Colin Wells, head of technical at Keylite looks at the issues around thermal bridging

AS THE UK braces itself for its first pandemic winter, the social and health benefits of improving the thermal efficiency of our homes has become more important than ever because people are at home more. Aspects of the design and materials used in a building all play a part in its overall thermal performance. But it’s not just about insulation, thermally efficient design is about taking time on the details.

Making our homes better places to live is about taking a holistic approach to design. In order to achieve thermal comfort, there needs to be a combination of building systems that effectively strike a balance between insulation measures, solar gain, thermal inertia, airtightness, and ventilation. When one aspect is ignored, the comfort of the user will be significantly reduced. For example, reducing a window size to maintain good thermal efficiency will reduce natural light but this goes against the fact that most homeowners want to increase the daylight in their homes. In fact, the combination of correct glazing specification and proper installation can provide a well-balanced heat gain and loss ratio by using free heat from solar gain, as well as maintaining natural light in the building.

Cold bridging

Thermal or cold bridging can be responsible for up to 30% of a dwelling’s heat loss. It occurs when there is a direct connection between the inside and outside of a building through one or more elements that are more thermally conductive than the rest of the building envelope. As a result, there will be wasteful heat transfer across this element. Furthermore, where warm, moist internal air comes into contact with the potentially cold surface,


“ Thermally

condensation and mould can form.

Thermal bridging in relation to roof window installation is caused by an ‘adjustment gap’ rec- ommended by all manufacturers so that the window can be positioned into the opening squarely. This gap between the roof and the window creates a cold section all around the frame due to the temperature differentiation between the inside and the outside. When left un-filled and exposed to colder temperatures, the gap could lead to condensation forming around the inside of the frame. In time this leads to mould accumulation which is a health risk, un-sightly and can cause physical damage to the frame and plasterboard if left untreated.

Most roof windows come with an optional thermal collar. This fits around the window to fill the gap and create a thermally efficient seal. Surely the onus should be taken away from the installer and products be designed and specified with optimum

thermal comfort in mind? Although a thermal collar is a known solution for preventing cold spots around a roof window frame, recent research reveals that only a fraction (less than 10%) of roof windows are fitted with thermal collars, with most companies selling them as an ‘extra’. For householders on a tight budget, this adds a layer of cost which might not have been accounted for. In certain cases, due to a lack of availability in merchant stores, roof windows have been fitted without thermal collars, compromising this crucial element of thermal design. Keylite has taken responsibility for insulating around a window by incorporating a patented expanding thermal collar into the frame as an integral component of the roof window. The expanded thermal collar fills the gap around the roof window ensuring the frame is fully insulated, which in turn helps eliminate the risk of condensation and mould. The thermal collar comes as-standard on all Keylite

efficient design is about taking time on the details. Making our homes better places to live is about taking a holistic approach to design.

Roof Windows and at no extra cost.

One of the most important things to remember is that our homes and buildings are built for people. When project costs are squeezed and materials, detailing and installation is compromised, all too often the performance of the building will suffer to the detriment of the occupier. Addressing thermal bridges around roof windows with an integrated solution such as Keylite’s expanding thermal collar, will go some way to addressing localised ‘cold spots’ in a building envelope, creating healthier and more energy efficient homes. BMJ November 2020

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