External Wall Insulation: A guide to retrofitting homes

External Wall Insulation: A Comprehensive Retrofitting Guide

Aron Coates, Architect, BA(Hons), BArch, ARB | Designs in Detail | March

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External wall insulation (EWI) involves applying an insulating layer to the exterior surface of an existing wall. This insulation can be finished with render or an alternative cladding material to shield it from weather impacts. While EWI is designed primarily for solid masonry walls, it is also beneficial for enhancing the performance of cavity walls.

Implementing EWI in retrofitted homes can significantly decrease our carbon output, yet it does come with certain challenges, particularly concerning condensation that could result in dampness and mould growth.

This article discusses critical factors regarding external wall insulation, offering insights to assist in your project planning.

The Case for Installing External Wall Insulation

In light of the ongoing climate crisis, it is essential to address the substantial carbon dioxide emissions resulting from home heating, which accounts for 14% of national emissions annually. This issue is especially pronounced in older homes.

A significant portion of a building's heat—over one-third—escapes through walls. EWI stands out as the most effective method for retaining this heat.

Beyond environmental concerns, implementing EWI alongside other retrofitting measures can provide numerous advantages:

• Protect the building from external weather conditions.
• Foster a healthier indoor living environment.
• Enhance the aesthetic appeal of poorly designed properties.
• Utilize thermal mass to minimize temperature fluctuations.
• Decrease cold bridging, which can significantly reduce heat loss and condensation.
• Enhance soundproofing capabilities.
• Improve airtightness and reduce draughts.
• Facilitate quicker installation compared to alternative insulation methods.
• Minimize disruption to the interior spaces of the building.

Disadvantages of External Wall Insulation

One of the most noticeable effects of EWI is the alteration of a building's appearance. Although this can enhance visually unappealing structures, it might detract from the aesthetic of well-designed homes.

Consequently, EWI faces challenges when applied to principal elevations in conservation areas, and it is often unsuitable for historically listed buildings.

Additional drawbacks of EWI include:

• Damp issues may arise in inadequately insulated areas.
• Installation costs can be high.
• The need for comprehensive improvement works can escalate expenses.
• Some finishes may not withstand the test of time, posing a risk for water intrusion if damaged.
• Modifications to roof eaves and external glazing might be necessary to accommodate new insulation thickness.
• Alterations to rainwater downpipes, plumbing, and other external services may also be required.

Which Homes Are Ideal for EWI?

EWI is most appropriate for older homes, especially those constructed prior to significant insulation regulations, typically solid wall structures.

It can also be utilized in residences built between the 1970s and 1990s that exhibit uninsulated cavity walls, provided proper moisture protection and ventilation measures are implemented.

Conversely, modern homes that feature partial cavity insulation (common in properties built from the late 1990s) do not benefit from EWI; in such cases, internal wall insulation (IWI) is a more suitable alternative.

Types of External Wall Insulation

EWI systems generally incorporate various insulation materials:

• Expanded Polystyrene (EPS)
• Polyisocyanurate (PIR)
• Phenolic
• Wood Fibre
• Cork

Plastic foam insulations like EPS, PIR, and phenolic boards are typically the most thermally efficient. However, their closed-cell nature makes them impermeable to water vapour, which may trap moisture inside the building, increasing the risk of damp unless appropriate measures are used. Additionally, plastic foams tend to have a larger carbon footprint.

On the other hand, timber insulation products, such as wood fibre or cork, are open-cell and allow water vapour to escape, making them a more sustainable choice with a lower carbon footprint. However, they require thicker boards to match the performance of plastic foams and can be pricier.

When choosing EWI, it is crucial to consider factors beyond thermal efficiency to ensure the selected system aligns with your project's requirements.

Choosing Finishing Materials for EWI

The most prevalent method of finishing EWI is through rendering. It is critical to assess whether the render system is permeable or impermeable during selection.

Additionally, various cladding materials can be used, including:

• Wood
• Zinc and other metal panels
• Brick slips
• Hanging tiles

Combined with quality finishes, EWI presents numerous design possibilities, significantly transforming the architecture of less appealing structures.

Addressing Damp Concerns with EWI

When properly installed and detailed, EWI will not induce damp conditions. In essence, EWI provides extra weather protection, shielding the building from outside moisture. The primary damp risks are often associated with internal sources.

Damp issues can emerge through interstitial condensation during colder months when warm moist air penetrates the building shell and condenses upon encountering colder surfaces. This is particularly true in spots with insufficient insulation or cold bridges.

To mitigate dampness, it is essential to ensure a consistent layer of insulation across the wall's surface, avoiding cold bridges. Open-cell, breathable EWI systems help trapped moisture escape outward, while closed-cell systems necessitate internal vapour barriers to limit moisture intrusion.

Appropriate heating and ventilation are vital to maintaining optimal internal moisture levels.

Best External Wall Insulation for Solid Buildings

For traditional solid wall structures, timber-based EWI is often the most effective option.

Older buildings, primarily those constructed before the late 20th century, were designed to function as 'breathing' structures, allowing moisture to move freely. Insulation systems compatible with existing building technologies can minimize damp and other long-term issues.

Insulation systems using wood fibre or cork effectively protect against weather while enabling warm moist air to escape during colder periods. A breathable internal finish can enhance moisture transfer, helping with evaporation during summer.

Timber-based EWI systems also boast a lower ecological impact, making them a sustainable alternative.

In contrast, closed-cell plastic foam systems, while providing robust insulation and weather protection, risk trapping moisture, necessitating enhanced ventilation.

To ensure optimal EWI installation, professional consultations are vital.

Can EWI Be Applied to Cavity Walls?

Modern cavity wall homes, primarily those constructed post-1980s, are generally not suitable for EWI, as installing it compromises ventilation. Conversely, homes built with earlier cavity walls, typically from the pre-1980s era without existing cavity wall insulation, can benefit from EWI.

For effective external insulation in these properties, the cavity must be entirely filled; otherwise, heat loss occurs through the air gap. EPS beading is often the most efficient retrofitting option, offering consistent cavity coverage.

Insulating cavity wall buildings can be technically complex, requiring careful site monitoring, underscoring the need to seek professional guidance beforehand.

Implications of EWI on Other Building Parts

Installing EWI increases wall thickness, impacting other architectural elements like eaves and window openings.

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It is crucial to verify that eaves are sufficiently deep to accommodate the insulation. Insufficient space may necessitate expanding the roof overhang, which could be prohibitively expensive for those on a limited budget.

In addition, insulation needs to extend into door and window reveals to prevent cold bridges. However, available space may limit the thickness of insulation boards, necessitating the use of thinner, more efficient panels or adjustments to the glazing unit.

Additionally, flues, vents, pipes, and other external fittings must be modified accordingly to allow for consistent insulation thickness.

Determining Required Thickness for EWI

The necessary thickness hinges on your desired performance and EWI type.

Aiming for EnerPHit Passivhaus accreditation demands significantly more insulation than merely meeting building regulations. Plastic foam insulations yield better R-values—indicating thermal resistance—compared to wood fibre insulation, achieving commendable U-values with thinner boards. Lower U-values reflect superior insulation performance.

For single brick solid walls measuring approximately 250 mm overall, installing 70 mm of PIR, 90 mm of EPS, or 100 mm of wood fibre insulation can achieve a U-value close to 0.3, meeting building regulation requirements.

Simultaneously applying EWI alongside full-fill cavity insulation—50 mm of EPS cavity bead insulation combined with 50 mm of PIR—can attain a U-value around 0.18. An airtight layer, such as a 12 mm thick lime plaster, can facilitate achieving a U-value of 0.15.

Calculating U-values is essential in determining insulation thickness ahead of installation.

EWI Versus IWI: A Comparison

External wall insulation (EWI) is significantly more effective than internal wall insulation (IWI) in insulating building walls. EWI provides a continuous insulation layer across the entire building envelope, enhancing thermal mass exposure, thus improving heating and cooling efficiency.

In contrast, IWI entails adding insulation to the interior wall surfaces, primarily through insulated plasterboards, which disrupts the thermal layer at junctions with floors and ceilings and diminishes overall performance.

While EWI is the preferred insulation strategy when feasible, IWI may be warranted in cases where EWI is restricted, such as listed buildings or properties in conservation areas, or where external constraints exist.

Planning Permission for EWI

Since EWI alters a building's appearance, planning permission is obligatory in certain circumstances, such as:

• Flats, maisonettes, and multi-occupancy homes.
• Properties located in conservation areas or designated land.
• Listed buildings or structures within the curtilage of listed properties.
• Changes in exterior materials.

Obtaining planning approval for EWI on the front facade of a building situated in a conservation area is unlikely, given the emphasis on preserving streetscape visuals.

Though typically not permissible on listed buildings, there are situations where EWI may qualify as permitted development if the proposed cladding resembles existing materials. For clarity, consider applying for a Certificate of Lawful Development before proceeding.

Building Regulations Approval for EWI Installation

Every EWI installation requires notification to local Building Control authorities. A certified installer can self-certify compliance with building standards, streamlining the process.

When the EWI installation impacts over half of the wall or more than a quarter of the total building envelope, building regulations necessitate enhancing insulation for the entire thermal element to achieve a maximum U-value of 0.30.

Additional Permissions Required for EWI Installation

When planning EWI work on shared boundary walls (party walls), notifying neighbours and securing their written consent through a party wall agreement is essential.

Should the walls of your property directly abut public highways—encompassing pavements—certain oversail licenses are necessary for any installations that extend over these areas. Local councils grant these licenses for nominal fees, which may necessitate site inspections before and after work completion to ensure public safety and accessibility.

Cost of Installing External Wall Insulation

The installation cost of EWI in the UK varies, influenced by factors such as home size, insulation type, and installation complexity.

On average, the expense ranges from £100-£200 per square metre, factoring in finishing materials and installation. Rendered EPS EWI tends to be the most budget-friendly solution, while wood-based EWI with premium cladding materials can drive costs higher.

Variability in pricing may also arise based on property type, location, existing wall conditions, and other necessary adjustments like modifying eaves or windows. Thus, obtaining quotes from multiple installers is prudent for accurate project cost assessments.

Grants for External Wall Insulation

As of now, the UK has one grant option aimed at EWI installation: the Energy Company Obligation (ECO) scheme, which funds home improvements for eligible lower-income households and properties that struggle with heating.

For those ineligible for the ECO Scheme, Parity Trust offers secured loans for various home enhancements, including EWI, in partnership with multiple local authorities.

Selecting Your EWI Installer

Engaging a qualified contractor for EWI installation is best practice. Verify that the insulation system has either British Board of Agreement (BBA) certification or European Technical Approval (ETA).

For ECO subsidy eligibility, ensure that installations come with a guarantee from the Solid Wall Insulation Guarantee Agency (SWIGA) or an equivalent body.

For sustainable insulation systems and building products, companies like Back to Earth and Mike Wye offer valuable advice.

Registered EWI installers can be found at The Installation Assurance Authority.

Assessing the Suitability of EWI for Your Home

When considering energy efficiency innovations for solid wall homes and early cavity wall properties with full-fill cavity insulation, EWI represents the most effective strategy.

For poorly insulated homes, EWI can significantly reduce heat loss and lower energy expenses, making it a savvy investment over time. Additionally, participation in the ECO scheme can provide support for low-income households and those experiencing fuel poverty.

However, keep in mind that EWI represents a substantial upfront cost, and additional concurrent building work may increase financial demands. A thorough evaluation of how EWI will alter your home's appearance is also vital; working with an architect can ensure it's a suitable choice.

Images of projects showcasing external wall insulation:

About the author: Aron Coates has over two decades of architectural experience, with projects ranging from historic building restorations to modern home designs.

The information provided in this article serves solely as general guidance and educational insights. At Designs in Detail, we recommend hiring a proficient residential architect to obtain tailored expert advice for your undertaking.

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