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How to Properly Insulate Your Flat Roof


13th April 2018

Flat roofing insulation will have a huge impact on the energy efficiency of your property. The type of insulation required will depend on the immediate environment in your area and specific building regulations.

Implementing the right level of protection will help to maintain the other materials used in the construction of the flat roof, which means it will fulfil its projected lifespan. If this is your first time thinking about insulation, then our guide will help you understand the basics of insulating a flat roofing system.

What is Flat Roofing Insulation?

Insulation for flat roofing is used to make a structure more energy efficient and minimise heat loss. There are many options available to fulfil this requirement as you will find out below.

Laying Flat Roof Insulation - Roof Stores
Laying Flat Roof Insulation

As a rule, any roof with a pitch below 10° is classified as a flat roof. In most cases, there will not be a loft in a building that features flat roofing. This means that your approach to insulation will differ from a standard pitched roof.

There are two main types of flat roof that require different approaches to installation:

  • Warm Flat Roofing: Insulation material is placed on top of the roof, rather than in-between the ceiling and the decking.
  • Cold Flat Roofing: Insulation is placed in-between the joists underneath the roof surface above.

Both these variants require different approaches to insulation which will be covered in this guide.

How to Insulate a Flat Roof

As there are two types of flat roofs available that require different types of insulation, the methods used understandably vary. Professional installation companies are always the best choice for these type of jobs, as they will have the skills and experience necessary to produce the very best results.

  • Warm Flat Roof: One of the main benefits you will receive from installing a warm flat roof is condensation will not become an issue because of the positioning of the insulation. At least 100mm of rigid board insulation must be positioned on top of the vapour control layer (the vapour control layer stops any moisture within the building contaminating the insulation). The waterproofing system is then applied over the top of the insulation.
  • Cold Flat Roof: Many properties use this option, as there are no additional costs for scaffolding or organising how to gain safe access to the roof exterior. The insulation material is placed between the supporting joists, leaving a gap of 50-60mm between the roof and insulation. This will enable the airflow to circulate freely. If insulation is being added to an older building, then it is common practice for the ceiling itself to be taken down. For properties that only have shallow joists above the ceiling, spray foam could be the best alternative, depending on the judgement of the installation company.

What Types of Flat Roof Insulation are Available?

Here is a list of the most commonly used materials for flat roof insulation and how they differ to allow you to make an informed choice:

  • Wood Fibre: This is an organic board compromised of wood or cane, mixed with several binders and fillers. To enhance its water resistance, asphalt can sometimes also be coated onto the surface.
  • Perlite: Similarly, perlite is mixed together with organic fibres and binders, along with air cells which are trapped into the material to insulate the roof. An asphalt coating is also applied to reduce absorption into the insulation material.
  • Polystyrene: There are two options available for polystyrene insulation – expanded and extruded. A foaming agent is mixed with polystyrene to produce the expanded version, while extruded polystyrene is comprised of heated polystyrene that has been heated and then expanded.
  • Polyisocyanurate: This is placed between the non-organic and organic felt within the flat roof. Using a blowing agent, the foam material is expanded which then creates a high level of thermal resistance.
  • Cellular Glass: Crushed glass is heated after being placed into a mould and can be bought in flat or tapered blocks for use in insulation on flat roofs.

If you're unsure which material would best suit your flat roof project, always consult an insulation or roofing professional for advice.

How Much Does It Cost to Insulate a Flat Roof?

For the most up to date pricing on flat roofing insulation, follow the links in the section below.

In most cases, flat roofing insulation is included in the costings put together by installation companies when quoting for a new roof. Alternatively, existing insulation material may be reused when a new roof is installed if it's still in good enough condition.

Professional firms will be able to supply a specific cost for insulation materials if that is your only requirement. You will also find it detailed on the quote sheet as part of the job.

Powerdeck F Insulation Board - Roof Stores
Powerdeck F Insulation Board

If you are thinking of installing insulation into a warm flat roof yourself, the Powerdeck U Insulation Board is specifically designed for this purpose. It is suitable for use with any hot-applied waterproofing system (including torch-on) and any bitumen compatible single-ply membrane system.

Another option is Powerdeck F Insulation Board - a durable, market leading PIR insulation board for built-up felt, mastic asphalt and single ply membrane waterproofing systems.

Both these options offer industry-leading insulation solutions for flat roofing at a manageable price point.

What are the Benefits of Insulating a Flat Roof?

Now that you know the differences between warm and cold systems, the next thing to understand is their respective benefits. Much will depend on the type of building, its position on your land and the type of environment.


Warm Flat Roof

For optimal thermal efficiency, this is by far the best option of the two. Since the government laid out new thermal and insulation regulations in 2010, it has emerged that warm roof systems are the best way to meet the required standards.

This will also be reflected in the Energy Performance Certificate for your property which is a requirement for all buildings. The knock-on effect is that this will lead to lower energy usage and, of course, a reduction in associated bills.

Condensation is much less of a problem with this system and ventilation is also not a requirement. This is particularly useful for bathrooms, kitchens or any other areas of the building that produce high levels of humidity.

In general, the roof structure is better protected from the more extreme scales of hot and cold weather. It will also prevent warm air from drifting into the roof void.

Cold Flat Roof

The lower cost of installing a cold roof is usually one of its main advantages. In general, it is suited to external outbuildings. The potential heat loss will not have an impact on the liveable spaces and the quality of life within them.

This is also a preferred option for residents who live in hot countries or regions. The temperature dramatically increases during hotter seasons, making the property an easy place to reside in.

What is the Lifespan of Flat Roof Insulation?

In general terms, the insulation used for warm flat roofs will degrade faster than those used for cold roofs. This is due to their being positioned closer to the outside elements. However, there is little cause for concern. Insulation materials are particularly robust and will last at least as long as the waterproofing material itself. That can be anywhere between 25-40 years.

Flat Roof Insulation Cross Section - Roof Stores
Flat Roof Insulation Cross Section

A great example of this is Powerdeck F Insulation Board which is both rot-resistant and highly durable. Depending on the weather and usage of the roof during its lifespan, the insulation board may still be in a good enough condition to re-use when the time arrives to replace the roof itself.


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