How they differ
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Methodology
A Section J report uses a prescriptive approach that sets out specific requirements for various building elements such as walls, roofs, and glazing. This is a pass/ fail criteria per element, so it has no flexibility. If you fail one part you fail the whole report, or you have an element you think is too stringent (I.E. Glazing is required to be double or triple glazed, or the slab on ground requires to be an insulated slab system) there is nothing you can do to lessen this requirement.
A JV3 report uses a performance-based approach that evaluates the overall energy efficiency of a building through computer simulations. This method allows us to move around performance. For instance using the same scenario when the glazing needs to be double glazed or the slab on ground need to have insulation, we could either lessen this requirement (single glazing) or delete the requirement. We would then compensate for this loss of energy by say; increasing the insulation in the ceiling, or upgrading the wall insulation or many other possible scenarios. This enables us to keep construction cost economical. While a JV3 report might cost more, the overwhelming outcome of these reports is the cost of the construction saving will more than offset the cost of the report by many orders of magnitude.