Research: In-situ U-values

Following the completion of our low energy extension in Uffculme, which can be viewed in this link, we were fortunate to have the opportunity to monitor the external building fabric to calculate whether the design U-value had been achieved.


The design U-value was calculated to be 0.13W/m2K. This would be achieved using thick layers of wood fibre insulation, and was influenced by the PassivHaus standard maximum U-value for external walls of 0.15W/m2K.

 

To measure the as-built external wall for this extension, heat flux sensors were used. When used in combination with internal and external air temperature loggers, it is possible to calculate the in-situ U-value for a construction, which in this case was an external wall.


A thermal camera was used to find the best place on the external wall to locate two heat flux sensors (so as to avoid cold bridging through timber battens). These were then fixed to the internal face of the external wall using a high conductivity compound. The sensors were left in place for a four week period, measuring data every 15 minutes.


Data was collected at the end of the measurement period and calculated using a running average methodology. This led to the output of measured in-situ U-value for the wall. This U-value was 0.12W/m2K, which is similar, but better than both the PassivHaus standard and design based prediction.


This gives great confidence in the specification of this wall construction on other projects.


All measurements are conducted in accordance with ISO 9869-1:2014 Thermal insulation — Building elements — In-situ measurement of thermal resistance and thermal transmittance — Part 1: Heat flow meter method.

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