Hi Michael ...
In this particular case I plan to build it to replace one we already have in place. About 5 years ago I used some pvc pipe to make arches and then covered the greenhouse with special uv resistant poly that is supposed to be good for about 5 years. It is still going strong but nearing the end of its practical lifetime. The new one will be a little larger.
We live in Canada at about 3000 feet so our garden starts late and winter comes along again pretty fast. We get very early greens, early summer tomatoes, peppers, loads of cucumbers and many squash out of the small structure ... the outdoor garden provides berries, some fruit, beans, spinach and the cabbage family pretty well.
So, a little bigger greenhouse with automatic ventilation and a much longer potential life span makes really good sense.
Incidentally, I do have a client who wants me to design a greenhouse for him this winter ... larger and timber frame ... but the detail work and research I did for this one will be a bit of a preliminary prototype.
cheers,
eric