May 10, 2018
Japan's envelope-performance calculation ignores heat loss through the foundation slab.
Primary-energy-consumption calculations are done using this standard, and Zero Energy House certifications and the like are based on those numbers. As a result, the perception has spread that "slab insulation isn't needed."
But at Passive House levels, the impact of this heat loss becomes very large. It's no exaggeration to say that, without slab insulation, you cannot clear the Passive House standard.
On the other hand, if you push for the standard at all costs, the construction cost can exceed the energy savings. Few clients say "for the planet, I'll pay more in construction," so it's important to balance things and find the best point for the customer.