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Linear Thermal Transmittance – Howgate Close

In construction and building efficiency, understanding the intricacies of heat loss through thermal bridges becomes crucial for architects, builders, and homeowners alike. This blog delves into linear thermal transmission, a key metric in assessing and mitigating heat loss in buildings, using Howgate Close as a case study to illustrate the practical implications of optimised thermal performance in residential construction.

What is Linear Thermal Transmittance?

Linear Thermal Transmittance, often expressed as the Psi-value (Ψ-value), is quantified as heat loss through thermal bridges in a building. Thermal bridges are areas in the building envelope that allow more heat to pass through than the surrounding materials, often resulting in significant energy loss. These can be linear, relating to junctions between walls, floors, and roofs, or geometric, relating to corners or edges.

A lower Psi-value indicates a more efficient thermal performance, signifying reduced heat loss through these junctions. At Howgate Close, a residential development, meticulous attention to minimising linear and geometrical thermal bridges has resulted in exemplary thermal performance, setting a benchmark for energy-efficient design.

Howgate Close vs. Notional Building

To understand the impact of these measures, it’s instructive to compare the calculated Psi-Values of Howgate Close with those of a ‘Notional Building’ – a benchmark based on UK Building Regulations. This notional standard provides a “recipe” approach, ensuring minimum compliance if all standards are met. Howgate Close’s focus on reducing thermal bridges has led to significantly lower Psi-Values, indicating lesser heat loss and, by extension, a more energy-efficient and cost-effective building.

The perimeter heat loss from Howgate’s window and door frames has an average Psi-Value of 0.024W/m2K. This figure is notably lower than the notional standard, demonstrating that Howgate’s window/door junctions lose half as much heat as a building constructed to the minimum Building Regulation standards.

Surface temperature, mould growth, and health

Another critical aspect of thermal performance is the f-value, calculated alongside the Psi-Value. The f-value estimates the risk of surface mould formation within a building, a significant concern for occupant health. A higher f-value, approaching ‘1’, also signifies a lower risk of internal surface condensation and mould growth.

In the case of Howgate Close, the average f-value for window/door junctions is 0.94, surpassing the Notional Building compliance threshold of 0.75. This indicates a 19% reduction in the risk of internal surface condensation at these junctions compared to the Building Regulations standards, highlighting an improved internal environment and reduced health risks for occupants.

Understanding mould growth in buildings

Mould is a fungus that grows in damp and poorly ventilated areas, thriving on organic materials such as wood, paper, and fabrics found within buildings. Mould growth is not only unsightly but can have severe health implications, especially for individuals with pre-existing conditions such as asthma, allergies, or compromised immune systems.

Surface temperatures and moisture levels within a building directly influence mould growth. Thermal bridges, which create cold spots on internal surfaces, can lead to condensation when warm, moist air inside a building comes into contact with these colder areas. This condensation provides the perfect environment for mould spores to grow and proliferate.

Health implications of indoor mould

Mould exposure can lead to a range of health issues, including:

  • Respiratory problems: Inhalation of mould spores can cause symptoms ranging from nasal stuffiness, throat irritation, coughing or wheezing to more severe reactions such as asthma attacks in susceptible individuals.
  • Allergic reactions: Mould can trigger allergic reactions in some people, leading to skin rash, red eyes, and sneezing.
  • Immune system suppression: For people with weakened immune systems, mould exposure can lead to more severe infections.
Mitigating mould growth

The f-value in thermal performance calculation is a vital tool for predicting and preventing the risk of mould growth by assessing the likelihood of condensation formation. By designing buildings with higher f-values, closer to ‘1’, architects and builders can significantly reduce the risk of internal surface condensation and mould growth.

Howgate Close’s achievement in surpassing the Notional Building f-value compliance threshold exemplifies the effectiveness of this approach. The risk of condensation and mould growth is substantially reduced by ensuring that window/door junctions and other potential thermal bridges are designed to minimise heat loss and prevent cold spots. This results in a more energy-efficient building and a healthier living environment for occupants.

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