Types of Partial Fill Cavity Insulation

Partial fill cavities are some of the most commonly built in the UK. We have created a set of details to help in detailing this type of insulation.

In areas exposed to very severe wind driven rain, partial fill cavities are the more robust solution for both rendered and facing masonry. A partial fill cavity is one where there is masonry externally, either rendered or visible brickwork with an air gap behind, before the insulation and then internal masonry wall.

The purpose of the air gap within the cavity is to stop any moisture within the external masonry from coming into contact with the insulation and therefore the inner masonry. This both helps the external masonry to dry whilst keeping the insulation dry, improving the thermal performance of the wall in wet weather. 

01 DL315A Partial fill 150mm masonry cavity wall, trench foundation with beam and block floor - no cavity tray
DL315A – Partial fill 150mm masonry cavity wall, trench foundation with beam and block floor – from the Detail Library
In areas exposed to very severe wind driven rain, partial fill cavities are the more robust solution for both rendered and facing masonry.

With increased thermal requirements, 150mm full-fill cavities with a rendered masonry facade are also recommended for very severe exposure but partial fill is still more commonly used.  

This post with look at most common partial fill insulation boards: 

  • Phenolic boards (PF)
  • PIR insulation boards
  • Mineral rock and glass wool batts
  • Polystyrene sheets (EPS)
  • Carbon neutral and negative options (wood fibre and cork)

We will look at their thermal performance, fire certification and sustainability.

Whilst partial fill cavities are great at stopping water penetration into the inner structure, they must be built correctly with the insulation fixed strongly on the internal masonry leaf. Cutting boards around cavity trays can also be fiddly. The buildability of full-fill cavities can sometimes mitigate some of these issues. 

Insulation is always the first option, along with windows and other external fabrics such as the roof, when making a building more energy efficient. The better u-values or thermal performance a building has, the lower energy needed to heat and cool the building. Over the lifespan on a building, the carbon used to produce insulation greatly outweighs the carbon used to heat a home.

Phenolic boards


These are premium cavity boards with a fibre free rigid thermoset phenolic core. It is usually foil backed. It is the most energy efficient by thickness with a thermal conductivity (lambda value) of around 0.019 W/mK. This allows for a narrower overall cavity to achieve the same wall U-value, however as it is a premium product, it is more expensive that PIR insulation boards. 

02 Phenolic boards Kingspan Kooltherm K108-Cavity Board 1
03 Phenolic boards Kingspan Kooltherm K108-Cavity Board 2

Phenolic boards do not perform well in fires with some manufacturers achieving Euroclass D s1 d0, whilst others are Euroclass F. With updates to the fire regulations, it is always advised to check the fire rating on the most up to date BBA certificate or directly with the manufacturer, as many have been recertified with lower fire classifications.

Phenolic boards have a relatively high embodied carbon even adjusting for the use of less material to achieve the same thermal performance. These materials are primarily plastic based petro chemicals. Whilst some manufacture’s recycle offcuts in the factory, there is currently no widely adopted recycling scheme for this type of insulation post lifecycle. Cutting boards on site can cause microplastics to be deposited in the surrounding areas.

PIR insulation boards


PIR insulation is usually foil backed with a fibre-free rigid thermoset polyisocyanurate (PIR) core. These boards are some of the most commonly installed in the UK due to their relatively good thermal performance to cost ratio, they are slightly less energy efficient than phenolic boards but much cheaper. Most PIR boards tend to have a lambda value of around 0.022 W/mK.

04 PIR insulation Celotex partial fill cavity

These perform the worst in terms of fire classification. Although classification is dependent on each manufacturer, most PIR boards tend to be Class E-F with few detailing the spread rate and droplet rate. As the main material of these insulation boards are plastic, petro chemicals, spread rates and amount of droplets when burning tend to be high.

PIR insulation is a plastic based insulation and therefore has high embodied carbon. Manufacturer’s tend to recycle the offcuts but insulation during the demolition process will tend to go to landfill. Sawing the product on site can also cause microplastics to be blown into the site. 

Mineral rock and glass wool batts


Mineral wool is sometimes used to describe both mineral rock wool and glass wool. These types of mineral and glass wool insulation come in different forms, batt and quilt (or blanket).

For partial fill cavity walls, the batt insulation is used, as these are pre cut rectangular panels that are more rigid than the quilt version. The quilt versions usually come in rolls are are more suited to insulating ceilings and lofts. This type of insulation form does not have the rigidity required for partial fill cavity walls.

For partial fill cavity applications, mineral rock wool made from rocks is more common, with less manufacturer’s offering glass options. Glass wool options perform slightly better thermally with a lambda value of 0.032W/mK compared to 0.034W/mK for rock mineral wool. They are also commonly used for full fill masonry cavities and party walls as glass mineral wool performs better acoustically.

05 Mineral Wool Insualtion ROCKSILK RAINSCREEN SLAB - 455MM Knauf insulation Partial fill blockwork and render wall
06 Mineral Wool Insualtion ROCKSILK RAINSCREEN SLAB - 455MM Knauf insulation Partial fill stone
07 Glass wool insulation slab ISOVER Cavity Wall Slab (CWS) 32 partial fill

Both mineral rock and glass wool achieve a ‘non-combustible’ rating which can vary slightly from manufacturer.

This is usually a A1 or A2, s1-d0 rating. Most materials exposed to enough heat will combust. In accordance with EN 13501-1:2018, A1 and A2 are classified as non-combustible with s1-d0 classifying little to no smoke development in the first 10 minutes and no droplets produced within this time.

This means that mineral wool can be used for higher buildings or in situations where the client or insurers require non-combustible facades.

Sustainability features can vary from manufacturer. Both require large amounts of energy for production. Glass wool is sometimes the more sustainable option as it uses more recycled materials, sometimes up to 86% vs 70% for rock mineral wool. Both options have a high energy use for manufacture, but this is offset by the use of waste material in the manufacturing process, reducing the embodied carbon of the product down to below that of phenolic boards and PIR insulation. Recycling of these products post life is not currently common although they are not toxic in landfill. 

Expanded polystyrene (EPS) foam boards


This is not an insulation commonly used in the UK although it can be an even more affordable option to PIR insulation. Thermal conductivity can vary greatly depending on the manufacturer from 0.042W/mK to 0.030W/mK and therefore thicker panels and more material will be required to achieve the same u-value as a wall built with PIR or phenolic boards. 

Similarly to the other petro-chemical boards, EPS has a very bad fire rating, usually around Euroclass E-F.

Unlike PIR and phenolic boards, EPS is 100% recyclable as it does not contain other ingredients in its manufacture and can be shredded and recycled. The cutting of boards on site can cause plastic waste to fly away and cause localised pollution as the polystyrene balls are very light. 

Extruded polystyrene insulation (XPS) insulation is another form of polystyrene board which is more rigid but can also be fully recycled. As this form is more rigid, it is usually used for floors and flat roofs although some manufacturer’s supply partial fill cavity insulation boards made from XPS. 

08 EPS Exanded polystyrebe Jablite Jabwall

Carbon neutral or negative options


There are a number of carbon neutral or negative options with partial fill insulation. These include wood fibre and cork board insulation. These types of insulation are commonly paired with more sustainable super structures such as timber frame and CLT construction or retrofit projects as they are more ‘breathable’ than plastic alternatives. However partial fill cavity options for masonry do exist. Other carbon positive options such as sheep’s wool and Jute insulation batts rarely have partial fill options and are more suited to full fill cavity construction. 

The thermal conductivity of both panels is between 0.036 W/mK0.038 W/mK, meaning greater thicknesses of the material will be needed to achieve the same u-value as most over commonly used insulations. 

09 Wood fibre insulation GUTEX thermosage homogen
10 Cork Insulation Expanded Amorim

Fire classification can vary from manufacturer and can be anything from Euroclass B s2 d0 – E. Due to their relatively low production, these insulations can be expensive, sometimes more than twice the cost of PIR insulation per m2. It can also be hard to find supplies at short notice as most production is outside of the UK.

Both wood fibre and cork boards are sustainable, recyclable, compostable, use no nasty chemicals and sequesters the CO2 the tree absorbed. Wood fibre boards are made with the waste products of timber manufacturing such as waste chippings and sawdust. Cork insulation boards are carbon negative and no trees are cut to harvest the cork, only the bark of the tree. 

U-Value Data for Partial Fill Cavity Walls Using Different Insulation

We have taken each of the different types of insulation and calculated the u-values of each partial fill cavity wall build up. Below we look at which assemblies will comply with the new build wall requirement of 0.18W/m2K, and which insulations will require additional help with internal insulation.

Calculations will vary according to thermal conductivity values of each material specified. We recommend you carry out your own calculations based on the exact products that have been selected for your project. The calculations below are a rough guide and must not be taken as approved and final u-values. All the u-values below are subject to change depending on materials, manufacturers and conditions.  

Phenolic Boards

11 Phenolic Boards

Mineral Rock Wool

13 Mineral Rock Wool


15 EEPS Board Insulation


12 PIR Board Insulation

Glass Wool

14 Glass Wool Batts

Wood Fibre

16 Wood Fibre


There are a huge variety of insulation boards for partial fill cavities in masonry construction. PIR boards tend to be the most affordable option and require a smaller thickness to achieve the required u-value.

Phenolic boards are more expensive but if a wall build up needs to be reduced, for example, on a very tight urban site, these will provide the thinnest buildup for the required u-value.

EPS boards can be an even cheaper alternative to PIR insulation, however, the higher lambda value means you will need a thicker panel to achieve the same u-value and therefore the cost-benefit of a wider cavity, longer ties, etc. may mean that PIR is a better suited option.

All petro-chemical based insulation, PIR, phenolic and EPS boards are made from plastic and therefore have bad fire ratings of Euroclass E-F. EPS is the only plastic based board that can be fully recycled post use. 

For non-combustible, mineral rock wool or glass wool slabs can be installed. These usually have a fire rating of Euroclass A1 or A2, s1-d0 r. These do not perform as well thermally and the cavity may need to be widened to achieve the same u-value. PIR boards and mineral or glass wool slabs are very similar in price. 

There are a number of sustainable options such as wood fibre and cork board insulation. Currently. These options can sometimes be very expensive and require a slightly thicker cavity than the mineral rock wool or glass wool slabs.

Download the Guide

27 Types of Partial Fill Cavity Insulation - Resource

Get the Details

We have created a set of details that demonstrate just one insulation option for partial fill cavity wall construction. These details can be adjusted to suit your own project requirements. See the complete set below.

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Written by Aida Rodriguez-Vega, architect and researcher. At the Detail Library, Aida keeps busy by carrying out technical research and drawing new details for the ever-growing library.

Links and Image Credits

Source URL: https://www.kingspan.com/gb/en/products/insulation-boards/wall-insulation-boards/kooltherm-k108-cavity-board/

Image URL: https://www.kingspan.com/content/dam/kingspan/kil/products/kooltherm-k108-gb-and-ireland/kingspan-kooltherm-k108-product-render-block-block-en-gb-ie.png/jcr:content/renditions/cq5dam.web.1280.1280.png

Credit: Kingspan


Source URL: https://www.kingspan.com/gb/en/products/insulation-boards/wall-insulation-boards/kooltherm-k108-cavity-board/

Image URL: https://www.kingspan.com/content/dam/kingspan/kil/products/kooltherm-k108-gb-and-ireland/kingspan-kooltherm-k108-product-render-brick-block-en-ie-gb.png/jcr:content/renditions/cq5dam.web.1280.1280.png

Credit: Kingspan


Source URL: https://www.knaufinsulation.co.uk/products/rocksilk-rainscreen-slab-455mm

Image URL: https://www.knaufinsulation.co.uk/sites/ki_gb/files/styles/product_main_image/public/externals/f4196d710103bb76b9cdd6b2544fff87.jpg?itok=ASoiclHh

Credit: Knauf insulation


Source URL: https://www.knaufinsulation.co.uk/products/rocksilk-rainscreen-slab-455mm

Image URL: https://www.knaufinsulation.co.uk/sites/ki_gb/files/styles/product_main_image/public/externals/fe2f4b42fa29ce0331e8bd5699d19b7f.jpg?itok=dftBQl-t

Credit: Knauf insulation


Source URL: https://insulation-uk.com/wall-insulation/external-walls/partial-fill-cavity-walls-masonry

Image URL: https://insulation-uk.com/assets/8b_walls_partial-fill-cavity.png

Credit: Isover, Saint-Gobain


Source URL: https://www.jablite.co.uk/application/partial-fill-cavity-wall-insulation/

Image URL: https://www.jablite.co.uk/app/uploads/2018/10/Jabwall.jpg

Credit: Jablite insulation


Source URL: https://gutex.co.uk/product-range/products/product/gutex-thermosafe-homogen/

Image URL: https://gutex.co.uk/fileadmin/_processed_/csm_1_Gutex-Thermosafe-homogen-200mm-Ecke_d2de1e28c0.jpg

Credit: Gutex


Source URL: https://www.amorimcorkinsulation.com/en/products/Expanded-Insulation-Corkboard/60/

Image URL: https://archello.s3.eu-central-1.amazonaws.com/images/2021/04/14/amorim-cork-insulation-expanded-corkboard-wall-panels-and-cladding-archello.1618390637.5609.jpg

Credit: Amorim