Old walls deliver energy efficiency surprise

An initial study suggested conventional industry practices were struggling to accurately represent the thermal performance of traditionally built walls.

Ultimately, this could have negative consequences for historic buildings as calculated theoretical U-values (suggesting a poorer performance) may 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, MSc student at the University of Portsmouth, compared the in-situ U-values (U-value is the universally known unit to describe the rate of heat transmittance or loss through a wall/roof/floor etc) of various traditional vernacular walls against the theoretical U-value for these walls using the class-leading 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 timber, cob, 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 may not be as energy inefficient as the building industry has generally understood them to be.

This SPAB research is not a criticism of the calculation methodology or U-value modeling software, but it does highlight the difficulty of modelling and calculating the thermal performance of traditional walls using conventional techniques.

Jonathan Garlick, SPAB Technical Officer and project leader said: "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.

"Energy efficiency is becoming the key issue for people working with historic buildings. If we aren’t basing our approaches on the right figures to begin with, then we could, unintentionally, be doing untold, invasive damage."

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