Heat losses through repeating thermal bridges, such as insulation bridged by cavity wall ties or stud framing, within a structure are accounted for by the calculation of the U-value of the plain element in question, however, thermal bridges (known as non–repeating or linear thermal bridges) also exist at junctions between elements or where the continuity of the external fabric insulation is interrupted (e.g., at junctions with external walls, floors and roof). With framing systems the effect of the non-repeating thermal bridging can be quite significant due to the presence of additional structural elements in place of insulation at these locations.
The relative effect of these non-repeating thermal bridges becomes more significant as U-values are driven down, to the extent that heat losses due to non-repeating thermal bridges can be up to 50% of the fabric only heat loss. Use of H+H aircrete can significantly reduce the thermal bridge effect at junctions as it will have a far better thermal resistance than denser materials.
Additional heat losses associated with these thermal bridges are required to be accounted for in the SAP energy assessment calculations, (which in Appendix K gives a procedure based on a linear thermal transmittance value, ψ (Greek letter Psi – pronounced "si").
The ψ value is a property of the thermal bridge junction and is the rate of heat flow per unit length of the thermal bridge. Table K1 of SAP gives values of ψ applicable to different types of junctions detailed in accordance with the Accredited Construction Details (ACDs) or onerous Default values where approved Details do not exist. The Aircrete Products Association has had a comprehensive set of junctions assessed by Constructive Details Limited the results of which show that use of H+H aircrete can improve thermal bridge losses significantly over both the ACD and Default values without having to resort to costly enhanced detailing at these locations.
Additional thermal benefits can also now be realised by using aircrete in separating walls to limit heat loss at junctions with external elements. When used in conjunction with aircrete inner leaves, heat losses at thermal bridges can be reduced by around 50%, enabling CO2 emission targets to be more easily met or savings to be made on other parts of the insulated fabric without compromising the thermal performance of the dwelling. This reduction can be equivalent to the effect of adding 10-15mm of insulation to the external walls.