Many people are confused by the terms ‘Passive Solar’ and ‘Passive House’ (or Passivhaus, in German), and the reasons are hardly surprising: both expressions use the term ‘Passive’ in a similar meaning (i.e. passively heated by the Sun and/or other indirect heat sources), and both pertain in some way to the design of energy-efficient buildings. Whilst there is clearly some kind of relationship, PS and PH are not interchangeable. In fact, if anything the relationship is akin to the relationship between Philosophy and Science.
The physics of heat conduction and insulation has been the subject of scientific research since at least the early nineteenth century, including the work of such greats as Fourier and Maxwell. The importance of the Sun as a source of house warmth has been understood intuitively for thousands of years, and the first homes to be designed scientifically to maximize solar gain date from the 1930’s and 1940’s. These homes, known at the time as ‘Solar Houses’ positioned large amounts of glass towards the South (Northern hemisphere) to capture the Sun’s warmth, and made use of thermal mass and more insulation that was typical of the era. The energy crisis of the 1970's also spurred development of so-called "super-insulated" houses.
Meanwhile, the research that led to the initial PH standard was conducted in Darmstadt, Germany in the 1980’s, with the first true passive houses being built in 1990. The PH work heavily extended existing scientific research conducted over the preceding century and more, the goal being to produce buildings which provide a healthy environment and year-round comfort for the occupants, whilst using very little energy for extra heating and cooling. In contrast to PS buildings, a PH building considers the Sun to be just one source of heating. Heat is also conserved from internal appliances (e.g. TV, refrigerator, hot water, etc.), and any warmth in stale air is also recovered.
A PH building requires a continuous, unbroken layer of insulation around the outer perimeter, which defines the thermal boundary. The building must be designed so that heat energy losses (through the walls and windows, and from ventilation) are can be replaced by gains (from the Sun, electrical appliances, etc.). Achieving this delicate “energy balance” requires extra effort during building design and construction. The key elements - a near-airtight building, continuous insulation, no thermal bridges, high quality, correctly installed windows, and heat-recovery ventilation – are brought together in the PH design software to create a healthy building environment that offers year-round comfort anywhere in the world, with no or very little extra heating or cooling energy required, and in a cost-optimal way.
In summary, a correctly designed and constructed PH offers the certainty that it will perform as expected. The PH design software (PHPP) can model building performance that turns out to be very close to actual, measured values. Whilst a PS design approach can certainly deliver comfortable houses when things work well, PH delivers guaranteed results. And that to me is the real difference - with PS you adopt a philosophy and the results are often good. With PH, you follow a scientific method and test both the design and the as-built product, and the outcomes are certain.
Given the overlap in the key elements of PS and PH, I'm really not able to come up with any reasons why one might choose the PS approach over PH. If you have any ideas, please do share them in the comments.