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    BASIX #4. The Enviromental Impact of Metal Frames



    Thermal Bridging in Steel Framed Homes

    The concept of thermal bridging in residential construction has come under scrutiny, particularly when
    considering the efficiency of a home’s insulation system.

    In a recent podcast with the Undercover Architect (Does Size Matter in Achieving 7 Stars? With Jeremy Spencer, Positive Footprints [NCC Series] ( Jeremy Spencer of Positive Footprints sheds light on the critical differences between steel and timber frames and their respective roles in thermal bridging—a phenomenon that could significantly affect the energy efficiency of new homes.

    What Is Thermal Bridging?

    Thermal bridging occurs when materials that are poor insulators come into contact, allowing heat to bypass the insulation through these more conductive paths. This can lead to significant energy losses, affecting the temperature regulation inside a home and consequently, the comfort of its inhabitants.

    Steel vs. Timber: A Conductivity Contest

    The stark contrast in thermal conductivity between steel and timber frames is a focal point in discussions about thermal bridging. Steel frames are incredibly conductive, about 1000 times more than softwood and 500 times more than hardwood. This high conductivity means that if a steel frame is used, heat will readily transfer through the steel, effectively short-circuiting the insulation.

    Disadvantages of Steel Frames

    When plasterboard is fixed directly to the inside of a steel frame and cladding is directly fixed to the outside, even if insulation is used, thermal bridging can significantly reduce its effectiveness. The heat doesn’t just pass through the insulation; it travels around it. This can lead to cold lines on walls and, more
    problematically, condensation—where the plaster touches the steel—potentially resulting in mould growth and structural issues.

    Jeremy Spencer emphasises that the real-life performance of steel-framed buildings can be up to 1.2 stars lower than what is rated on paper due to thermal bridging. That’s a substantial decrease, not only reflecting higher energy bills and less comfort but also increasing the difficulty and cost of meeting the new 7-star requirement.

    Timber’s Role

    While timber does conduct heat, it does so at a significantly lesser rate compared to steel. It’s not an
    ideal insulator compared to insulation materials, but it outperforms steel by a wide margin, making it a more energy-efficient option for framing.

    The impact of thermal bridging in homes cannot be overstated. While steel frames may offer certain advantages in construction, their high thermal conductivity can undermine a building’s thermal efficiency. This is a critical consideration for builders and homeowners alike, especially those looking to minimise their environmental footprint and maximise energy savings.

    Timber emerges as the more energy-conserving choice, reinforcing the need for a thoughtful approach to
    selecting materials that align with energy efficiency goals. As the building industry evolves, accounting for these subtle yet significant factors is key to designing homes that are not only structurally sound but also thermally responsible.

    While timber frames offer numerous advantages, it's important to note some drawbacks, these include vulnerabilities to pests, potential rot over time, and susceptibility to swelling based on climatic conditions. However, builders can utilise effective treatments to ensure the durability and longevity of timber frames - a consideration that all clients should explore.

    Despite all of this timber is the most eco-friendly building material on Earth, timber framing is sustainable, renewable, non-toxic and organic. It's also carbon positive, which means the entire production process, from sapling to timber framing installation, removes more carbon dioxide than it emits. And that's great news for our planet.

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