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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 (show)

Brand: Celotex Model and Size: 1200 x 600 - 7.2m2
Delivery Time: 3 - 5 days
Celotex TC3000 provides a quick and easy method of achieving effective thermal insulation in built-up flat roofing applications including hot applied bituminous, mastic asphalting and torch-on systems. With bitumen polypropylene fleece facers on both sides, TC3000 is manufactured from rigid Polyisoc..
Brand: Cellecta Model and Size: 1200x2400mm - 2.88m2
Delivery Time: 2 - 4 days
62mm Cellecta Hexatherm XPLY is a high performance extruded polystyrene thermal insulation roof board bonded to a 12mm plywood laminate facing, also suitable for internal walls and floors. It combines the benefits of high performance extruded polystyrene with a facing board to produce a composite th..
Brand: Isoroc Model and Size: 1000mm x 600mm - 1.8m2
Delivery Time: 1 - 2 days
100mm Isoroc Isofas Mineral Wool Insulation Slab is a reliable and durable non-flammable insulation solution for universal use. These slabs can be used in external and internal wall insulation as thermal, acoustic and fire insulation. They can be used as a barrier for fire protection...
Guide Price: £41.00
£49.20 inc VAT
Brand: Recticel Model and Size: 600x1200 - 11.52m2
Delivery Time: 1-3 working days
30mm Recticel Powerdeck U is a high performance, rigid PIR board for use in warm flat roofs under torch-on with bituminous waterproofing systems for new build and refurbishment projects and in any bitumen compatible single-ply membrane system. It is core faced on both sides with a bituminous glass f..
Guide Price: £146.93
£176.32 inc VAT
Brand: Recticel Model and Size: 600x1200 - 14.40m2
Delivery Time: 1-3 working days
25mm Recticel Powerdeck F is a durable, high strength PIR insulation board for built-up felt, mastic asphalt and single-ply membrane waterproofing systems. It has a PIR core faced on both sides with a gas tight alkali resistant facing - perforated mineral coated glass fibre tissue to both sides. It ..
Guide Price: £101.78
£122.14 inc VAT
Brand: Soprema Model and Size: 1250x600mm - 10.50m2
Delivery Time: 2 - 3 working days
30mm Soprema XPS SL is an extremely durable, high strength extruded polystyrene thermal insulation board offering high water resistance for flat and pitched roofs. It can be stored and installed in severe wet weather conditions with no deterioration during its life. It is easy to cut and customise o..
Guide Price: £102.21
£122.65 inc VAT
Brand: Kingspan Model and Size: 1200x600mm - 4.32m2
Delivery Time: 3 - 5 days
50mm Kingspan Thermaroof TR24 is a rigid insulation board used for flat roof insulation, consisting of a rigid thermoset insulation core, covered with a glass tissue coated with bitumen. Its upper surface is finished with a thermo fusible polypropylene fleece, while the lower surface is covered with..
Guide Price: £92.33
£110.80 inc VAT
Brand: Cellecta Model and Size: 1.5m x 50m - 75m2
Delivery Time: 2 - 3 working days
Cellecta HexaTex is a high performance thermal improvement membrane suitable for use on inverted roofs finished with gravel ballast, and concrete pavers on appropriate supports. Dramatically minimises the cooling effect associated with rainwater flowing down through the insulation and draining away...
Brand: Icopal Model and Size: 1m x 5m - 120m2
Delivery Time: 7 - 10 days
Icopal Extra Strength Top Coat Bitumen Roofing Felt is an extra strength top coat roofing felt, made of heat-hardened polyester and thicker waterproofing on both sides for roofs and foundations. A latest generation, patented, specialised roofing felt for flat roofs. Due to the shorter heating t..
Brand: Icopal Model and Size: 1m x 10m - 200m2
Delivery Time: 7 - 10 days
Icopal  Roofing Felt Underlay 3mm Hydrobit V60 S30 Bitumen - Torch On is an extra strength base bitumen roofing felt, made of glass fleece reinforcement, coated with SBS modified bitumen blend with mineral filler, top side is finished with sand, bottom side is finished with foil. A latest ..
25mm Flat Roof Insulation Board Xtratherm FR-BGM (pack of 18)
Guide Price
Brand: Xtratherm Model and Size: 1200x600mm - 12.96m2
Delivery Time: 3 - 5 days
25mm Xtratherm FR-BGM is a PIR insulation board that is faced with a fleece finished bitumen/glass facers and is suitable for use below partially bonded bitumen based built up roofing systems. FR-BGM is suitable to apply torch on felt directly on the insulation. FR-BGM can also be used in mechanical..
Guide Price: £120.29
£144.35 inc VAT
Brand: Thermafleece Model and Size: 390mm x 10m - 11.7m2
Delivery Time: 2 - 3 working days
Thermafleece SupaSoft offers a safe, sustainable and cost effective way to insulate your loft. It contains 100% recycled polyester and is made almost entirely from recycled plastic bottles. SupaSoft contains no harmful chemicals or binders and is completely safe to handle. It can be used to ins..
Guide Price: £50.04
£60.05 inc VAT
50mm Rockwool Rockton Insulation Slab - 1000mm x 610mm (pack of 12)
Guide Price
Brand: Rockwool Model and Size: 1000mm x 610mm - 7.32m2
Delivery Time: 1 - 2 days
50mm Rockwool Rocton Slabs are a reliable and durable non-flammable insulation solution for universal use. These high density slabs can be used in partition walls, curtain walls, within steel and timber frame structures, over ceilings and between floor joists. Rockwool Rocton has the highest so..
Brand: Xtratherm Model and Size: 2400x1200 - 2.88m2
Delivery Time: 1-3 working days
56mm Xtratherm FR/TP Ply is a composite insulated panel of 50mm Xtratherm Polyisocyanurate core with a composite foil face, bonded to 6mm WBP grade plywood board. Xtratherm FR/TP Ply is designed to provide high levels of thermal insulation and decking in one operation for new and refurbishment flat ..
25mm Xtratherm Thin-R FR/ALU Flat Roof PIR Insulation Board (pack of 12)
Guide Price
Brand: Xtratherm Model and Size: 2400x1200mm - 34.56m2
Delivery Time: 1-3 working days
25mm Xtratherm Thin-R FR/ALU Flat Roof PIR Insulation Board is a flat roof insulation with enhanced vapour resistant performance. Thin-R FR/ALU board is faced with low emissivity aluminium facings both sides. Rot proof and will remain effective for the life span of the building. Suitable for use bel..
Brand: Thermafleece Model and Size: 16.7m x 1.5m - 25m2 - Free Delivery
Delivery Time: 3 - 5 days
Thermafleece Breather Membrane  is a light weight vapour permeable underlay for use in floors and walls or on top of insulation as a protective layer. Thermafleece Breather Membrane is lightweight and easy to handle membrane comprises of 3 layers; two outer layers made from a non..
Guide Price: £50.50
£60.60 inc VAT
50mm Polyfoam XPS Roofboard - Extra grade - Extruded Polystyrene Board (pack of 9) - 600mm x 1250mm
Guide Price
Brand: Polyfoam XPS Model and Size: 1250x600mm - 6.75m2
Delivery Time: 5 - 7 days
50mm Polyfoam XPS Roofboard - Extra grade is a high strength (kPA 300) extruded polystyrene board (XPS) with high compressive strength highly resistant to water absorption  suitable for roofing applications. It is a lap jointed board and withstands both occasional and long term s..
Brand: Polyfoam XPS Model and Size: 1200x600mm - 30.24m2
Delivery Time: 5 - 7 days
56mm Polyfoam XPS Upstand Board is a high strength (kPA 200) extruded polystyrene laminated board used for the thermal insulation and protection of upstand and parapet walls. Polyfoam XPS Upstand board consists of a 50mm high performance rigid extruded polystyrene (XPS) with a tough, weather re..
17.5mm Polyfoam XPS Laminating Board - (pallet of 68) - 1200mm x 2400mm
Guide Price
Brand: Polyfoam XPS Model and Size: 1200x2400mm - 195.84m2
Delivery Time: 5 - 7 days
17.5mm Polyfoam XPS Laminating Board is a high strength (kPA 200) extruded polystyrene board (XPS) that can be used as a thermal and structural insulation as a fabrication board for a wide variety of manufacturing processes and applications. This board has high resistance to water absorption an..
Brand: Polyfoam XPS Model and Size: 1220x3000mm - 204.96m2
Delivery Time: 5 - 7 days
21mm Polyfoam XPS Laminating Board is a high strength (kPA 200) extruded polystyrene board (XPS) that can be used as a thermal and structural insulation as a fabrication board for a wide variety of manufacturing processes and applications. This board has high resistance to water absorption and ..
Polyfoam Slimline Zero Membrane - 1.5m x 100m Polyfoam Slimline Zero Membrane - 1.5m x 100m
Guide Price
Brand: Polyfoam XPS Model and Size: 100m x 1.5m - 150m2
Delivery Time: 5 - 7 days
Polyfoam Slimline Zero Membrane - 1.5m x 100m is a high performance, thermally bonded tri-laminate of polypropylene waterproofing membrane  to be used over flat roof, inverted roofs and green roofs, to reduce the amount of rain water flowing between the insulation and the waterproofing mem..
Brand: Unilin Insulation (formerly known as Xtratherm) Model and Size: 1200x600mm - 12.96m2
Delivery Time: 3 - 5 days
25mm Unilin FR-BGM is a BBA certified high performance PIR insulation board that is faced with a fleece finished bitumen/glass facers and is suitable for use below partially bonded bitumen based built up roofing systems. Unilin FR-BGM is suitable to apply torch on felt directly on the insulation. FR..
Guide Price: £120.31
£144.37 inc VAT
Brand: Unilin Insulation (formerly known as Xtratherm) Model and Size: 2400x1200mm - 34.56m2
Delivery Time: 1-3 working days
25mm Unilin Thin-R FR/ALU Flat Roof PIR Insulation Board is a flat roof insulation with enhanced vapour resistant performance. Unilin Thin-R FR/ALU board is faced with low emissivity aluminium facings both sides. Rot proof and will remain effective for the life span of the building. Unilin Thin..
Brand: Unilin Insulation (formerly known as Xtratherm) Model and Size: 1200x2400mm - 34.56m2
Delivery Time: 1-3 working days
25mm Unilin XtroLiner XtroDeck XO/XD board has a core of modified polyisocyanurate (PIR) bonded to low emissivity embossed aluminium foil facings. Unilin XtroLiner XtroDeck XO/XD board offers excellent insulation performance with excellent thermal conductivity.Unilin XtroLiner XtroDeck XO/XD bo..
Brand: Celotex Model and Size: 1200x2400 - 37.44m2
Delivery Time: 1 - 5 working days
90mm Celotex TA4000 PIR Insulation Board (pack of 13) Size: 1200mm x 2400mm Area per pack: 37.44m2 Thermal conductivity: 0.022 W/Mk R-Value: 4.05m2K/W Compressive strength: ≥150kPa Water vapour resistance: >150 MNs/g Weight: 3.09kg/m2   Celotex TA4000 prov..
6mm Knauf Rooftop Aquapanel Cement Board - 900mm x 1200mm
sale!
Brand: Knauf Model and Size: 900mm x 1200mm - 1.08m2
Delivery Time: 5 - 7 days
6mm x 900mm x 1200mm Knauf Rooftop Aquapanel Cement Board is an extremely durable construction board highly resistant to moisture, mould and mildew growth. It can be used as a cover board above the insulation and as a substrate to take on the waterproofing layer. Knauf Aquapanel Cement Board Ro..
£69.50 (£83.40 inc VAT)
£59.50
£71.40 inc VAT
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