How to Insulate a Flat Roof in Various Ways

Can you put insulation on a flat roof? A flat roof, whether as part of an extension or covering the whole building, can be thermally insulated to reduce heat loss. To insulate your flat roof properly, you must be careful about the methods and materials used. You have two options when working on attic insulation in Toronto or insulating a flat roof: you can make a cold roof or a warm roof. The primary difference between a warm flat roof and a cold flat roof is the location of the insulation material. On a warm roof, the insulation material is on top, while on a cold roof, the insulation material is between the rafters and the roof deck.

Ways to Insulate a Flat Roof

There are several ways of insulating a flat roof:

1. Insulating Between Ceiling Rafters

Typically, two types of insulation are used for insulation between ceiling rafters. Spray foam insulation or dense-pack cellulose insulation. The spray foam insulation can either be closed or open cell. It is often challenging to properly ventilate a flat space since air doesn’t move in a straight line. It is common for moisture to accumulate in flat spaces; if trapped, it can cause extensive damage. It is better and more efficient to have non-ventilated space and instead fill the full cavity with the dense-pack cellulose or even seal the entire area with spray foam.

When choosing between spray foam insulation and dense-pack cellulose insulation, you must consider the rafter height. You will only be able to use close-cell spray foam insulation if you have 2×6 rafters existing due to the R-value restrictions. You can use open-cell spray foam or dense-pack cellulose if your rafters are at least 2×8’s.

The cost of open-cell foam is around $2.75/square foot, while that of the closed-cell spray foam is around $4/square foot. You should never use fibreglass insulation in an area like this.

2. Insulating the Exterior of a Roof Surface

Insulating a flat roof from the outside is mainly common for commercial roofs but can also work for residential roofs. This insulation procedure involves applying a strong adhesive to a foam board and laying the board on the roof surface. The insulation is secured using washer head nails. A waterproof rubber membrane is placed on the surface of the foam board. To achieve the required R-value, you have to use two or more layers of foam board. No ventilation needs to be present for this type of insulation. The ceiling joists may remain empty with this type of application. Always remember that if you choose this method, you must insulate the rim joists to keep the insulation continuous from the roof to the wall and avoid air and moisture infiltration issues.

3. Insulating the Ceiling

This insulation method is not always recommended because the ceiling height will drop at least 4 inches to adhere to the code requirements. It also makes it difficult to add hardware and lighting in the future.

How to Insulate a Warm and Cold Flat Roof

A warm flat roof is insulated from the outside. The top layer consists of a waterproofing membrane, which sits on top of the insulation board that has a vapour control layer beneath it. This is secured over the timber decking while the roof joists beneath remain uninsulated. You can top the roof joists with plasterboard to allow you to decorate your living space. With the vapour control layer in place, condensation and moisture will not pass through the roof structure. On the other hand, the waterproof membrane protects the building or flat roof extension from excess wind and rain.

A cold flat roof is insulated the same way as a warm flat roof, only from the inside. With this insulation, only the waterproof membrane lies on top of the timber deck. The insulation boards are fitted within the joists of your roof, while the vapour barrier is fitted below them, with a plasterboard ceiling beneath that.

Choosing a Flat Roof Insulation

If you have an existing flat roof but want to improve its insulation, you may want to understand various ways to insulate a flat roof. For example, should you choose a cold or warm flat roof? The best choice of flat roof insulation will depend on several factors.

You should opt for a warm flat roof if you want the best thermal performance. Warm flat roof insulation is also easier to install. On the other hand, the main advantage of a cold roof installation is that it takes up less space. Therefore, cold roof insulation may be best if you have a balcony or a door opening onto a flat roof. A cold flat roof also offers better support for foot traffic since users will stand directly on the faced board instead of standing on the insulation board that could be easily compressed.

Insulating a Flat Roof from the Inside

If you have a fat roof but want to improve its insulation, you may want to know how to insulate an existing flat roof from the inside. After all, it is costly to remove the top layer of your roof to lay down the new insulation, especially for large commercial establishments.

However, before you commence the insulation, it is important to understand what you are signing up for. Insulating your flat roof from the inside is not always the best idea. If the procedure is not done well, you might end up damaging your roof over time. Here’s what you should know before you decide to insulate your roof from the inside:

Ventilation Matters

When insulating a flat roof from the inside, you could accidentally create a hot roof. The industry standards call for cold roofs because cold roofs last longer and will not develop the moisture issues that come with hot roofs. Cold roofs are well-ventilated through the layer of insulation, waterproofing, and the rest of the roof. This ventilation allows moisture and heat from the building to leave the roof instead of causing moisture issues inside.

Hot roofs have moisture problems, most resulting from insulation improperly installed from the inside. If not well installed, the new insulation could block off the ventilation and trap moisture and condensation inside. The following are the common problems of flat roofs:

The insulation you install might end up being useless and ineffective due to the moisture from the buildings
Mould and fungus could accumulate on the moist insulation and roofing, affecting your building’s air quality. Poor air quality could cause health issues for the building occupants.
Because a hot roof often has moisture problems, it will not have a longer lifespan than a cold roof with proper ventilation.

Insulation Materials for Flat Roofs

Insulating an existing flat roof is often a labour-intensive process. While there are many insulation options to choose from, some work better than others. What is the best material to cover a flat roof? The ideal insulation option will depend on the specific building and the specific roofing system. The typical options include:

Polyisocyanurate — this is the most popular flat roof insulation material. Since it is a closed-cell foam, it has a high R-value even from a minimal thickness. If you are working with an existing flat roof with limited space, you need this thinness.
Expanded polystyrene — this material works by expanding into crevices, making it ideal for new flat roofs. Expanded polystyrene may not be an ideal insulation material for existing flat roofs. When installing this insulation, care must be taken to avoid cutting off or damaging the ventilation.
Extruded polystyrene — this insulation is easier to install into existing roofs. However, it is not as high performing as the expanded version.

Adding insulation to an existing roof may be expensive, especially for a large commercial building. However, the insulation cost is much lower than the cost of repairing or replacing a flat roof that fails early.

Refurbishment Options When Insulating a Flat Roof

There are different refurbishment options depending on the condition of your flat roof. For example, you may opt to only replace the damaged or aged waterproof if the deck is structurally sound. You may opt for a complete re-do if the whole roof is compromised.

Waterproofing goes hand in hand with insulation. You can choose from the following waterproofing methods:

Single Ply Membrane — The lightweight coverings are a popular choice because they can work on a wide variety of flat roofs.
Built-up felt — Made from three layers of bituminous roofing felt, built-up felt offers a high level of weather protection.
Mastic asphalt — This is a long-used waterproofing substance that is easy to repair and strong enough to withstand foot traffic.

There are limited options available that are truly ideal ways of insulating a flat roof. This explains why flat roofs/flat ceilings are not a common construction method for residential houses. However, if you use the proper insulation methods and materials, you will have a perfectly insulated flat roof. If you are unsure how to insulate a flat roof, it’s always a good idea to contact a professional. At Bernardi Building Supply, our experts will inspect your flat roof and advise you on the most suitable insulation method.