this rises to 51%, meaning it can take decades for some buildings to pay back their carbon debt.

The most effective way to avoid these embodied carbon emissions is to extend the lives of existing buildings through refurbishment and retrofit instead of demolishing them and building anew.

Environmental: UK residential greenhouse gas emissions in 2019 were estimated at 67.6 million tonnes CO2e (making up 15% of the country’s total). Non-domestic buildings account for 17% of our energy consumption, responsible for a further 12% of the UK’s total greenhouse gas emissions.

As part of its net zero ambitions, the government is aiming for all homes to be upgraded to EPC band C by 2035 and has mandated that privately rented homes reach EPC band C by 2028.

In England, 17 million homes are EPC band D or below and will need upgrading. The best way to do this is to improve the thermal performance of the building fabric first. This not only has a direct impact on CO2 emissions but offers improved resistance to extremes in temperature.

Lower energy bills: Upgrading these homes can have a big impact on people’s energy bills too. This is important considering how much gas and electricity have gone up recently, and how much they are forecast to rise once the price cap is reviewed later this year amid the current energy crisis.

Comparing the difference, the annual fuel bill average for EPC band D and below is more than three times that for bands A to C. This has a substantial impact on fuel poverty.

Fuel poverty relates to households that need to spend a high proportion of their income to keep their home at a reasonable temperature. Among fuel poor households in England, 90% are living in properties rated EPC band D and below.

Improved health: With 3.18 million fuel poor households in England – and that number set to increase significantly as the effects of the current energy crisis hit – lifting people out of fuel poverty does not just help them financially but can also have a tremendous impact on their health.

Cold homes directly contribute to excess winter deaths, by raising the risk of heart attack, stroke, respiratory illness and depression. They also cause an increase in poor diet when households must choose between heating and eating.

In October 2017, Public Health England shared guidance on a cold weather plan for England, making the case that planning for cold weather is essential to health and wellbeing. Retrofitting insulation is essential.

Types of systems used

There are various ways to thermally upgrade houses. Roof insulation, draughtproofing and glazing should all be considered. Airtightness is key but it must work with suitable ventilation. It is important to pay attention to how energy-efficiency measures interact. Adding better glazing and draughtproofing will lead to damp problems if there is not suitable ventilation.

With about a third of all the heat lost in an uninsulated home escaping through the walls of the building, retrofitting insulation is essential. If a house was built after the 1920s, it is likely to have cavity walls. Made up of two layers with a small gap, the system enables the gap to be filled with an insulation material.

Internal wall insulation will result in a loss of internal floor space. Rigid insulation boards can be fitted to existing walls, and stud walls can be created and filled with insulation material, but measures need to be taken to help prevent damp penetration.

External wall insulation provides the added benefit of protecting brickwork from the elements, as well as offering less disruption to the home life of occupants during the construction process and the opportunity to improve the external aesthetics with a render finish.

A common method is to fix insulation directly to the external wall and apply render over the insulation. It is ideal for solid-wall buildings, where cavity insulation is not possible and internal insulation would mean sacrificing too much space.

A rainscreen cladding system is made with a bearing wall, an insulation layer and a cladding material that is fixed to the building using a supporting structure. Its advantages make it popular among specifiers.

House rainscreen is an exterior wall detail where the siding (wall cladding) stands off from the moisture-resistant surface of an air barrier applied to the sheeting. House ventilated facade.

Advantages of insulated rainscreen cladding

Ventilation: A rainscreen system is typically a wall system where the outermost layer is formed by a weathertight non-structural cladding – usually panels but sometimes masonry – fixed in a way that leaves a ventilated cavity between the cladding and the insulation fixed to the load-bearing wall.

The cladding protects the load-bearing wall from the elements by providing a capillary break. This allows drainage of moisture, as well as ventilation and air movement through the cavity to aid drying. The main goal is to protect the load-bearing wall and increase the building’s durability and longevity.

Thermal performance: The continuous external blanket of insulation is better for moisture control than internal wall insulation, with a lower risk of condensation issues. There is an additional benefit for building occupants in incorporating a cavity: in warm weather, the air in the cavity gets warm enough to rise, drawing in cooler air, which has a cooling effect on the building. Conversely, in winter the air in the cavity is not rising and so helps the insulation retain heat inside the building.

Acoustic upgrade: Over 80% of people report being exposed to noise pollution in their homes, leading to sleep disturbance, stress and a number of other health concerns. The main source is road traffic noise, followed by aircraft noise and that arising from construction activity and roadworks. However, there is now a recognition that noise pollution is more than just an annoyance.

In office buildings, research has demonstrated the adverse impact environmental noise can have on worker productivity. More worryingly, studies show that the stress and sleep disturbance caused by noise is linked to serious illness. Improving the acoustic performance of the facade is needed, and a retrofitted rainscreen system can help. However, specifying the right materials is essential.

Noise is reflected by the cladding. If there is mineral wool present in the cavity, that will act to absorb noise. The improvement will depend on the performance of the base wall: typically, the worse the existing wall is, the bigger the benefit of the cladding system.

Acoustic through-wall testing of rainscreen systems undertaken by Rockwool has shown that adding a facade system with cladding to a base wall can improve acoustic performance by up to 10dB, even after factoring in a correction for road traffic noise. That equates to a perceived halving of the volume of noise.

A rainscreen system is typically a wall system where the outermost layer is formed by a weathertight non-structural cladding

Key design and planning considerations

Condition and detailed design: The first consideration is the condition of the building. Existing defects need to be addressed first and detailed design is crucial, allowing all foreseeable issues to be solved before work begins, reducing the performance gap and offering greater control over quality and cost.

Each building will need an individual assessment and a bespoke detailed design package, taking into consideration the condition of the existing wall. Uneven surfaces or architectural detailing can result in gaps behind the insulation layer. Air movement in these gaps can degrade the thermal performance of the building, as well as provide a potential path for smoke and flames in the event of a fire. Choosing a resilient insulation that can accommodate an uneven substrate is essential.

Wind washing – air movement driven by wind pressures, passing through or behind the thermal insulation – is another element that needs careful consideration. When working on tall buildings, exposed sites and corner details, it is important to minimise gaps. A solution could be specifying mineral wool insulation with a density that exceeds 64kg/m³ or dual-density.

Funding: There are a few schemes and grants for small businesses working in the private sector. Bodies like Ofgem offer advice, and schemes such as Business Energy Scotland provide small and medium-sized businesses with free assessments on their energy use.

The biggest pots of funding are usually made available for social housing, with the biggest of those being the Social Housing Decarbonisation Fund (SHDF). Available across England to registered providers of social housing, whether private or local authority, the fund is worth £3.85bn delivered over 10 years.

The government has launched a service called the Social Housing Retrofit Accelerator, which helps providers put together their SHDF bids. The amount of data and assessment required to apply is comprehensive and time-consuming, so forward planning is essential.

PAS 2035: This is the overarching document in the retrofit standards framework that provides a whole-house, best-practice approach to improving the energy efficiency of existing buildings. Schemes applying for funding must comply with PAS 2035. It is also a requirement of the TrustMark quality scheme and a growing requirement for non‑subsidised schemes.

Its goal is to ensure a project is designed properly from the start and that the right measures are in place to deliver performance improvements and cost savings. This is done through the four roles of retrofit assessor, retrofit co-ordinator, retrofit designer and retrofit evaluator.

The Retrofit Academy and the Social Housing Retrofit Accelerator service offer PAS 2035 training, webinars and workshops, found online.

Regulations and guidance: A series of Approved Documents outline the government guidance on how to comply with Building Regulations. Fire safety is covered in Approved Document B, split over two volumes. Volume 1 covers dwellings and volume 2 other buildings.

Section 10 of Approved Document B1 outlines the requirements regarding fire for external wall constructions. In specifying a material’s reaction-to-fire performance, it uses the Euroclass classification system. Through laboratory testing, this classification system assesses several key performance indicators to make a full assessment of a product’s reaction to fire.

The ratings run from A1 to F – and for A2 and below, are suffixed with s and d ratings relating to smoke emission and burning droplets. Only Euroclass A1 and A2 products are tested in accordance with the non-combustibility test, EN ISO 1182, to identify that they will not, or will significantly not, contribute to fire. Products that are rated B or below are combustible – meaning that they will burn, and may char or flame.

The first step is to determine whether the building in question is classed as relevant under regulation 7(4). In England and Wales this covers any building 18m or taller that:

  • Contains one or more dwellings
  • Contains an institution, such as a care home or hospital
  • Or contains a room for residential purposes (as of December, this includes hotels, hostels and boarding houses).

For relevant buildings, all materials making up the external wall construction should have a Euroclass rating of either A2-s1 d0 or A1, with the exception of some non-substantive materials such as gaskets and membranes.

For buildings taller than 11m not covered by regulation 7(2), there are three possible routes to compliance:
– Buildings with a storey 18m or more in height require insulation with a minimum of A2-s3, d2 or better.
– Alternatively the system can undergo a BS 8414 fire test, which is then assessed against BR 135. Note that the results of these tests are specific to the exact construction tested.
– Buildings that include a residential purpose and are 11m or more in height require insulation materials with a minimum rating of A2-s1 d0.

Type of insulation: There are a number of factors that specifiers should consider before selecting the insulation materials, which include reaction to fire, thermal and acoustic performance. It is important that the insulation selected has a positive impact for the building’s occupants and does not compromise safety.

Project teams may choose to specify rock and glass mineral wool insulation materials for their resilience to fire, acoustic properties and moisture resistance. Due to its fibrous open cell structure, mineral wool can hold increased amounts of air, making it an excellent insulator.

An alternative option is rigid foam insulation, which comes in three main types: PIR (poly-isocyanurate) boards, phenolic boards, and EPS (expanded polystyrene) boards. Rigid insulation has a closed cell structure which gives a low thermal conductivity. However, these combustible insulants are not suitable for relevant buildings above 18m, and must be subjected to specific testing if they are to be used on buildings outside of those quoted in regulation 7(4).

Specifying the correct insulation for each building’s requirements is important. Upgrading building fabric insulation is essential for reaching net zero, and insulated rainscreen systems are an effective solution. A focus on maintaining or improving building safety should always be a key consideration when planning a retrofit.

When upgrading the fabric insulation of a building, insulated rainscreen systems are an effective solution. The right materials improve the performance of the building and create safe, comfortable spaces for occupants.

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