Thermal performance and energy efficiency
Aircrete offers the lightest form of masonry construction due to millions of tiny air bubbles formed by a chemical reaction during the manufacturing process. In addition to making it easier to handle on site, its lightweight structure also helps reduce heat loss by working alongside other construction materials and insulation products to deliver very low U-values. Aircrete also provides good airtightness, adding to its inherent insulating properties and ensuring Part L requirements are more easily achieved, whilst at the same time keeping the width of external walls to a minimum. The use of aircrete in beam and block floors and foundations also helps reduce heat loss to the outside environment.
Thermal bridging and construction details
A thermal bridge describes an area within the building envelope that has significantly higher heat loss than the surrounding fabric due to the geometry or the presence of materials with poor insulating properties. This creates a bridge for heat to escape, which for dwellings can represent up to a third of their overall heat loss. Consequently, thermal bridging sits alongside airtightness and insulation as a critical aspect in the design of energy efficient buildings.
The simplest and most cost effective way to minimise thermal bridging is to use high performance construction details for key junctions such as those that occur at lintels, party walls, ground floors etc. The Aircrete Products Association has developed a comprehensive set of such details that are freely available to download. These use materials readily available at builders’ merchants. Each detail has its own calculated heat loss rating (psi value) for use in the SAP/SBEM Part L compliance tools, and is accompanied by a simple 2D drawing showing how it should be constructed.
Use of high performance aircrete construction details, offers a straightforward means of optimising thermal performance, particularly when compared to the alternative option of using the default thermal bridging value in SAP/SBEM, which may result in heat loss from junctions that is up to 60% higher.
Aircrete provides a useful level of thermal mass that is typically around six times more than that offered by lightweight construction methods such as timber frame. Thermal mass has the natural ability to absorb and release heat within a building, helping deliver a consistent internal temperature and comfortable environment. In contrast, lighter-weight structures tend to provide a less stable internal temperature that undergoes more significant fluctuations in response to changing internal and external conditions.
The thermal mass provided by aircrete is particularly useful in helping to prevent summertime overheating, which is achieved by allowing it to soak up internal heat during the day and then using cool night-time air to ventilate the building and purge the heat from the structure. This technique is recognised in the SAP overheating check, making compliance more straightforward and less expensive in buildings that have thermal mass.
The beneficial thermal properties of aircrete
• Low thermal conductivity, which is in the range of 0.11 – 0.19 W/mK, providing beneficial insulation to the structure.
• Useful level of thermal mass, with a typical k-value of 52 kJ/m2K; around six times more than timber-frame construction.
• Minimal thermal bridging using
aircrete details, enabling very low dwelling y-values of around
0.04 W/m2K or better.
• Low air permeability, helping minimise heat loss from air leakage.