Flat roof insulation is used to enable a room to feel warmer in the winter and cooler in the summer months. Foam products such as Polyisocyanurate (PIR) bonded with foil or glass tissue facings works particularly well and is often found in modern constructions. Flat roof insulation will improve the thermal conductivity of the residential or commercial building. A roof is typically classed as ‘flat’ if it has a pitch below 10° Celsius.

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Celotex 100mm PIR Insulation Board – GA4100 (2.4m x 1.2m)

£36.20 ex VAT

100mm Kingspan Thermaroof TR24 PIR Flat Roof Insulation Board (1200 x 600mm)

Call for Price

120mm Celotex Insulation Board – XR4120 (2.4m x 1.2m)

£46.50 ex VAT

120mm Kingspan Thermaroof TR24 PIR Flat Roof Insulation Board (1200 x 600mm)

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50mm Celotex Insulation Board – GA4050 (2.4m x 1.2m)

£20.69 ex VAT

75mm Celotex Insulation Board – GA4075 (2.4m x 1.2m)

£30.96 ex VAT

Kingspan Thermaroof TR26 120mm Flat Roof Insulation Board

£159.62 ex VAT

Knauf Rocksilk RS45 Slab – 100mm (1200 x 600mm)

£359.07 ex VAT

150mm Celotex Insulation Board – XR4150 (2.4m x 1.2m)

£57.25 ex VAT

70mm Celotex Insulation Board – GA4070 (2.4m x 1.2m)

£29.50 ex VAT

80mm Celotex Insulation Board – GA4080 (2.4m x 1.2m)

£34.33 ex VAT

Kingspan Thermaroof TR26 100mm Flat Roof Insulation Board

£203.85 ex VAT

Kingspan Thermaroof TR27 100mm Flat Roof Insulation Board

£137.11 ex VAT

Knauf Rocksilk RS45 Slab – 50mm (1200 x 600mm)

£337.27 ex VAT

Knauf Rocksilk RS45 Slab – 60mm (1200 x 600mm)

£371.90 ex VAT

Knauf Rocksilk RS45 Slab – 75mm (1200 x 600mm)

£333.42 ex VAT

Insulation For Flat Roofs

At Trade Insulations, we take pride in providing the nation with top brands at trade prices. We sell insulation for flat roofs. The purpose of flat roof insulation to help stop heat escaping from the roof ceiling of the building. Flat roof insulation products can be bought directly from us.

What are the benefits for flat roof insulation?

  • Heat rises, roof insulation will prevent heat escaping through the ceiling and improve the energy efficiency of your home as well as saving you money on energy bills.
  • Up to 30% of your heating can be lost through roofs that do not have the required insulation installed correctly.
  • Roof insulation applied correctly can mean cooler summers and hotter winters, making a more comfortable living or working space at more controlled temperatures.
  • Specific insulation materials designed for certain requirements can also prevent mould, rot or damp, can add an acoustic advantage and add extra protection with the prevention of fire.
  • Roof insulation when installed correctly can have a lifespan that lasts for as long as the building itself.
  • Flat roof insulation is installed externally. This means it can be installed at any time without affecting your homes interior.

A factor to consider when insulating a flat roof is whether the property has a warm or cold roof structure.

Insulating A Flat Roof

Flat roofs are insulated by externally layering insulation materials on top of the roof providing it is a warm roof. Plywood and vapour check is fixed to the roof and then insulation is applied and tightly butted together and secured using foil tape. Other methods then follow such as more battens, plywood, vapour check (weatherproof sheet), and drip trims etc. This method means no mess or changes need to be made to your home internally and will be an easier form of application for the property owner.

Cold Roof vs Warm Roof
  • Cold roof is when the insulation is fitted in between the rafters of the roof and timber deck is applied over the top along with weatherproof coverings. This method is referred to as cold roof as cold spots can occur at the rafters causing thermal bridging.

Using a cold roof technique can means having to replace boards on the roofs surface.

  • Warm roof is when timber deck is applied directly to the rafters and then insulation is layered on top. This method is referred to as warm roof as due to application of insulation it prevents thermal bridging.

Overall cold roof is more work, takes longer and does not provide great thermal conductivity. Warm roof is a quick and easy application that provides great thermal conductivity. Warm roof is a method used by most building professionals.

When installing roof insulation take safety measures to ensure insulation is fitted correctly around electrical sockets, wires and ventilation spaces, or the correct procedures are put into place to remove these components.

Always consult a building professional when insulating your roof to prevent the wrong insulation being installed and the possibility of damaging your property.

Some roofs are harder to reach and not accessible in which case a specialist can be called in with the correct tools to insulate the roof.


How thick should flat roof Insulation be?

An uninsulated flat roof has a U-Value of around 1.5 W/m2K. When installing insulation, you're aiming for a value of 0.25 W/m2K or less. You should insulate your flat roof with insulation boards totalling around 150mm thickness to achieve this.

How do I stop my flat roof from sweating?

Installing a good vapour barrier underneath the rafters in a roof will help to prevent warm air from flowing into the roof void from below. Multifoil insulation has reflective layers to keep heat inside the building, ensuring no risk of condensation.

How long does flat roof insulation last?

Insulation materials tend to be extremely strong and durable. You can expect flat roof insulation to last the lifespan of the building or anywhere between 25 and 40 years – when installed correctly. It’s worth noting that warm roof insulation tends to degrade faster than cold roof insulation due to the materials being exposed to the elements. However, if the waterproofing materials are installed correctly this shouldn’t cause too much concern as the insulation will last just as long.

Flat Roof Insulation Products

Kingspan Thermaroof TR24

Kingspan TR24 is part of the Thermaroof range designed for the insulation of flat roofs. Specifically flat roofs, waterproofed with fixed single-ply waterproofing. The Kingspan TR46 PIR board is suitable for new builds, extensions, and refurbishments. The installation technique creates a flat roof insulation solution for fast-track building programmes.

  • 1200mm x 600mm
  • Thicknesses, 30mm – 150mm
  • Thermal Conductivity: 0.022W/mK – 0.027W/mK
  • Material: Polyisocyanurate, Glass tissue, Thermofusible Polypropylene Fleece
Celotex PIR Insulation Board

Celotex GA4100 is a multi-purpose insulation board, which forms part of the GA4000 range. he high-performing PIR board is environmentally friendly. It is designed to reduce energy bills and therefore reduce carbon footprint. This, therefore, makes this product a great choice in construction projects of all scales.

  • 2400mm x 1200mm
  • Thicknesses, 25mm – 200mm
  • Thermal Conductivity: 0.022W/mK
  • Material: Polyisocyanurate (PIR), Low Composite Foil
Recticel Eurodeck Flat Roof Insulation Boards

This product is extremely durable and when installed correctly can provide reliable long term thermal performance that will continue to last for the entire lifetime of the building. Eurodeck helps to provide a more temperature-controlled environment meaning warmer winters and cooler summers.

  • 2400mm x 1200mm
  • Thicknesses, 25mm – 160mm
  • Thermal Conductivity: 0.022W/mK
  • Material: Polyisocyanurate (PIR), Low Composite Foil