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External Wall Insulation & Fire Resistance

Many people often question whether external wall insulation is safe. A concern that often plagues these conversations is the topic of fire resistance. The tragedy of the Grenfell Tower is still at the forefront of national news. As such, the concerns about the fire resistance of cladding and insulation persist. Considerations for fire resistance must be examined at every planning stage, from the selection of materials to the installation of the systems. Some insulation materials can contribute to the spread of fire if not correctly specified or installed. However, many forms of insulation act as active fire breaks.

The gold standard of fire resistance

Rockwool, also known as mineral wool, is a type of insulation that is highly resistant to fire. It’s made from basalt rock and slag which is a by-product of steel and iron production. These materials are consequently melted and spun into fibres. The fibres are then compacted into dual-density battens or rolls.

The reasons behind Rockwool’s gold standard of fire resistance:

  1. Non-combustibility – Rockwool is non-combustible, which means it does not burn. It can withstand temperatures up to approximately 2150°F (1177°C), well above typical house fire temperatures.
  2. No toxic smoke – Rockwool does not produce significant quantities of smoke when subjected to fire. Smoke can be one of the most dangerous elements of a fire in a residential or commercial building.
  3. Doesn’t contribute to fire spread – Rockwool’s fibrous structure and non-combustible nature mean it doesn’t contribute to the spread of fire. Moreover, it can even help to slow it down.
  4. Melting point – Rockwool has a high melting point, so even in a severe fire that melts other materials, Rockwool tends to maintain its integrity.
  5. Euroclass A1 – This is the highest classification for fire-resistant materials in the European Standard for reaction to fire. Rockwool insulation typically achieves this classification, marking it as a non-combustible material with no contribution to fire, no smoke emission, and also no production of flaming droplets or particles.

For all these reasons, Rockwool is often used in applications where fire resistance is especially important, including fire-stopping in walls and floors, and insulation in commercial buildings. As with any building product, it should be installed in accordance with the manufacturer’s instructions and relevant building codes to ensure its fire-resistant properties are effective.

Rockwool’s benefits are not limited to fire resistance. The dual density also provides acoustic insulation to the property. Crucially, Rockwool is also infinitely recyclable…into more Rockwool! The concept of circularity is crucial to the mission statement:

Rock is one of the most abundant raw materials on the planet, but we still need to make better use of our planet’s resources. We have developed our technology in a way that allows us to use waste from other industries as alternative raw materials.

The building sector produces a third of all waste, much of which ends up in landfill today. So we also have to prevent our building products from becoming waste. We do that by making recyclable, long-lasting products and offering a recycling service. Our products can be easily removed when a building is renovated, or demolished and recycled back into new products.

In fact, stone wool can be recycled again and again into new stone wool. This is an important element of a “circular” business model – another way you can be a part of our vision for sustainable cities of the future.

Rockwool, Releasing the natural power of stone PDF
fire resistance as part of the 7 strengths of stone
Rockwool’s 7 Strengths of Stone

Fire resistance of other insulation materials

Mineral wool will always have superior fire resistance. However, this does not mean that other materials do not also possess resistance to fire. Kingspan K5 is a form of phenolic foam which offers a reasonable level of resistance. This is in addition to superior thermal performance. For reference, you only require 60mm of phenolic foam as opposed to 110mm of mineral wool to achieve a U-value of 0.3W/m²K.

The characteristics that give phenolic foam resistance to fire are the following:

  1. Fire resistance: Phenolic foam is self-extinguishing and doesn’t readily ignite. When it does burn, it releases very little smoke and usually doesn’t drip or produce flaming particles, which could otherwise contribute to the spread of fire.
  2. Limited oxygen index: Phenolic foam has a high limited oxygen index (LOI) – typically around 30-35%. Kingspan boards can have a LOI of up to 50%. This means it requires an atmosphere with an unusually high proportion of oxygen (30-35% compared to normal air’s 21%) to sustain combustion.
  3. Euroclass Classification: Phenolic foam usually achieves a Euroclass B rating, meaning it has very limited contribution to fire and only a tiny amount of smoke production.

However, while phenolic foam’s fire resistance is generally superior to that of other foam insulations, it can still be combustible and should be appropriately protected or isolated from potential ignition sources in construction assemblies.

DefinitionGradeSmoke propagationFlaming droplets
Non-combustible materialsA1
Non-combustible materialsA2s1d0
Combustible materials: Very limited contribution to fireBs1d0
Combustible materials: Limited contribution to fireCs1d0
Combustible materials: Medium contribution to fireDs1d0
Combustible materials: High contribution to fireEE-d2E-d2
Combustible materials: Easily flammableFFF
Fire ratings explained

Natural insulation materials

Natural insulation is not a topic we cover often, however, some will offer fire resistance. The level to which they do so varies depending on the type of insulation. Some of the most popular types of natural insulation include sheep’s wool, cork, straw, and cellulose.

Sheep’s wool is naturally flame-resistant due to its high nitrogen and water content. When exposed to flame, wool chars and self-extinguishes once the flame source is removed. It also does not melt or drip.

Cork is fire-resistant due to its cellular structure, which contains a gas nearly identical to air, and because it chars rather than burns with a flame. It also releases less smoke and toxic gases compared to synthetic materials.

Strawbale construction has surprisingly good fire resistance due to the density of the bales and the plaster usually applied to them. However, loose straw is highly flammable.

As a plant fibre, cellulose would normally be quite combustible, but when used as insulation it’s typically treated with borates, which are fire-retardant.

Despite these materials’ varying levels of fire resistance, it’s crucial to follow building codes and standards to ensure the overall assembly – not just the insulation – is fire-safe. This often involves integrating fire stops, barriers, or other elements that help slow the spread of fire.

Fire barriers and fire break fixings

Fire breaks, also known as fire barriers, are an important safety feature designed to slow or prevent the spread of fire.

A fire break is a layer of fire-resistant material that is integrated into the EWI system to compartmentalise the insulation and stop a fire from spreading across the insulated surface. The fire breaks are usually arranged in a grid-like pattern. Therefore, any fire is contained within one section of the grid. Materials used for fire breaks in EWI systems need to be non-combustible and capable of withstanding high temperatures. Mineral wool is a commonly used material.

Fire break fixings are used to secure the fire breaks in place. They must also be capable of withstanding high temperatures without failing. The fixings also need to penetrate through the insulation to secure the fire break to the underlying wall. They may also be designed to secure the fire break to the surface of the insulation. In both cases, they need to be able to keep the fire break in place even if the insulation layer is compromised by fire.

If you have any questions about fire resistance, leave us a comment below!


2 thoughts on “External Wall Insulation & Fire Resistance

    1. Hi Jeff, it certainly performs best in acoustic and fire-resistant facets. However, in terms of thermal performance, other boards achieve better U-values at lower thicknesses. So it’s not the most cost-efficient

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