Low-energy building science and building science in general have both come a long way in the last 40 years. Energy codes for US housing have improved, and retrofitting has been done and is getting better in its methods. We lag **some** EU countries on building energy use (per HDD) and much more can be done, but the amount of energy to heat and cool US homes has decreased over the last 40 years, despite significant increases to population and average home square footage per capita. This 'ok' trend will no doubt continue for some time. Simple easy/cheap energy retrofits could easily drop US home HVAC demands by 25-40% below current, and could easily be accomplished over a decade or two (since they usually only require a few days work at each house). Current payback on these retrofits varies with the price of fuel...electric resistance, oil or propane, ROI is <5 years, nat gas or heat pumps more like 10 or more.
The homes we are discussing that break from this trend are all the result of some sort of calculation.....if there is no limit to how efficient we can make a house, what goal are we trying to reach during design?
For Earthships, IIRC, the goal is to achieve close to 100% passive solar heating and natural cooling, at minimum cost, through high thermal mass, earth tempering and large amounts of direct solar gain. The designs only seem to work well in certain climates, and are very unconventional in appearance.
For Passive houses, the goal is to have close to 100% passive heating and cooling, using only normal structural elements as thermal mass, and superinsulating, airsealing and HRV'ing to a level where (Northern European) BTU loads match typical body heat, appliance heat, and small amounts of direct solar gain. This works out to be about 1 BTU/sqft.HDD and is a bar (one of several) that must be met to be certified a 'passive house'. The resulting designs do require special and expensive detailing and windows, but are close to conventional in appearance.
The goal of 'Net-Zero Energy' is obvious enough, but not sufficient to constrain the design for PV versus insulation. Optimizing for lowest cost at current PV prices does not require passive house insulation, and the optimum is moving towards less and less insulation as PV gets cheaper. Before several years ago, the cost optimum was close to passive, now it isn't. Certainly many going Net-Zero still go Passive House too, mostly for bragging rights, not cost optimization, so the ideas are often conflated, but they needed be. In the future they certainly won't be.
Still another design option is just to ask what insulation levels minimize total cost of ownership on a 30 year horizon, including current heating and cooling energy costs, with certain assumptions regarding financing (which correspond to something more like a 10 or 15 year simple payback). What is that called? It is roughly the house design called for by the most current energy codes in the US. This is exactly the calculation that DoE does to develop housing codes. Of course, most locations use old insulation codes, but I would still argue that there are far more houses being built at 2013 codes in the US, than the other three kinds above put together. And as modern code adoption moves forward through different states and towns, I expect that will continue to be true.
So, which is the 'right' design? If it became possible to buy 100% carbon free RE tomorrow at prices comparable to current electrical rates, then those 'lowest cost Net-Zero' houses would be built to the same insulation specification as '2013 code houses', and the only difference would be whether the RE came from an on-site array or an off-site RE source! Both would be zero carbon for energy, and cost the same amount. IOW, once industrial PV reaches grid parity, then any 2013 code house that buys that power becomes just as zero carbon energy as any other net-zero or passive house, and at lower cost and embodied carbon that those other kinds (by virtue of using less materials).
So, what is the difference then....its all in bets on the future. IF you are a doomer, and think society will collapse to 19th century tech...then an Earthship is your way to go. If you think energy will be available but scarce in the future, and will be much more expensive than now (for any set of reasons), then you build an expensive Passive House, and say it will pay for itself in that future. If you are worried about AGW and CO2, you build Net-Zero, on whatever budget and PV/insulation mix you prefer. If you are the DoE, you think (inflation corrected) energy costs will be about the same for the next 30-50 years, and you come up with the 2013 energy code design.
You can look at the DoE/current code position as a Business as Usual (BAU), ostrich head in the sand approach if you want (esp if you want to sell expensive Passive Houses, Deep Energy Retrofits and MREs to doomers). You can point to the Germans (worried about future energy costs and availability) and say they are making the wiser choice.
Instead, I propose that cheap solar (and storage) changes all the calculus a lot. When we say 'solar will soon reach grid parity cost', we are saying the German's have made the wrong bet. And the US DoE has made a better bet (perhaps for all the wrong reasons, but whatever). If PV gets big after it gets cheap (the usual order of events), then the cheapest way to make a house Net-Zero is to build a house much closer to a 2013 energy code than a passive house, and power it with PV (either on site, or from a grid supplier, your choice).
For example: I energy retrofit my 1960 split level place to a performance roughly the same as current energy codes, switched it from heating oil heat and DHW to air-source heat pump for both, fully electrified, and buy 100% of my electricity for the house and the LEAF from PA wind farms, at $0.124/kWh, a price that is currently $0.01/kWh cheaper than my conventional utility. My site is too shaded for PV. The energy retrofit cost $6k and some DIY, the HVAC switch cost $18k (or $10k more than installing central air, which I would have done anyway), and I save ~$4k per year on home energy costs. By some reckoning, my 1960 house is now 'net-zero carbon energy', and I am getting a 25% ROI on my energy investments.