Greenhouse newbie - many questions.

Hi. I’d really appreciate some help on the next steps for my greenhouse. The foundation (see photo) is 14x22’ and it’s located at 4100’ in NE Oregon, Zone 5a. The front of the foundation faces directly south. It, and most of the east & west sides are apx. 2’ below grade and back wall is about 5’ below. Before backfilling I plan on installing a French drain along the exterior. Any thoughts on rigid PVC vs the flex drain pipe? Do you recommend using some kind of coating on the foundation before backfilling?

For the floor I’m thinking of adding a few inches of gravel, then a top layer of bricks, maybe with a bit of sand to hold them, for any areas that aren’t raised beds. Will try to get the height of the bricks to be just above footer height. How’s that sound?

I plan on using stick construction with 3x6’ double pane windows horizontally on the S. side, which should give a little over 6’ of interior headspace on the low (south) side. Will also use some windows on the east & west sides.

Planning on a shed roof and using multi-layer polycarbonate. What pitch would you recommended for a fairly snowy climate at about 45º latitude? And do you think it’d be good to extend the roof beyond the south wall by a foot or two to avoid rain and snow running down the south wall? Would you recommend double or triple layer polycarb and do know of any good sources for long sheets?

What do you recommend for ventilation. Summer temps are often in the 80’s and sometimes 90’s here. And what about winter - would it be good to include a fan to circulate air even if the outdoor vents are sealed up?

Hoping to at least overwinter some greens. Planning on including a small Cadet-type wall heater to protect against the coldest weather. (Only have 110 wired to the greenhouse so probably can’t put in a stronger heater.) And if having the greenhouse partially in the earth is enough to keep the temperatures moderate in the winter I hope to be able to grow a citrus tree.

Would really welcome your thoughts and suggestions on any of the above. Thanks!

I don't know a lot about greenhouses specifically, but I have a fair amount of construction experience so I'll at least comment, and maybe someone else will post more information for you. As far as pipe, definitely go with the rigid PVC, SO MUCH BETTER than the black single wall corrugated pipe if that is what you are talking about. That stuff gets so damaged, so easily it's not funny. Collapses, torn up if you ever run a eel / snake through it, etc... PVC isn't that much more imho, and well worth it. As far as roof pitch, definitely don't go less than a 4/12 pitch, and keep in mind that if you go very steep, when the snow melts off, it's coming down in sheets unless you put some type of ice breakers on the roof. Maybe between a 4/12 and 8/12 pitch? Personally I'd extend the roof 12 to 18 inches at most. You also don't want to create a situation where you are "shadowing" your wall with the roof, kind of defeats the purpose of the greenhouse.

Lastly, you are not going to do much with a 120V heater, even running at max possible in a greenhouse that size, even with good insulation. Consider a gas heater, maybe small propane tank just outside, use the electric for a blower / fan for the heat maybe? Hope this gives you some things to consider.

Thanks LGR - really appreciate your suggestions. That's all very helpful. You make a great case for rigid pipe. Thinking either a 5/12 or 6/12 roof. Too bad about the 120V heater not doing much, especially since using gas is not an option for me (ethically). I'll have to hope that the warmth of the earth, the insulation and a small contribution from a 120V heater will be enough to at least keep things from freezing. All suggestions are appreciated!