21st Nov 2022 -

A flat roof, whether covering the whole of a building or as part of an extension, can be thermally insulated to limit heat loss. When insulating a flat roof you will have two options, making a cold roof or a warm roof. These flat roofs can then be covered by a variety of waterproofing membranes. If looking to add insulation to an existing flat roof the process becomes more expensive and time-consuming.

Warm Flat Roof vs Cold Flat Roof

The difference between a warm flat roof and a cold flat roof is to do with the location of the insulation material in each. In a warm roof, the insulation layer is on top, meaning that the roof structure is on the warm side of the insulation. In a cold roof, the insulation is between the rafters and the roof deck, leaving a cold gap under the solid board of the roof. Whether your whole building has a flat roof, or if you have a flat roof extension to a pitched roof house, you can opt for either of these options.

Should You Choose a Warm Flat Roof or a Cold Flat Roof?

The best choice of flat roof insulation for you will depend on a couple of factors. If searching for the best thermal performance, then you should opt for a warm flat roof. This will also be easier to install. The main benefit offered by cold roof insulation is that it takes up less space, which may be vital in cases where there are balconies or doors opening onto flat roofs. Cold flat roofs also offer better support for foot traffic, as users will be standing directly on a faced board, rather than on an insulation board that can be more easily compressed.

insulation boards

How to Insulate A Hot Flat Roof

A warm flat roof will be built from six sections and insulated from the outside of your roof. The top layer will be a waterproofing membrane, which will sit on top of an insulation board, with a vapour control layer below it. This will be secured over the timber roof decking. The roof joists below will be uninsulated, and can be topped with plasterboard, allowing you to decorate your room.

The vapour control layer will help to avoid condensation and moisture passing through the flat roof structure. The waterproof membrane is designed to protect your flat roof extension or building from any excess wind and rain.

How to Insulate A Cold Flat Roof

A cold flat roof is comprised of the same elements, just insulated from the inside instead. In it, only the waterproof membrane will sit on top of the timber deck. The insulation boards will be installed within the joists of the roof, with a vapour barrier installed below them, and a plasterboard ceiling below that.

Your Options For Refurbishment

There will be different options for refurbishment based on the condition of your flat roof. When the base deck is structurally sound you can opt to replace an aged or damaged waterproof membrane. If the roof is wholly compromised then you would need to opt for a complete re-do, removing all components of your flat roof and installing replacements. Installing roof insulation alone cannot be done in either warm or cold flat roofs. This can be quite an expensive process, and if your flat roof is structurally sound and offers some degree of thermal protection, you may wish to replace the waterproof membrane or simply leave it as is. In some cases, spray foam insulation can be retrofitted within flat roof voids, though this can lead to issues with condensation. 

Your Options for Waterproofing

Once you have built your insulated flat roofing system you’ll need to add a permanent covering to it. There are three main options for this, single-ply, built-up felt, or mastic asphalt.

  • Single Ply Membrane: The most popular choice, these lightweight coverings can work on a wide variety of flat roofs. They are durable and offer a series of resistances like fire, chemicals and UV light. Single-ply membranes are easy to install and repair.
  • Built-up Felt: This option is made from three layers of bituminous roofing felt, offering a high level of weather protection. Built-up felt boasts a long lifespan, with high-performance felt designed to last for roughly 30 years.
  • Mastic Asphalt: Mastic asphalt is a long-used roofing substance, with waterproofing benefits. It is seamless, and easy to repair while being strong enough to withstand heavy foot traffic.

Insulation Boards For Flat Roofs

There are a variety of rigid insulation boards designed specifically to be installed within a flat roof. Kingspan produces a range of boards such as the Kingspan Thermaroof TR26, intended for use in flat roofs faced with mechanically fixed single-ply waterproofing. Recticel also produces a range of insulation boards for flat roofs, such as the Recticel Powerdeck F which is compatible with built-up felt, mastic asphalt and approved single-ply membranes. Ensure the type of roof insulation you buy is designed specifically for use within flat roofs. You will be looking for around 120mm of insulation to get your home to the desired U-value.

How to Insulate a Dormer Flat Roof

The flat roof of a dormer can be insulated in much the same way as a larger flat roof, building either a warm or cold flat roof using insulation boards, vapour control layers and a waterproofing surface. This will limit the heat loss through your dormer flat roof.

Our Range of Flat Roof Insulation

Building Materials Nationwide stocks a huge range of thermal insulation and insulation accessories. We have a comprehensive range of insulation boards that can be used within a flat roof, both a cold and a warm design. For more of our how-to content, take a look at our series of guides. We aim to offer everything that could be needed in building supplies, with a huge selection of products, that can be delivered nationwide, all sold at competitive prices.

For our customers in the industry, we offer trade accounts. These will get you access to our full range, our express delivery options, and your own dedicated account manager. Our dedicated account managers can be spoken with via phone and email, assisting you with large and unusual orders, and sourcing needed products from our network of suppliers. Sign up for a trade account online today.

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