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Warm Flat Roof Fascia Detail

A warm roof contains an insulation layer atop the roof meant to provide maximum thermal protection and warmth. Insulation is laid on the decking, rafters, and vapour control layer. The vapour control layer restricts the transfer of moist, warm air to the fabric, rather than allowing it to exit the building. Creating a warm roof is quick and simple, making it more cost effective than other options, as long as the existing roof deck is in satisfactory condition.

Thermal Bridging in an Insulated Roof

All around the upkeep of a roof is essential for safety, functionality, and the overall look of a home. One common tweak that people make to their roofs is making changes to attempt to increase the warmth in an area or the entirety of a structure. This can be an ultimately fulfilling plan.

However, it is important to be properly educated on how to construct and maintain these changes. Many of these adjustments can result in thermal bridging, which should be avoided as it can be a major detriment to the quality of your roof and home.

Thermal Bridging in Roofs

A roof’s thermal functionality is an essential component in ensuring overall good thermal performance across the entirety of a building, home, or structure. Therefore it is essential to address thermal bridging when completing any sort of construction. Many areas have made their thermal codes stricter to maintain top performance.

Thermal bridging is when heat travels through an object that is more conductive than the materials it is located near, creating a path for heat to travel through (1). This causes energy loss, as well as potential health hazards like mould and mildew.

Reducing thermal bridging is great for reducing energy bills, preventing issues with condensation, keeping temperatures reasonable, and extending the longevity of your roof.

Before you can complete any roof construction and reduce thermal bridging, it’s important to know the relevant parts of your roof. 

Roofing Joists: Roofing joists are horizontal boards that run across the top of the structure. They provide proper weight distribution and support for your roof. 

Fascia: The fascia is a board that runs along the border of your roof’s overhang. Its purpose is to add a finished look to the roof and provide a transition between the home and the roof. It also supports the gutters and shingles, keeping moisture out of the home.

Decking: Decking is the wooden boards that make up the structure of a roof, giving it its shape.

Rafters: A rafter is a long wooden beam meant to provide support to the deck, shingles, and other parts of a roof. Each rafter is one component of a series of sloped pieces supporting the peak of the roof.

There are two common types of roofs in which thermal bridging occurs: warm roofs and cold roofs.

Warm Roofs and Thermal Bridging

A warm roof is a roof containing an insulation layer that lays on the decking and rafters.The goal of a warm flat roof construction is to provide and maintain continuous warmth and thermal protection to an area. Warm roof insulation is extremely simple to install as you can simply place the new boards and insulation on top of the old roof, or strip the existing roof and place the insulated boards over the joists. 

One difficulty with a warm roof is that the insulation is located above the existing roof level, making the additional height a factor that you’ll have to deal with. Additionally, although warm roofs are extremely common, there is an important area that is commonly missed when installing them, that often results in thermal bridging.

The space beneath the newly placed insulation boards located behind the fascia board is often left without insulation, causing it to remain cold. Therefore, when the warm air rises through this area, it will be moist. This ultimately results in condensation in cold areas, creating a dew point as the air attempts to rid itself of moisture. All of this can cause issues, such as rotting and mould.

The best way to prevent condensation in the space behind the fascia is to place insulation in the gaps between the joists so that you have continuous insulation starting from the ceiling and ending at the point behind the fascia.

Additionally, to avoid thermal bridging, it is a good idea to incorporate a vapour and or an air control layer underneath the insulation. Control layers restrict the flow of air, vapour, and heat through your home or roof. These are essential building components to prevent the degradation of your building or home. 

This type of air waterproofing will prevent any moisture from being pushed into the insulation by the thermal force from within the structure. 

Cold Roofs and Thermal Bridging

In a cold roof, the insulation is placed in between the roof joists. This means that there is no loss of height when using this method. The negative of this type of roof is that it is necessary to take and apart from your roof to install a cold roof. You will either have to take the ceiling down or remove all the existing decking from the top and place the insulation from the inside.

Therefore, this method is more complicated and costly than a warm roof.

It is also important to note that for a cold roof, some space needs to be used for ventilation. One common mistake people make when creating cold roofs is not creating any or enough ventilation in both the front and back, rather than just the back. Additionally, it may be difficult to determine where to stop the insulation. Taking the insulation too far forward can cause a blockage, restricting the air vents and stopping or greatly reducing the ventilation.

To prevent thermal bridging in a cold flat roof, ensure that the insulation is carried over the wall and that there is plenty of ventilation. Make sure that any of the materials that you utilize for insulation support are low conductive.

Want To Know More About Warm Roof Construction?

If you would like more information relating to warm roof construction, please contact us or call us now on 01277 375 511. One of our friendly flat roofing specialists will be delighted to help.