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Interstitial Condensation – Howgate Close


Interstitial condensation presents a significant challenge. This phenomenon, which occurs between the interface layers of roofs, walls, and floors, can undermine the integrity of a building’s fabric, leading to material degradation, mould formation, and a decline in air quality. However, innovative design and construction methods can mitigate these risks, as demonstrated by the developments at Howgate Close.

What is interstitial condensation?

Interstitial condensation occurs when moisture-laden air penetrates the building envelope and condenses within the structure’s layers. This problem is not merely a superficial or aesthetic issue; it represents a serious threat to the building’s longevity and the health of its occupants. Persistent moisture within the walls, floors, and roofs can degrade building materials, making them more susceptible to damage and decay. Moreover, the increased moisture provides a breeding ground for mould, which can proliferate and significantly decline indoor air quality.

Innovation at Howgate Close

At Howgate Close, a forward-thinking approach to wall construction demonstrates how modern design principles can effectively address the challenge of interstitial condensation. In this project, the traditional cavity wall is eschewed in favour of a modified solid wall with external insulation. This design strategy places the insulation on the external face of the solid block wall, fundamentally altering the dynamics of moisture movement and condensation risk.

Risk Assessment

Its proactive risk assessment approach is critical to Howgate Close’s success in managing interstitial condensation. A comprehensive Condensation Risk Analysis was conducted to evaluate the likelihood of interstitial condensation within the wall construction. This analysis offered a detailed examination of the wall’s performance in various environmental conditions.

What is a Condensation Risk Analysis?

A Condensation Risk Analysis (CRA) is a crucial assessment process in building design and construction. It’s aimed at predicting and preventing the risk of interstitial condensation within the building fabric. This analysis is particularly important in climates with significant temperature variations between seasons. These fluctuations can increase the likelihood of condensation occurring within walls, roofs, and floors.

The CRA is conducted using a combination of theoretical calculations and computer modelling based on the principles outlined in standards such as BS 5250, “Code of Practice for Control of Condensation in Buildings.” The analysis takes into account various factors.

  • The Building’s Location and Climate: Temperature and humidity data specific to the location simulate the conditions the building will face throughout the year.
  • Materials and Construction Details: Different materials have different thermal and moisture properties, affecting how moisture moves and condenses. The CRA considers these properties and the construction details to predict how moisture behaves within the building elements.
  • Internal Conditions: The analysis also considers the internal environment of the building, including temperature and humidity levels, influenced by occupancy and usage patterns.
  • Ventilation and Airflow: Proper ventilation is critical to managing moisture levels. The CRA evaluates the building’s ventilation strategy to ensure it removes moisture-laden air before condensing.
Conducting the CRA

The process involves collecting comprehensive data on the building’s design, materials, climate, and intended use. Specialised software then models the building, simulating how heat and moisture travel through its fabric across different seasons. This analysis identifies potential condensation risks by predicting areas where the temperature could fall below the dew point. The final step involves interpreting the results to make informed recommendations for design modifications or construction practices to mitigate these risks. This might include changing materials, improving insulation, or enhancing ventilation strategies. Conducting a CRA ensures the building design complies with relevant building regulations. UK Building Regulation Part C mandates effective moisture control.

Findings and implications

The findings of the Condensation Risk Analysis at Howgate Close are encouraging and instructive. The analysis concluded that the external wall detail effectively avoids critical surface moisture, eliminating the danger of mould growth—a common concern in traditional wall constructions. Perhaps most importantly, the study found no risk of condensation forming at any interface within the wall at any time of the year. This result demonstrates compliance with UK Building Regulations and also signifies a significant advancement in building design and construction practices.

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