Third Greenhouse and glazing angles

[Bob VonMoss]

Take the Phoenix Earthship. That may not have first greenhouse glazing
at all, so the bedrooms merge into the 2nd greenhouse glass, like most
earthships. The 2nd greenhouse glass is slanted to match the solar
noon sun angle on Dec 21 to avoid reflecting precious sunlight away at
solar noon. Then the third greenhouse was added on. In that case they
used some kind of trusses, but tilted them down to match the frame
holding up vertical glass at the stem wall. So the direct sunlight
enters through the glass on the roof on the south facing part of the
trusses. For Taos, NM at solar noon on Dec 21 the sun is 31.5 degrees
high on the horizon. If the second greenhouse glass was slanted
perpendicular, the face would be at 58.5 degrees. The roof trusses on
the Phoneix look like south facing and north facing form 90 degrees
and it's tilted south just a tad, maybe 5-7 degrees, making it 50-52
degrees, not 58.5 (almost). Does it even matter that much? The sun
won't enter the top part of the 2nd greenhouse glass on June 21 at
solar noon. Maybe that's OK.

With the glass at slightly different angles, the sun may not penetrate
as strongly, even in winter, though it might be warmer anyway, since
there is another airlock. How about that guy's video on youtube. He
writes it was "-20", though "+17F" during the middle of the day, and
shows the thermometer in the 3rd greenhouse with "+90F" that he claims
said +100F before he opened the door. It's probably cold at night too
with no solar gain.

What if the 3rd greenhouse is somewhat narrow with say bottle walls on
the sides? or narrower than the main building? There is the potential
for more shadows. Maybe that doesn't matter if the 3rd greenhouse has
windows on some side walls.

What interests me is another diagram of the 3rd greenhouse with a flat
roof. A flat roof will collect more rainwater into the north cistern.
It is simpler, probably isn't as heavy duty as the vigas, probably
cheaper than the Phoenix custom trusses, and is anchored on the main
roof. In the drawing the glass is vertical. So let's say the 1st
greenhouse glass is vertical, the 2nd matches the winter angle, and
the 3rd is vertical. Is the 2nd greenhouse angle even necessary? Would
it be cheaper to just do that vertical too?