Greenhouse newbie with lots of 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!

Hi Leon,
I will start by saying that I am not a greenhouse expert, but I have read a lot, and I have had a couple greenhouses that I used.

I think overall it sounds fine, but there are some thoughts on specifics.

If you want the effects of a partially underground greenhouse, then you do not want to build up to the footer. You want to keep that 2ft depth as much as possible. If you build the inside up to ground height... you loose a lot of the benefits.

As for venting, I am a huge fan of the automatic vent openers. They use a wax/oil material to open and close vents based on the temperature without any timers or electricity. A fan would probably be good to have anyway... but I would try to put in enough vents so you wouldn't need a fan. I think I read somewhere for passive venting the vents sqft should be about 1/3 of the sqft of the floor space? Maybe someone more knowledgeable can correct me on that.

As for the roof pitch... if I'm reading it right, you are planning to have the roof be clear as well? If so... I would only overlap the roof a bit, and I would keep it nice and steep so the snow falls off easily. If the roof is solid, then I would look into the roof overhang and angles so that you can have direct sun in the winter and shade in the summer. I think you are supposed to add 15 to the lattitude and that is the angle of the windows. Again, maybe someone can fact check me on that, but that is what I remember.

You plan to build a clear roof but I would suggest you look into how deep winter greenhouses are built.  It may change your mind.

Traditional greenhouses have wild temperature swings and need lots of added heat in the winter. Or even to get through cold spring nights.

https://www.youtube.com/watch?v=24X-qdeeZUQ

This design maximizes heat gain in the winter and limits it in the summer.  It is heavily insulated on 3 sides and the roof to help store that winter heat gain.

I would suggest automatic vent openers (and more than one so you have backup if one fails)  It is very common to have cold mornings but by afternoon the greenhouse can heat up quickly and kill the plants.  By having the openers work automatically you are less likely to come home from work to find burned plant.

Make sure your windows don't have any type of low energy coating on them.

Thank you Matt & J - really appreciate your replies.  

I am planning on a clear polycarbonate roof and using automatic vent openers.  J’s feedback has me questioning having polycarbonate on all the roof vs having at least the upper few feet be solid roof with polycarbonate from there down.  Or maybe half and half.

What benefits would I loose by filling the floor to the top of the footers?

Also curious to know why would windows with low e coating would be a disadvantage.  

I need all the help I can get on this.  Thanks so much!

Hi Leon,
If you fill in up to ground level, then you no longer have the ground acting as insulation around the greenhouse. It would be about like you built it at ground level. I'm sure there might be some benefit with the ground temp seaping into the foundation... but for simplicity... lets just say, you lose your partially underground benefits of insulation and temperature if you fill it in.

Could I suggest a different shape for your greenhouse? I didn't have time to do up a nice one on the computer, but check out the shape below. The long front is all clear for the sun to come in. The small part of the roof slanting to the north is solid and insulated. As is the back wall. The side walls could be solid and insulated depending on your needs.

Was going to just fill in the bottom 6" which would bring the floor level just a smidge above footer level.  That would still leave 2' of foundation below grade in the front & sides and 4' in the back (on the north wall).  

I like your design.  It was actually my original one and the guy helping me build the greenhouse (who's an experienced builder but who's not familiar with greenhouses) recommended the shed roof to save cost.  Your suggestion does having me wonder if it'd be best to go back to the original design.

Thanks Matt!  

Low E coatings are designed to stop heat transmitting through the glass.

In the winter the sun is low it shines more directly on the front wall allowing lots  of heat gain.

In the summer when the sun is high it shines less directly on the front wall allowing less heat gain.

At night everything inside the building is losing  heat by radiating IR.  Glass is fairly reflective to IR and bounces the heat back into the greenhouse.

Polycarbonate  is translucent to IR so it allows a lot more heat to leave the greenhouse at night than glass would.

You also should look up how much venting you need.  With a clear roof people often way underestimate the amount of venting a greenhouse needs.

Originally you asked about a fan to circulate air.  That is probably a good idea.  You can use it to blow warm air from up high down to where the plants are on cold days.  Also the wind helps make the plants stronger when it blows them around and they have to resist it to stay straight.

Was going to just fill in the bottom 6" which would bring the floor level just a smidge above footer level.  That would still leave 2' of foundation below grade in the front & sides and 4' in the back (on the north wall).    

Ok, I'm following you now. Yeah, I think that would be fine.

And I would seriously consider a greenhouse shape. There are several variations. Yes a shed form would be cheaper to build. And yes, it would be way better than no greenhouse. But if you can do it, I think you would be happier with a building designed for being used as a greenhouse, rather than a shed that was converted to a greenhouse.

Thanks Matt!   Curious - I like your design more than the shed design on aesthetic grounds but what makes that design better for a greenhouse on practical terms?

Also curious to know if installing a heat exchange (GAHT)system will be worth it considering:
I'm done with heavy equipment in the area and not sure if the existing trenches are deep enough and long enough to make it work. Here's the data: The greenhouse will have apx. 1600 cubic feet of interior space. There's 25' of trench on the exterior of the north wall that's 4' below grade. Was planning on putting the French drain down there but could possibly add heat exchange tubes down there before backfilling also (see any reason why not?). If so, the tube could probably be doubled back on itself which would give another 25' of length. From there it could run along the trench outside the east wall, which is another 15', though only 2' below grade. From there it could run out a 15' trench (under a berm) that the French drain will run out. There will probably be around 3' of fill over that. So if that last 15' run is double with the pipe coming back to the greenhouse that would add another 15'. So the total would be roughly 25 + 25 + 15 + 15 + 15 = 95', with about half of that being 4' under grade and the other half about 2-3' under. Being in zone 5a the frost likely goes deeper than at your place. Is there some kind of equation to figure out if this would provide enough of a heat exchange? (The photos show the trench along the north wall and the trench through the berm that the French drain will run out to.)
Thanks.

Hi Leon,
The angle of the glass has a lot to do with how much light comes in. Also, tilting it gives more glazing to get more light in the building. There are quite a few articles on this, but I do not know it well enough to spit it back out. But the light can be reflected more if the angle is different. Probably not that big of a deal at the home scale, but worth noting.

As to the heat exchanger, I don't have any kind of calculation to point you to. Sorry. My general thought is that any heat storage is better than none... but I do not have enough experience to tell you if that is enough to keep it above freezing.

Thanks Matt.

Leaning towards going with a 6/12 roof to minimize snow accumulation.  Reckon light will be more at premium in the winter, so even if that's not the most efficient angle for light in general it should be better than having snow block a lot of light.  The roof will be polycarb and there will also be a fair amount of vertical glass glazing in the walls.

Also, I might try the Swedish Skirt method since the foundation's still exposed. Might be able to get a bunch of partial pieces of ICF, cut the middle plastic brackets off and use the foam for insulating the foundation and those wings that angle into the ground. It'll obviously be more labor intensive than buying full sheets of rigid foam but the price and the environmental impact will sure be less.  Any thoughts on that?