Keeping the wolf from the door
The three little pigs discovered (at great cost to two of them) that bricks make better walls than straw or sticks when under attack by a voracious predator. However, does the same apply to energy efficiency, an important factor for an EPC and the cost of heating a home?
The types of materials used to build homes have changed radically over the centuries. Historic buildings generally reflect the availability of local materials such as granite, flint, clay or chalk, but today we more typically use imported bricks or system-built houses that are assembled on site from units manufactured elsewhere.
The ease with which heat passes through a wall, or its ‘thermal conductivity’, is stated as a U value. This is measured in W/m2K, representing watts of heat that pass through a square metre for each 1°C temperature difference between the inside and outside of the wall. The lower the U value, the less heat is escaping.
The current Building Regulations Part L1A requires a U value of 0.30 W/m2K or better for an external wall in a new domestic property in England, though this alone will not guarantee a positive Standard Assessment Procedure (SAP) outcome.
When undertaking an EPC, it is relatively easy to derive an accurate U value for walls in new-build and modern buildings. Each element of the wall’s construction (for example the plasterboard, bricks and cavity insulation material) has a laboratory tested R value, a measure of its inherent thermal resistance. The U value is calculated by combining the R values, with an allowance for the thickness of each.
Modern brick walls are designed to achieve the 0.30 W/m2K standard. Timber frame suppliers typically use a 140mm stud and 140mm mineral wool insulation specification to deliver a U value of 0.29 W/m2K. For those with greater aspiration, some Passivhaus dwellings achieve U values below 0.12 W/m²K, with off-site system-built components providing more reliability than those put together from scratch on-site.
The improvement in wall U values has been one of the success stories in achieving better energy efficiency in new homes. The solid brick walls of the 1930s (around 225mm thick) have a U-value of just 2.70 W/m2K, while a typical unfilled cavity wall from the 1970s is likely to achieve 1.00 W/m2K at best.
It is more difficult to determine U values for the walls of historic and vernacular properties. The average wall width can be measured but its exact composition will be uncertain. For example, the proportions of straw and clay in a cob wall vary from building to building, or even across an individual wall. Solid stone walls differ in aspects such as the proportion of mortar, the different types and density of stone, and the presence of internal voids.
In situ measurements using thermal testing equipment rather than mathematical formulae suggest that traditional limestone can achieve a U value of 2.00 W/m2K at 200mm thickness to 1.20 W/m2K at 650mm. The corresponding figures for granite are 2.20 W/m2K to 1.25 W/m2K. Historic solid brick walls have been measured at between 2.50 W/m2K (100mm thick) and 0.75 W/m2K (600mm thick).
Many champions of traditional English cottages promote the qualities of cob walls, and their measured U values come in at around 1.00 W/m2K, irrespective of thickness (400mm to 700mm). By comparison, traditional timber-framed walls with in-fill tend to perform quite poorly. Measured results include 2.48 W/m2K for a 100mm brick infill and 2.03 W/m2K for 100mm of wattle and daub.
However, the walls of one historic timber-framed property with a 210mm hemp and lime infill yielded a respectable 0.40 W/m2K. History does not record whether hemp and lime can withstand the huffing and puffing of a wolf!
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