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Energy efficiency and building performance

Words:
Caroline Rye

Shortlisted: Design and Technical

What is the effect of insulation on the performance of old buildings?
What is the effect of insulation on the performance of old buildings?

The SPAB Building Performance Survey (BPS) was first established in 2011 to address the dearth of information about energy efficiency in traditional buildings. In particular, there was an absence of evidence about the performance of traditional buildings and, by extension, a lack of understanding as to what effective and risk-free interventions could be made in the name of energy conservation. Of specific concern were the long-term consequences relating to damage to fabric and occupants’ well-being as a result of insulation and the effects of reductions in ventilation/air infiltration. The SPAB Building Performance Survey 2015 Report has measured: fabric heat loss, air leakage, indoor air quality, wall moisture behaviour, room comfort and fabric risk conditions. Following refurbishment, measurements were repeated and specifically wall moisture behaviour has been monitored continually since 2011. Findings from this research are made available on a yearly basis in the form of SPAB research reports.

Early on in the study measurements of thermal transmissivity (U-values) confirmed what had already been established by an earlier piece of SPAB research. This was that U-values measured from solid walls differed markedly from those calculated following the standard U-value calculation procedure (BS 6946). The difference indicated a lesser degree of heat loss from measured U-values than that predicted by the conventional calculation method, in some cases less than half. This has profound implications for the refurbishment of solid walls particularly if internal wall insulation is being considered. If low improvement targets, such as 0.3 W/m2K are aimed for, a standard calculated U-value will suggest large quantities of insulation are required. Insulation then applied to the internal side of a wall could over-cool the fabric beyond the insulation layer where the solid wall is exposed to wind and rain. The dewpoint of the wall can thus be reduced long enough for moisture to accumulate within the wall fabric. Furthermore, predictions of potential energy and carbon savings may be overstated and not reflect performance in reality. For this reason it should be imperative that an architect has a thorough understanding of the likely heat loss of a solid wall and, if in doubt, or when dealing with buildings of particular significance, the measurement of U-values should be an important aspect of refurbishment design practice.

Another major finding from the BPS is one that relates to moisture within walls. Via long-term measurements of temperature, relative humidity and material moisture it has been possible to establish moisture behaviour trends for different solid walls with different refurbishment treatments. One of these walls has been insulated internally with polyisocyanurate board, a common choice of insulation material, due to its performance and affordability. This wall has shown a trend of rising relative humidity within the centre of the wall regardless of seasonal trends since the application of insulation in 2011. Air at this location is approaching saturation (100%RH) indicating the possibility of condensation or the presence of liquid moisture. Measurements of pre and post refurbishment U-values showed that the application of 100mm of PIR had reduced the U-value of the wall from 1.24 to 0.16 W/m2K. The insulation material is also encased in a metallised film forming a barrier to moisture movement. Both these factors may play a part in the increasing humidity profile measured for the wall and suggest that the use of impermeable insulation materials and/or low post-refurbishment U-values may not be suitable for some solid walls.

The Building Performance Survey has measured the pre and post-refurbishment performance of traditional buildings for over five years. The evidence collected is detailed and wide ranging providing examples not only of solid wall performance but also air leakage, indoor air quality and internal room conditions. The survey is therefore a valuable source of information for any professional engaged in the refurbishment of these types of buildings.

Research by Caroline Rye and Cameron Scott, ArchiMetrics


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