Traditionally built solid stone walls can be as much as three times better at keeping in the heat than is generally supposed, according to SPAB research.

Normally, the heat efficiency of walls is calculated using BuildDesk U 3.4 software or something similar. But this theoretical U-value can be wildly at odds with the actual performance of the wall, research by the Society for the Protection of Ancient Buildings (SPAB) has discovered.

They have gone out and measured the heat loss through a variety of traditionally built walls and compared it with the theoretical value produced by BuildDesk. The results raise serious questions about the treatment of historic buildings.

A report based on the initial study can be downloaded from the SPAB website (see address at the end). It suggests current industry practices are struggling to represent the thermal performance of traditionally built walls accurately.

Ultimately, this could have negative consequences for historic buildings, as calculated theoretical U-values suggesting a poorer performance than empirical evidence shows they actually achieve could lead owners and professionals to adopt disproportionate energy saving interventions that may not only be unnecessary but also invasive and potentially harmful to the fabric of a building.

SPAB’s report, written by Dr Caroline Rye, a student at the University of Portsmouth, compared the in-situ U-values of various traditional vernacular walls against the theoretical U-value for these walls using the BuildDesk U 3.4 software. Importantly, the theoretical value obtained from the U-value calculations is used by professionals as the base-line for assessing thermal performance of different types of constructions. However, SPAB’s on-the-spot research suggests that 79% of the traditionally built walls sampled – including walls of limestone, slate, and granite – actually perform better than expected.

Even taking into account a possible error margin of up to 10%, SPAB’s findings show that old buildings are not as energy inefficient as the building industry has generally understood them to be.

SPAB say their research is not a criticism of the calculation methodology of U-value modeling software, but it does highlight the difficulty of modeling and calculating the thermal performance of traditional walls using commonly employed techniques.

Jonathan Garlick, SPAB Technical Officer and project leader says: “Amazingly, this research has not been carried out before in England. Accepted theoretical performance figures have long been used as a standard base measurement by professionals and homeowners when old buildings are being up-graded, altered or even assessed for Energy Performance Certificates. But are they correct? We believe that with some traditional materials our in-situ results prove that they are not. We appear to be actually underselling the thermal performance of our old buildings by not fully understanding them.”