Hi, What is your climate like? How many months do you have to heat your house in winter, and how many months do you have uncomfortable heat in summer? These are the first things to clarify before you make your design.
I have many years experience in living in houses that are passively heated and cooled, with some similarities to earthships but several differences.
Yes, vertical south-facing windows are
much better than sloping glass, in my experience. Sloped glass overheats horribly in summer, but vertical glass gets much less sun in summer because the sun moves high overhead in summer.
An overhang over the south windows or wall reduces summer overheating. Depending on your latitude (how far north you are) and how much overheating you might suffer from, you design your overhang. I suspect that in Portugal the sun is not as high as in the tropics, and you may have very hot summers, so you might want a very large overhang to reduce overheating. I think you can probably find a
solar design website that lets you calculate how wide an overhang is needed to shade a window of X height at X latitude. The lovely thing about passive
solar heating is that the sun moves lower in the southern sky in the winter, so in winter the sun shines under the overhang and into your house.
East, west and north facing windows get sun during summer mornings and afternoons, so minimising windows on those 3 sides reduces overheating. West-facing windows are the worst for overheating, because they add direct solar gain during long summer afternoons when your house is already warm from the hot air of the day. Exact south-facing windows with a good overhang get no direct sun at all in summer. It's nice to have some windows on all sides, for nice cross-ventilation when you open the windows, and for the aesthetics of the rooms, but make them small, and try to shade the 3 sides with plants or
trees.
An aspect of earthships that I like is thick earthen walls for thermal mass. Our buildings use rammed earth (not filled tires) and this thermal mass stabilises the temperature summer and winter. Other materials than earth are possible, but thermal mass is
essential for a passively heated and cooled building. Today on 11th June our buildings are nice and cool inside even when it gets up to 28C outdoors at this time of year. By late summer, August, the thermal mass is no longer so effective, as it will have heated up by then. Similarly, for the first half of winter, our buildings stay warm because they still hold the heat of summer.
A solar-heated wood-framed building without major thermal mass suffers from cooling too quickly and heating too quickly. Insulation does not help this problem at all. Thermal mass walls can be stone or
concrete, but earthen walls have nicer qualities in my experience, though there are places and reasons that make stone or concrete necessary.
Water is also a very effective thermal mass material, and so tanks of water (possibly including plants and fish) can be used. I've seen three successful houses using water tanks for thermal mass in passively solar heated houses in Cape Cod, US, where the houses were wood framed with insulation.
If your heating needs in winter are not very heavy, with south facing vertical windows and good thermal mass, you may need very little or no backup heating, so a small
wood stove might serve your needs better than a rocket mass heater. A small stove can heat up a room up quickly on those few nights or mornings when you need additional heat, and some of its heat will be stored in the thermal mass walls, if you have those. Rocket mass
heaters sound great for cold northern climates with long winters, many months of needing to heat houses, and cloudy winters with low sun angle making solar heating less effective. I don't know a lot about Portugal but I think maybe you don't have those conditions.
