Flat Roof Insulation

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Hot air rises but that doesn't mean your bills have to. Flat roof insulation is a no-brainer measure to boost indoor comfort and crank down your heating bills. Between your property's ceiling and roofing membrane is where you would find flat roof insulation. The two main types are warm roof insulation and cold roof insulation; the former tends to be more popular due to the fact that it presents less condensation risk. 

Find Out More About Our Types of Flat Roof Insulation
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What Is Flat Roof Insulation? 

By definition, a flat roof, according to BS 6229, is “having a pitch less than 10° to the horizontal.” 

Insulating a flat roof is an entirely different process from that of a traditional pitched roof. However, doing so can have significant economic benefits such as lower energy use and decreased household bills. Additionally, insulation provides protection against condensation buildup.

Not only are flat roofs susceptible to leaks, but they are also often impacted by condensation, especially domestic-scale timber roofs.

It is important to understand the relationship between thermal performance, air leakage and condensation before designing a flat roof and purchasing flat roofing solutions.

Please note: If there are any signs of moisture present in the existing roof insulation, it must be replaced before the new insulation is installed.

The Benefits Of Flat Roof Insulation

  • Heat retention/prevention - Roof insulation controls heat transfer to help a build retain heat in the winter and cool in the summer. This, in turn, reduces energy bills and optimises the overall energy efficiency of a building. 
  • Sustainability - A byproduct of energy efficiency is sustainability. The less energy a building uses, the less pollution you emit into the environment. 
  • Longevity/durability - Multiple structural issues can present themselves in an uninsulated roof such as rot, damp and condensation. Installing roof insulation is a preventative measure to ensure the durability of your build. 
  • Depending on the insulation used, other benefits of insulating your flat roof could include an improvement in sound insulation.
  • Tapered insulation can be used to create a fall which eliminates ponding water on a flat roof, ensuring water drains away from the roof. This type of insulation is compatible with many different roof types. 
  • Flat roof insulation ensures that the cold air remains outside and the warm air inside.

Types of Flat Roofs

Flat Roof Insulation 

Unlike pitched roofs, flat roofs don't have loft space, so the insulation must be installed between the ceiling and the roof membrane. Flat roofs are often insulated with rigid insulation boards that are then covered with a weatherproof layer.

The insulation boards work to prevent heat transfer, thereby inhibiting heat from escaping in winter and entering in summer. If you're looking to install flat roof insulation, it's best to do so from the outside.  

Flat Roof Insulation can also be installed from the inside, but condensation issues may arise if it is not installed correctly.

Preferably, a flat roof should be insulated from above. Install a layer of rigid insulation board either directly on top of the roof's weatherproof layer or above the timber roof surface, with an additional weatherproof layer placed on top of the insulation board. 

Flat roof insulation costs will depend on which insulation method you choose and the amount of roofing insulation required. 

Warm Flat Roof 

Warm roof construction/built-up roof (BUR) is one where all the insulation is above the joists/roof deck and below the waterproofing layer, making it part of the warm fabric of the building. 

The insulation layer is positioned immediately below the waterproof membrane. The vapour barrier rests on top of the decking but underneath the insulation material; insulation can be adhered, mechanically fixed or loose-laid.

Cold Flat Roof 

A cold roof space sees thermal insulation placed beneath the structural deck and roof joists, allowing it to remain cold.

Cold flat roofs require a vapour control layer to function effectively, in conjunction with adequate ventilation to allow water vapour to escape before it condenses on the cold deck.

In this system, a cavity should be left between the insulation and the waterproofing layer to assist in ventilating. Cold flat roofs are typically a less popular method than warm flat roofs because of this and for reasons relating to space, labour costs, etc.

Frequently Asked Flat Roof Insulation Questions

What Is The Difference Between Warm Deck & Cold Deck Roof?

The terms "warm deck roof" and "cold deck roof" simply refer to the position of the insulation in an overall build-up. 

A warm deck roof, for example, places insulation above rafter level, indicating that the rafter space beneath is "warm".

Whereas, insulation in a cold deck roof, is installed beneath the rafters. The term "cold deck" doesn't mean that the roof itself is cold, but rather that anything above the insulation is subject to colder temperatures. This happens because there's no barrier over the top of the rafters to prevent air from seeping in and circulating through them.
Let's break down each method below. 

Cold Deck: 

Installing insulation in a cold deck roof at ceiling level creates a colder temperature void, especially during winter, which can contribute to the formation of condensation. This can lead to rotting timber, damp ceilings, and damaged structural integrity. As a result, it's critical to make sure the void is well-ventilated. 

Warm Deck: 

In a warm deck roof, the insulation is installed below membrane on top of the roof deck--usually with a vapour control layer placed underneath the insulation. With this type of construction, there's much less risk of condensation because there is no cold surface upon which the condensation can form and the void remains at a similar temperature to the roof.

What Is The Recommended Depth For Loft Insulation? 

Current building regulations recommend a minimum insulation depth of 270mm (blanket insulation such as mineral or glass wool) in the loft.

Multiple layers can be added to the build-up to achieve this. 

Do You Need To Vent A Flat Roof? 

We highly suggest that you ventilate a flat roof; regulations advise that you leave a 50mm gap between insulation and the underside of the roof covering. This applies to both warm and cold roof spaces. 

What Are The Four Main Materials Used in Roof Insulation? 

  • Fibreglass insulation
  • Insulation boards/insulation panels
  • Glass mineral wool insulation
  • Spray foam insulation (PUR)

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