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Mind the gap

Mitigating the risk between thermal performance on paper versus reality

Mind the gap

Acoustic, light, economic and thermal performance all have a significant impact on making our homes better places to live. Whilst it’s important to recognise that these elements do not work in isolation and that a holistic approach should be taken to building design, in this particular article I’ll be focusing on thermal comfort and good design principles that should be adhered to.

Thermal comfort is the outcome of a well-balanced combination of building systems that strike a balance between insulation measures, solar gain, thermal inertia and airtightness and ventilation.

If one aspect is ignored, then user comfort can be significantly reduced. For example, some builds reduce window size in order to maintain good thermal efficiency, yet this also lessens natural light. In fact, increasing daylight is the top of the list for many homeowners.

However, using the 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.

For many architects, specifiers and housebuilders compliance is the dreaded c-word. It can be all too easy to get caught up in aesthetics and build timelines, but you can’t rest on your laurels when it comes to compliance – the cost of non-compliance far outweighs the price of getting it right first time.

With thermal performance I’m looking primarily at Conservation of Fuel and Power Part L, section 6. Part L is split into two parts, part L1A for new build dwellings and part L1B for existing dwellings, but I’m going to focus on L1A.

The purpose of approved document L is to ensure energy efficiency in buildings. The Target CO2 Emission Rate (TER) and Target Fabric Energy Efficiency (TFEE) rate are the minimum energy requirements for a new-build dwelling. To showcase that a building complies with regulation requirements, Dwelling CO2 emission Rate (DER) and Dwelling Fabric Energy Efficiency (DFEE) rates are calculated, and both must meet or exceed the TER and TFEE.

However, whilst a product or design may say it meets with Part L requirements, is that really good enough? The ultimate question of course is, are you creating a house or a home? Because your answer to that question will determine the detail in your design and the products you specify.

The detail is in the design, but that detail shouldn’t stop at the drawings, it should also translate into your specification and the installation. Whilst a product may say it meets Part L or has a certain u-value, once this is installed will this still be the case and how can you help mitigate the gap between performance on paper and actual performance of a finished build.

Architects and housebuilders should look to work with SAP assessments that integrate with Registered Construction Details, which can give a thorough and more accurate insight of the expected building performance.

When it comes to thermal comfort and performance one of the biggest issues for developers and architects is thermal bridging and condensation, and all too often double glazing is seen as the main culprit of this. However, despite meeting relevant standards it’s often in the installation of a product where cold bridging can occur, leading to unpleasant mould left on windows and reducing its thermal efficiency.

Installation of roof windows is often left to the manufacturer’s discretion. With most stipulating that a 20mm gap is required around roof windows to enable for onsite adjustments and ease of installation. This gap is supposed to be ‘filled in’ after installation to reduce the impact of thermal bridging. However, this is rarely stipulated in building designs and the retrofittable product of a thermal foam is often not on specifications, likely because at this level of detail there is not the awareness or the incentive to purchase extra materials.

If this area is not insulated, then this can lead to cold bridging and condensation on roof windows. The gap between the roof and the window creates a cold section all around the frame due to the temperature differentiation between the outside and the inside, which can eventually lead to mould and contributes toward heat loss.

In order to make houses more comfortable and energy efficient the onus should be taken away from the installers and products should be designed and specified with optimum thermal comfort in mind. Manufacturers should design in features that eliminate thermal bridging reduces the risk of non-compliance and delivers a build that is closer to the expected energy performance.

The difference between the on-paper specification and the as built performance often occurs because of attention to detail and the lack of awareness that a product will only perform as specified if it’s also correctly installed. Keylite is aware of this issue surrounding the installation of roof windows and has taken the lead in ensuring it removes the possibility of this happening. Meaning its products are designed in such a way that architects and installers don’t have to consider the ‘gap’ when specifying or fitting roof windows.

With this principle in mind, Keylite Roof Windows introduced an expanding thermal collar for its products, the only system of its kind and the only company in the UK to address this issue.

The thermal collar is already attached to the roof window, so when onsite installers can fit with a 20mm gap for ease of installation and then simply pull the tab on the window, once fitted, to release the expandable thermal collar, which then bridges the gap between the frame and building envelope. This not only saves time and cost, but also creates more thermally efficient and comfortable homes. Keylite Roof Windows has helped to bridge the gap between design and build, giving architects and specifiers peace of mind that what they design and specify on paper can be achieved when built.

Performance and comfort work in unity, you could seek to achieve the absolute minimum thermal standards in line with Part L, but would that create the best experience for the occupants? Thermal comfort is impacted by correct detailing, material selection, glazing and ultimately installation. When all these elements are taken into account you are building homes not just houses.

For more information on Keylite Roof Windows expanding thermal collar, please visit The Gap

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