I have heat element windows, 3 glasses, two innermost are sealed and have argon gas between them, which makes them quite superior compared to windows that have only air between them.
Windows are typically around 10-20% from heat loss, but if there are large window surfaces that will be easily higher number, at old days people put small windows to houses as it was common to have only 2 glasses, now I have heat element windows that are small in size as those old windows, which makes heat loss from windows relatively small.
Now there are many better materials than sand for insulation, rockwool is one of the best, but at old ages peat has been used and straw is rather good too.
2 meters is not quite enough of sand, sand ground is frozen to 1.5 meters deep during winter and that is with nearly infinite land mass, at southern part of country much less of course, but to get it insulate, not loose heat one would need to have it enough thick that walls are not needed to heat by interior air.
Rockwool is just something that probably can't be used well in earthship kind of design, humidity issues can be problematic.
I use 100mm at kitchen door which leads to warehouse that is not insulated. there is thick frost between door and insulation, humidity from inside air freezes to surface of door, there is +10C in kitchen during winter time, between door and insulation it can be as cold as -20C during coldest days and that is just 100mm insulation, putting 200mm to walls and 400mm to top (50% heat loss is from top) would almost remove need to heat housing.
Here many builds today passive heating houses, those are heated by waste heat from electronics, cooking and humans, very little if any heating is required, kind of similar to earthship, but still bit different, those seem to work rather well here:
Passive house - Wikipedia, the free encyclopedia
There is also link at beginning of that article to passive solar building design and in references earthship is also mentioned, so there are similarities, but I think that modern methods excel at situations where solar energy is limited as you can get away without any solar heat at all if needed. Ideas itself are very workable, but for me using sand as insulation/heating battery is bit problematic at extreme conditions.
At some point wooden outer layer needs to be replaced in my house at that time it will be easy to add enough insulation to outer walls to make house to be passive heated model as even now I'm reaching low energy model levels, but it will take some time, until then I'm rather happy to have 1/4 of total energy consumption compared to many newer houses.
I consume less than half of water compared to typical value, also I produce hardly any waste as I choose what I buy so that I don't need to put anything to waste bin and so on, not really eco minded, but just using common sense instead of running after desires.