This building is built on the edge of the cycle path in Easton, it was previously an empty plot and is used for infill housing, the plot was sold specifically for the purpose based on the plans for an environmentally exemplary family home.
An AECB talk on air tightness was arranged and the air pressure test was undertaken, throughout the design and build process Michael Drake and David ffrench had worked to ensure that the thermal envelope is continuous and the air pressure test showed the evidence of what they had achieved. The development achieved an air tightness of 0.57 m3/ hr. It can be very hard to achieve such a low air tightness and the envelope was checked and repaired throughout the build process usually by David, they invested in a higher quality thermal vapour permeable layer, and checked it at separate stages in the build process. The architects carefully considered and individually drew each thermal bridge to ensure that even edges retained the same level of heat, a thermal imaging camera was used at the event and it showed a fairly uniform level of heat retention throughout.
This was achieved through careful design, they were thinking about the thermal bridges from the early stages, they were advised by Piers Sadler who is a Passive House consult and has a strong understanding of thermal bridging and air tightness and he explained the strategy as tracing a pencil around all the edges to ensure there were no gaps. The thermal bridges were carefully considered and drawn in both auto cad and sketchup 3d modelling, the benefits of the sketchup model being that you can see how different materials fit around each other. During the build there are more challenges as the air tightness levels were more than in a standardised build it was difficult to explain the air tightness strategy to each sub contractor, and often there would need to be repairs made after each sub contractor had completed their task.
M Sustainability undertook providing the EPC and SAP calculation for the finished house. This was challenging because after all the work that Michael and David had put into making the house sustainable and environmentally friendly they were keen to achieve an A rating. However the basis of SAP rating is the cost to heat a property, and the chosen fuel is wood pellets, which is more expensive than other fuels, which meant that despite low u values and high levels of air tightness, the SAP rating was not as high as expected.
Many of the junctions Accredited and Enhanced Construction details consider the junctions in two dimensions; but to fully understand the construction process on site and ensure that all junctions reduce heat transfer successfully they should be modelled in three dimensional CAD drawings. This would ensure that all junctions can be correctly constructed on site and avoid delays.
Initially the house was modelled using default values for the Psi Values. There was considerable effort put into junctions at Design stage and during construction in order to reduce the heat loss, each junction was modelled with heat conservation and air tightness in mind. This could have been overcome by undertakings full calculations of Psi values but would have represented a considerable cost, and would not have made significant design changes at that stage. Although these calculations would have provided a more accurate (and energy efficient) depiction of the house within SAP, it would not have made any changes to the house in reality.