How much of a GH wall has to be glass?

I have seen a lot of nice designs here for making various kinds of greenhouses.  I am studying them to learn but have a question that is not addressed.  How much of a gh wall has to be clear/transparent?  If the base of the plants is 18" off the ground, is there any reason to make the transparency go all the way to the ground?  Most of the discussions I have read about gh design, heat is the biggest factor to manage.  Reason for the question: transparency is more expensive and more time consuming to repair/replace.

I'm not an authority on this, but I do believe there is no particular reason to have greenhouse glass below the level that gets light to the plants' leaves. The greenhouse I am building has raised beds and is bermed on the outside to 6" above the bed level. The sloped glass has to be a bit away from the soil anyway, and I see no reason to build 6" of unusable foundation structure.

While I cannot give you a specific figure, I have personally found the my greenhouses tend to have too much glass.   This leads to them quickly overheating.   Another problem is that glass closer the ground tends to get broken/damaged more easily.  So far, in my green house/high tunnel construction I have never had a problem with too little glass/ plastic.  I have had problems with too much.

My passive solar greenhouse is about 1/3 glass (E, W, N and N roof are solid).  The foundation wall comes up about 1-2' from grade.  I do have a raised bed inside that is 6" below that stem wall.  With my winter snow, any glazing below 3' tall is covered anyway during the season where I need max sunlight.  

If you're in the northern hemisphere, the north wall doesn't need to be transparent. In fact, making that wall solid and insulating it, then adding some kind of thermal mass, will help even out the temperature inside.

(In the southern hemisphere, it's the south wall.)

As for how far down the glass needs to go, I think that's more situation-dependent. If it's easier for you to make it transparent all the way to the ground, go for it. Personally, I like having the bottom shady so I can use it for storage space, and so wildlife is less likely to run into the walls. (Currently I have a greenhouse that is clear all the way to the ground. In the summer we frequently get rabbits trapped in there, because the door is open for ventilation, and they don't realize they can't go out through the sides.)

It also depends on how far from the equator you are, if you plan on using it in winter. When the sun is low, you might get more light through the sides than through the top.

Generally though, it's only the leaves that need sunlight, not the pots. So if it's easier for you to limit the transparent parts, just make sure you're not shading the leaves.