I'm looking for information on building a half hoop house / walipini into a hillside (newbie)

Sorry I'm new here and still getting used to this forum format, hopefully this is correct

I just have some questions regarding building a hoop house into a hillside that I haven't been able to find answers for online. We have a flat, south facing pasture that backs up into a hill on the north side. We'd like to build an approximately 15x50 greenhouse/hoophouse/walipini (not too concerned with the exact materials as long as it is durable and affordable) that utilizes the hillside as the back wall, and has a roll up side ventilation since we get a lot of sun in the Summer. I haven't seen any instructions that address reinforcing the hillside once you dig it out. We get a lot of rain here, so flash floods and landslides are something to consider as well. I'm not even sure this is a viable option for our location.

I'm just an absolute beginner and have no idea what to search to get the information I am after. I have found plans for walipinis and hoop houses, just nothing that effectively combines the two. I'm not sure how you would anchor the hoops into the hill, or reinforce the inner wall of the hoop house. Would a wood frame work? Could we use glass and cob? We have a barn full of beer bottles we can use. I saw mention of compacting the berm, but I'm not sure how. Is there a hot house style that might work better with our land? I'm open to absolutely any advice.

Thank you so much! I can provide more information if needed. We are located at 2550' in the mountains of WNC. Our hillside has a lot of clay in it.

Welcome Dakota!

If I understand what you are after correctly, it sounds to me like earthbags might be something to investigate. Wood could work, either dimensional or timbers/roundwood. Photos of the site might help as in providing some ideas or guidance. so specifically;
- Wood frame: Yes, it could work
- Glass and cob: Not sure, but it might. Would need to flesh this out a good bit more.

I attached a sketch of one idea I had assuming I understand what you're after correctly. Hope this helps give you direction.

A walipini is actually underground, so if your water table is too high you may end up with a swimming hole. My suggestion there would be to watch where the water flows and pools. Also dig a three foot deep hole in the spot where you want to put the walipini and watch it. Does it fill up in the rain? Does it drain easily afterward? Does it fill from beneath when there is no rain? What you build with is going to be determined by the amount of water it will be in contact with, and how long. If you're determined to do walipini, there are ways to do it but you need to be aware of what risks need to be mitigated so it can be built into the design.

Please provide a picture of the spot so we can see what you're working with.

Caleb Mayfield wrote:Welcome Dakota!

If I understand what you are after correctly, it sounds to me like earthbags might be something to investigate. Wood could work, either dimensional or timbers/roundwood. Photos of the site might help as in providing some ideas or guidance. so specifically;
- Wood frame: Yes, it could work
- Glass and cob: Not sure, but it might. Would need to flesh this out a good bit more.

I attached a sketch of one idea I had assuming I understand what you're after correctly. Hope this helps give you direction.

Nice illustration!
I especially like the diversion swale.

Earth bermed/underground shelters raise a question in my mind.
Do we want to tie the shelter to the earth, thermally,  or just use the earth as insulation or siding?
Either way it behooves us to alter the nearby earth,  by shielding it from water and/or insulating it from the rest of the soil.
Here is an illustration:

https://permies.com/forums/imageCache/image/92511dc69c7e2de5add1cbbc5af5f5ab/image002.jpg

In the greenhouse sketch, maybe enhancing the water shedding of the swale/berm with a buried plastic/rubber membrane would be a way of heating such an "umbrella".
Layers of cardboard could be a cheaper alternative to foam insulation, and I think Paul's wofati uses woodchips and or sawdust.

I'd be tempted to not dig down at the south wall, just have the floor level to the exterior on that side.  Then dig down to that same depth on the north side (let's pretend it takes 2' of digging).  Then build a 5' high earth bag or rock wall in that trench.  Dig out the floor of the greenhouse flat and toss that dirt on the uphill side of the wall.  By doing that you get your swale, you get an earth bermed north wall and you don't have to dig out as much dirt or delve into the water table as deeply.

I'd be a little concerned about what happens with 4 inches of rain that flows downhill and hits a 50' wide swale.  I have no experience with that sort of thing but maybe someone else does.

But maybe I'm overthinking this.  If you just set a hoop house on the hill, maybe it would work just fine.  You'd have a sloped floor but you could build terraces to plant in and walk on.  I guess it depends what you want the space to do.  

Hoop houses set on hills, without compensating so the hoops are properly vertical, do not appreciate snow loads!

I've lived in earth bermed houses for the past 20+ years, and also used solar greenhouses all that time.

The north wall is a retaining wall. Find out about what is needed for that. Here where I live, stone masonry is common and cheap so we always use that in our foundations and retaining walls. We tend to build them slightly sloped back into the wall. We don't have any groundwater to worry about, nor any building codes. A wood frame, earth bags, or something innovated with bottles doesn't sound structurally great to me, but it's a greenhouse and you won't have children sleeping in it so you maybe can afford to be innovative and risk it failing.

The cross section idea offered above looks sweet, but in my experience of solar heated greenhouses for producing vegetables, any wall along the south side, no matter how short, does cast a shadow in mid-winter and reduces the warm growing space when you need it the most.

I suspect in summer in North Carolina (is that where you are) it might get way too hot and cook your perennials, so some way to actually remove the glazing would be ideal. We use UV-resistant polythene film and remove it for the summer, attach it for the winter. Instead, being able to open out both ends entirely might be enough ventilation, but I don't know. You can also paint the inside of the glazing with a thin slurry of any kind of clay you've got around, and then wash it off with a hose when the full sunlight is needed again.

Rebecca Norman wrote:Instead, being able to open out both ends entirely might be enough ventilation, but I don't know.

Our (very small) greenhouse has a door on either side, in the direction of the prevailing winds. The ventilation is enough that the temp inside seldom gets more than 5 degrees above the outside with both doors open. It is also built against a block retaining wall, with the hill behind it for thermal mass. Whether this will work depends on which direction the winds blow. If from the north or south, you're unlikely to get the necessary effect. In that case rolling up or removing the glazing / film during the summer would probably be the best option.

We're planning a greenhouse/sunroom attached to the south side of our house, so there are some similar challenges (although not so much flooding on a hillside, of course). My mom has a south-facing sunroom on her house in northern Arizona, and there, having a door at each end to open in the summer is more or less sufficient; but here in southern Arizona, I think it may not be. We saw a great implementation of Rebecca's suggestion of removable glazing the other day in a sunroom built down here a few decades ago. There are clips in the wooden framework, and the glazing (with aluminum frames and felt padding on one side) slides right into the frames and clips in for the winter. We think we'll do something like that with all our mismatched glass (partner is on the phone bartering for some more as we speak). Of course, none of this is a hoop house. For that, the UV-resistant polythene film would be better, and lots of people just roll up the sides rather than removing it entirely in the summer (although removing it and storing it somewhere shady and protected would probably prolong its life).

There's plenty of literature out there about the structural stability of earth bags and cob, with or without bottles, so although I don't have the books handy right now, you could read up on how to make it into a good solid retaining wall if those are materials you have plenty of.