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Flat Roof Insulation

The type of insulation used in a new flat roof is dependent on the roof specification. On a new concrete slab, with a screed, semi-rigid insulation boards such as expanded polystyrene board, extruded polystyrene board, glass fibre batts, mineral fibre batts, urethane foam board or phenolic foam board are laid under the roof covering. In a new timber structure, glass fibre quilt and mineral fibre quilt can be laid between the joists. In case of an existing flat roof, insulation can be increased externally with extruded polystyrene or foamed glass, or internally with an insulated lining board such as mineral fibre or polyurethane foambacked plaster-board.

Three choices are available for insulating a flat roof depending on the roof specification:
  • Cold roof
  • Warm roof
  • inverted roof

It is essential to understand the difference between those specifications to be able to install insulation in the right place to create a vapour barrier in the right area and prevent rotting of the roof joists.

Warm roof

A warm roof is when the insulation spans across the top of all the joists; it must be continuous and span over every joist. This means that any air rising from underneath would make the void between the joists warm hence a ‘warm roof’.

Cold roof

In a cold roof the insulation is put at the bottom of the joists or below the joists. This way sufficient ventilation is maintained which allows air to circulate above the insulation (below the roof). The void above the insulation should be at least 50mm thick. In this case the decking with the roof on it, which is above the joists, would be cold hence a ‘cold roof’.

Common mistake in the roof specification is when the insulation is installed inside the joists (close to the top part of the joists). This makes the roof neither cold nor warm, creating the void below the insulation. In this case there is no ventilation which makes the whole roof ‘sweat’ and causes the joists to rot.

Inverted roof

The inverted roof concept which is also known as protected membrane or upside down roofing. This method ensures insulating both the water proofing membrane and the reinforced concrete slab from extreme thermal stress. This is achieved by placing an extruded polystyrene thermal insulation board above, instead of below the waterproofing membrane and can be used for insulating flat topped buildings or balconies, retail or office developments, flats, hospitals, airports, hotels and residential housing in areas where roof is exposed to high temperatures.

Insulationshop.co offers a wide range of high density flat roof insulation from manufacturers such as Xtratherm, Celotex, Knauf, Kay Metzeler, Cellecta and more. For more information about particular products please see product description.

CHOOSE MANUFACTURERS or scroll down to see products

59mm Plydeck Xtratherm (PIR with OSB Board) - 50mm PIR + 9mm OSB Board
Guide Price
Brand: Xtratherm Model and Size: 2400x1200 - 2.88m2
Delivery Time: 1-3 working days
Xtratherm Plydeck is a composite insulated panel of 50mm Xtratherm Polyisocyanurate core with a composite foil face, bonded to 9mm OSB board. The Xtratherm Plydeck provides a high level of thermal insulation and decking in one application.APPLICATIONPlydeck is designed to provide high levels of ther..
Guide Price: £77.01
£92.41 inc VAT
50mm Xtratherm Flat Roof Board FR-MG (pack of 10)
Guide Price
Brand: Xtratherm Model and Size: 1200x600mm - 7.2m2
Delivery Time: 1 - 2 days
Xtratherm FR-MG is a mineral coated glass faced board. Xtratherm XT/FR MG boards should be laid over the vapour control layer in a break bonded pattern. The long edges of the boards should be laid at right angles to the direction of the substrate and all edges must be fully supported by the deck. Th..
Guide Price: £114.35
£137.22 inc VAT
56mm Kingspan Thermaroof TR31 PIR bonded to Plywood (pack of 20) - 50mm PIR + 6mm Plywood Sheet 56mm Kingspan Thermaroof TR31 PIR bonded to Plywood (pack of 20) - 50mm PIR + 6mm Plywood Sheet
Guide Price
Brand: Kingspan Model and Size: 1200x2400 - 57.60m2
Delivery Time: 3 - 5 days
56mm Kingspan Thermaroof  TR31 is the flat roof insulation comprising a 50mm high performance rigid PIR thermoset insulation core faced with an FSC certified 6 mm nominal thickness WBP exterior grade plywood on its upper surface and a low emissivity composite foil facing on its lower surfa..
96mm Recticel Plylok PIR Flat Roof Insulation Board - 90mm PIR + 6mm Plywood Sheet
Guide Price
Brand: Recticel Model and Size: 1200x2400mm - 2.88m2
Delivery Time: 1-3 working days
96mm Recticel Plylok PIR Flat Roof Insulation Board is a rigid PIR board consisted of a few bonded elements contributing to its thermal performance and rigidity. The closed cell rigid 90mm PIR foam core is faced on both sides with multi-layer aluminium foil and bonded to 6mm WBP plywood. It is ..
Guide Price: £131.60
£157.92 inc VAT
96mm EcoTherm Eco-Deck - Insulated Flat Roof Decking
Guide Price
Brand: EcoTherm Insulation Model and Size: 1200x2400mm - 2.88m2
Delivery Time: 2 - 3 working days
96mm EcoTherm Eco-Deck is a tough, structural, pre-insulated thermal composite board for flat roofs. It comprises a 90mm thick fibre free polyisocyanurate (PIR) insulation core faced with 6mm thick tough, exterior grade plywood and is ready to fix and finish using basic tools and traditional roofing..
Guide Price: £114.09
£136.91 inc VAT
20mm Celotex TB4000 PIR Insulation Board - TB4020
Guide Price
Brand: Celotex Model and Size: 1200x2400mm - 2.88m2
Delivery Time: 1-3 working days
Celotex TB4020 is targeted at cut-to-fit applications between rafters or joists and enables users to achieve lower U-values with a single layer of insulation than has previously been possible. It is also appropriate for use in floors and walls. With low emissivity foil facings, TB4020 is manufa..
Guide Price: £23.48
£28.18 inc VAT
50mm Celotex Crown-Fix Flat Roof Board (pack of 24) - pallet of 2 packs
Guide Price
Brand: Celotex Model and Size: 2400x1200mm - 138.24m2
Delivery Time: Out Of Stock
50mm Celotex Crown-Fix is a brand new product for use in flat roof systems. Representing the next phase of Celotex innovation, Crown-Fix is a high performance rigid polyisocyanurate (PIR) insulation which delivers excellent dimensional stability and added robustness in a mechanically fixed single pl..
Guide Price: £2,102.07
£2,522.48 inc VAT
50mm Celotex Crown-Bond Flat Roof Board (pack of 11) -pallet of 12 packs
Guide Price
Brand: Celotex Model and Size: 1200x600mm - 95.04m2
Delivery Time: Out Of Stock
50mm Celotex Crown-Bond is a brand new product for use in flat roof systems. Representing the next phase of Celotex innovation, Crown-Bond is a high performance rigid polyisocyanurate (PIR) insulation which delivers cutting edge thinness, increased dimensional stability and added robustness in a bon..
100mm Celotex Crown-Up Flat Roof Board - pack of 6 - pallet of 12 packs
Guide Price
Brand: Celotex Model and Size: 1200x600mm - 51.84m2
Delivery Time: Out Of Stock
100mm Celotex Crown-Up is a brand new product for use in flat roof systems. Representing the next phase of Celotex innovation, Crown-Up is a high performance rigid polyisocyanurate (PIR) insulation which delivers increased dimensional stability and added robustness in a built-up flat roofing system...
Guide Price: £1,593.81
£1,912.57 inc VAT
25mm Celotex FR5000 Fire Resistant PIR Insulation Board (pack of 16)
Guide Price
Brand: Celotex Model and Size: 1200x2400mm - 46.08m2
Delivery Time: Out Of Stock
Celotex FR5025 is one of the available dimensions of the Celotex FR5000 PIR insulation boards made from polyisocyanurate rigid core and faced on both side with low emissivity aluminum foils for improved thermal performance. The product is applicable to different types of constructions incl..
25mm Kingspan Thermaroof TR26 (pack of 12)
Guide Price
Brand: Kingspan Model and Size: 2400x1200mm - 34.56m2
Delivery Time: 3 - 5 days
25mm Kingspan Thermaroof TR26 is a high performance, fibre free, rigid thermoset PIR insulation core faced on both sides with a low emissivity composite foil facing which is used for insulating flat roofs and is fully compatible with most mechanically fixed single ply waterproofing systems and ..
Guide Price: £420.45
£504.54 inc VAT
25mm Kingspan Thermaroof TR27 (pack of 12)
Guide Price
Brand: Kingspan Model and Size: 25mm TR27 - 8.64m2
Delivery Time: 3 - 5 days
25mm Kingspan Thermaroof TR27 is a PIR insulation board made from highly effective rigid PIR core faced with coated glass tissue on both sides added during manufacturing. The product offers substantial durability and long life span depending on the condition of the supporting structure. Th..
Guide Price: £101.65
£121.98 inc VAT
Brand: Kingspan Model and Size: K7 2400x1200 - 34.56m2
Delivery Time: 3 - 5 days
25mm Kingspan Kooltherm K7 Pitched Roof Board is the rafter/joist level insulation comprising a premium performance rigid thermoset phenolic insulation core faced on both sides with a low emissivity composite foil facing. This reflective, low emissivity surface improves the thermal resista..
30mm Kingspan Styrozone H350R Flat Roof XPS Board (pack of 14)
Guide Price
Brand: Kingspan Model and Size: 1250x600mm - 10.50m2
Delivery Time: 3 - 5 days
Kingspan Styrozone H350R Flat Roof XPS Board is highly effective rigid insulation board made from extruded polystyrene core covered with smooth skin on both sides. It is especially designed for protected membrane flat roofs, but is also used in green roofs as a popular alternative to tradi..
Guide Price: £74.38
£89.26 inc VAT
Brand: Kingspan Model and Size: 3m x 100m - 300m2
Delivery Time: 3 - 5 days
Kingspan Aquazone is a high performance non-woven polyethylene membrane suitable for use on inverted roofs finished with gravel ballast, green roof systems and concrete pavers on appropriate supports. Kingspan Aquazone can can dramatically minimise the cooling effect associated with rainwater f..
£566.21
£679.45 inc VAT
25mm Rocksilk RS45 Universal Insulation Slab Knauf (pack of 20)
Guide Price
Brand: Knauf Model and Size: 1200x600 - 14.4m2
Delivery Time: 2 - 3 working days
25mm Rocksilk RS45 Universal Slabs are rock mineral wool slabs, available in a range of densities and designed for use in multiple thermal and acoustic applications where density is critical. The standard product is supplied unfaced, but slabs can also be manufactured with a factory applied foi..
Guide Price: £41.63
£49.96 inc VAT
50mm Rocksilk RS60 Universal Insulation Slab Knauf (pack of 9)
Guide Price
Brand: Knauf Model and Size: 1200x600 - 6.48m2
Delivery Time: 1 - 2 days
50mm Rocksilk RS60 Universal Slabs are rock mineral wool slabs, available in a range of densities and designed for use in multiple thermal and acoustic applications where density is critical. The standard product is supplied unfaced, but slabs can also be manufactured with a factory applie..
Guide Price: £41.63
£49.96 inc VAT
50mm Rocksilk RS100 Universal Insulation Slab Knauf (pack of 6)
Guide Price
Brand: Knauf Model and Size: 1200x600 - 4.32m2
Delivery Time: 2 - 3 working days
50mm Rocksilk RS100 Universal Slabs are rock mineral wool slabs, available in a range of densities and designed for use in multiple thermal and acoustic applications where density is critical. The standard product is supplied unfaced, but slabs can also be manufactured with a factory appli..
Guide Price: £35.43
£42.52 inc VAT
Brand: Knauf Model and Size: 1200x600 - 8.64m2
Delivery Time: 2 - 3 working days
50mm Earthwool Flexible is a non-combustible, inorganic rock mineral wool slab, used for thermal and acoustic insulation and specifically designed for installation by friction fitting. Earthwool Flexible is odourless, non-hygroscopic, rot proof, does not sustain vermin and will not encourage th..
£45.31
£54.37 inc VAT
Brand: EcoTherm Insulation Model and Size: 1200x2400mm - 2.88m2
Delivery Time: 1 - 5 working days
25mm Eco-Versal is a foil faced universal solution for insulating walls, floors, pitched roofs, dormer cheeks and ceilings. It is ideal for insulating timber frame walls and can also be used to insulate solid walls internally. EcoTherm Eco-Versal comprises a fibre free rigid polyisocyanurate (PIR) i..
£26.34
£31.61 inc VAT
60mm Rockwool Hardrock Multi-Fix (Dual Density ) Fleece Faced Board (pallet of 20)
Guide Price
Brand: Rockwool Model and Size: 1200x1000 - 24.00m2
Delivery Time: 2 - 3 weeks
60mm Rockwool Hardrock Multi-Fix Dual Density Fleece Faced Boards are manufactured from renewable volcanic stone which produces a non-combustible roofing insulation board. A uniquely versatile solution suitable for all roof covering types - torch, pour and roll, single ply, EPDM, liquid applied and ..
Guide Price: £522.27
£626.72 inc VAT
30mm Rockwool RW3 Slab (Prorox SL930) - pack of 15
Guide Price
Brand: Rockwool Model and Size: 1200x600mm - 10.8m2
Delivery Time: 2 - 3 working days
30mm Rockwool RW3 Slab (Prorox SL 930) is a semi-rigid resin bonded slab, providing great thermal, acoustic and fire resistance, designed to be used in various applications in floors, walls and roofs, including special applications in industrial buildings for insulation of  ships, offshore plat..
Guide Price: £58.64
£70.37 inc VAT
50mm Rockwool RW5 Universal Insulation Slab (Prorox SL960) - pack of 4
Guide Price
Brand: Rockwool Model and Size: RW5 50mm - 2.88m2
Delivery Time: 2 - 3 working days
50mm Rockwool RW5 (Prorox SL960) is a high quality resin bonded rigid slab for a wide range of thermal, acoustic and fire insulation requirements both within buildings and for industry. They are suitable for many applications including thermal insulation for floors, walls, roofs and boiler rooms. Th..
Guide Price: £42.82
£51.38 inc VAT
Brand: Rockwool Model and Size: 1200x600mm - 8.64m2
Delivery Time: 1 - 2 days
50mm x1200mm x 600mm Rockwool Flexi Insulation Slab is a unique semi-rigid thermal and acoustic insulation product made from volcanic rock. It has a flexible edge, specifically developed using patented technology for a fast and easy, perfect friction fit between timber and metal frame syst..
£42.50
£51.00 inc VAT
25mm Rockwool RWA45 Slab (Prorox SL920) - pack of 16
Guide Price
Brand: Rockwool Model and Size: 600x1200mm - 11.52m2
Delivery Time: 1-3 working days
25mm Rockwool RWA45 is a high quality resin bonded flexible slab for a wide range of thermal, acoustic and fire insulation requirements both within buildings and for industry. They are suitable for many applications including thermal insulation for floors, walls, roofs and boiler rooms. The str..
Guide Price: £28.50
£34.20 inc VAT
50mm Rockwool Flexi Insulation Slab 1200 x 400mm (pack of 12)
Guide Price
Brand: Rockwool Model and Size: 1200x400mm - 5.76m2
Delivery Time: 1 - 2 days
50mm x1200mm x 400mm Rockwool Flexi Insulation Slab is a unique semi-rigid thermal and acoustic insulation product made from volcanic rock. It has a flexible edge, specifically developed using patented technology for a fast and easy, perfect friction fit between timber and metal frame syst..
Guide Price: £30.15
£36.18 inc VAT
25mm Yelofoam X2i Cellecta - XPS Insulation
Guide Price
Brand: Cellecta Model and Size: 2500x600mm - 1.5m2
Delivery Time: 2 - 4 days
25mm Cellecta Yelofoam is made of extruded polystyrene (XPS) foam blown with a mix of HFA & CO2 gases, containing a yellow pigment, flame retardant additive and nucleating agent. Cellecta insulation products are manufactured to the highest possible specification. Ex All product’s’ physical prope..
Guide Price: £17.72
£21.26 inc VAT
50mm Cellecta Hexatherm XROOF 300L Thermal Roof XPS Insulation Board
Guide Price
Brand: Cellecta Model and Size: 600x1250mm - 0.75m2
Delivery Time: 2 - 4 days
50mm Cellecta Hexatherm XROOF 300L is an extruded polystyrene thermal insulation board designed specically for inverted roof applications where a high resistance to compression and water absorption is essential. XROOF roof insulation boards are ideal for a multitude of roofing requirements, inc..
50mm Cellecta Hexatherm XDRAiN Drainage Channel Inverted Roof Board - XPS Insulation 50mm Cellecta Hexatherm XDRAiN Drainage Channel Inverted Roof Board - XPS Insulation
Guide Price
Brand: Cellecta Model and Size: 600x1250mm - 0.75m2
Delivery Time: 2 - 4 days
50mm Cellecta Hexatherm XDRAiN is a high performance extruded polystyrene inverted thermal roof boards, featuring integrated drainage channels on their underside, which speeds up the shedding of rainwater from the roof, eliminating the need for a separating drainage mat, making them ideal for green ..
56mm Cellecta Hexatherm XMD Vertical Up-Stand Roof Board - XPS Insulation
Guide Price
Brand: Cellecta Model and Size: 1200x2400mm - 2.88m2
Delivery Time: 2 - 4 days
56mm Cellecta Hexatherm XMD is a high performance extruded polystyrene thermal insulation board bonded to a 6mm thermal laminate facing, suitable for vertical upstands. It combines the benefits of high performance extruded polystyrene with a facing board to produce a composite thermal board to suit ..
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