It might be easier to just pour a concrete pad. What is the cost of what you want to do?

Add conduits for water and electric.
Add steel anchores to hold down the GH against wind force.

dcarch

Yes, I see a problem: a lack of proper drainage. Poking holes in the plastic, then laying the foam board, then plywood over it won't give you drainage, only eventually, rotting wood and mildew.

Few things are more important in a GH than drainage and if you can't connect to a drain line beyond your foundation, a dry well beneath it will also serve. It means digging a deep hole to below your frost line, but once you make the effort, you will be forever grateful that you did.

Hi Christian,
Here's what I did with my 8x16 Sunshine Gardenhouse foundation. My site slopes about 10-12 inches front to back, so I raised the front 4 inches above grade and leveled the back to match that height. I laid landscape fabric on the ground level, which blocks weeds but allows good drainage; the poly you mention would block drainage and create an anaerobic situation underfoot. Imagine replacing your cotton socks, or underwear, with poly--permanently--breathing is a good thing.
I placed 4x8x16 cement blocks over compacted decomposed granite base as a perimeter for the foundation. The composite boards for the base sit on top of the blocks, just at the inside, so I can drive the anchors into the ground for wind stability. Spring storms here in north Texas get ugly, and the gh has made it through one season so far.
Those 4x4 boards you mentioned for the base, are they pressure treated CCA boards? The CCA stands for Copper Chromium Arsenate, some pretty toxic elements for yourself and your greenhouse inhabitants...think about cedar, or stone, or concrete....
Hope you get an idea or two from the photo. I'm in a much warmer climate than yours, so take that into consideration, and get some feedback from folks in cooler climates than mine. And have a ton of fun!

Here is a link that might be useful:

Christian,
Here's a shot of the greenhouse on top of the foundation...

Here is a link that might be useful:

Christian,
For reference, here's the landscape fabric over ground level, starting to apply decomposed granite base...

Here is a link that might be useful:

I just did a web search on 'SunGlow greenhouse' and didn't find anything...is there a website somewhere that has info on it?

If I had it to do over, I would have done my hobby greenhouse just like my commercial ones, with a crushed stone (unwashed so it will pack) base, then ground cover cloth, the commercial kind, not the cheap stuff. Like Dewitts is a good one to use. for a small installation like that you can order cuts from Morton's greenhouse supply online--they are in Tennessee and will cut and ship small pieces by the foot.
You can sweep it, water drains, it's awesome. Insulate the side walls, but leave the center be. Once it gets warm, it stays warm that way.
One thing, if you can plan ahead on where your electric, water etc will be, do that first. It sucks to put in all your benches and then need to move some of them to do other projects! But it's do able anyway.

Here in Maine, IF I can come up with the funds to build a greenhouse...I plan to dig a 2' deep trench that is about 3' wide. simple concrete footer 6" thick and 2 layers of concrete block for the foundation. 2 layers of 2 inch foam insulation over the concrete block AND another 4" layer of foam insulation laying on the ground extending 2' out from the greenhouse. Then lay some corrugated drain pipe in a silt sleeve all around that, cover with gravel and viola! The type of foundation is called a Frost Protected Shallow Foundation. Should work, I think, and much less expensive than pouring a 4' concrete foundation to get below frost line, yet does essentially the same thing due to the 4" R30+ foam insulation. of course, I'll have to take the footings into consideration. In theory you can do this with virtually no actual foundation, just the insulation.

zengeos: You just pretty well described the insulation under and around my GH, but mine is erected on timbers bolted to concrete footers. We needed to avoid the least movement from ground heave to both prevent any warping of the GH frame and accomodate my desire for a waterproof, finished floor, which is glazed quarry tile over concrete board- over OSB- over foam- over a vapor barrier- over crushed limestone- all sloped to the middle to a floor drain that spans the width.

That drain feeds to a sump pit with a pump under the sink that drains to a 100 ft. long grey water drain set below our frost line. It took a lot of time and effort to get the foundation ready for the GH, but it proved well worth it, and- it worked.

My GH is heated and due to all the ground insulation, it not only stays warm, I can walk in it barefoot in the coldest weather without a chill. Matter of fact; I did just that early this morning, when I dashed out there wearing only a warm robe over my nightie and my "Wicked Good's" then realized I needed to top off some fish tanks, so removed the slippers to keep them dry. My feet got wet as I dumped buckets of water, but not especially cold.

I couldn't drain a GH foundation as you described however: Our soils are too hydric, so I presume yours are far more permeable.

Hi Christian
I`d go with the landscape fabric instead of poly as it will breathe and drain.
Use the rigid insulation either vertically or horizontally at the perimeter only, but don`t cover the floor with it.
If you do, you`ll lose the benefit of the (insulated) soil inside and below the gh which provides a fair amount of thermal mass.

Brdwidow, you may be onto something fr me re's the perfpipe. How ARE you draining water away from the outside perimeter of your greenhouse?

Mark-

Mark:

The water that goes into the floor drain is connected to a sump pit under the sink that also drains into it and is pumped out with a standard basement sump pump. The sump pit connects to a grey water line about 100 ft. long, the same way any basement sump is drained, at least here in an area that just 200 years ago was a vast tallgrass (wetlands) prarie over a lot of peat and saturated clay.

So the only perf pipe is in the long grey water drain line. All of the pipe inside the GH and leading out, is solid.

If that seems excessive for ordinary GH use, please understand that my GH is actually a fish room and when I water change, I may drain as much as 300 gal. of water within a few hours, and as we calculated that I could release about 200 gal. an hour before the gray water line became overloaded, I rotate changes on the big tanks, to make sure it doesn't.

But with just plants and reasonably permeable soil, all you might need is a dry well. If so, it's just a deep hole to below your frost line, some large plastic pipe for a liner, large stone to fill and a screen under a removable grate at the top, to prevent solids from clogging the well.

I am always amazed that anyone would construct before they know what is UNDER what will be their foundation, but then, we are no longer an agrigulcural society, so unless they are farmers, most people don't know that dirt isn't just dirt, and that they have a Soil & Water Conservation District to call on, but that is where you will find the soil maps for your property, along with soil experts, courtesy of the feds.

Once you know the permeability and structure of your soils, calculating the perk rate is easy, and you can plan your drainage needs from there.

Birdwidow,

My soil structure is a bit odd. I have a rather high water table. In Spring through June the water table is not even 18 inches below ground. My soil is heavy, but rich soil at least the top 12 inches or so. Below that it's a little hardpack. I think the issue s the land was used as a farm for many years, and I suspect the soil got compressed and packed dwn over the years. My veggies in general seem to like the soil. Even the sweet potatoes I planted for the first time seemed to produce a reasonably good yield. The ting I need to watch is turning it into a plant bathtub deathtrap....with the water all percolating TO the greenhouse. Ahh well...no worries right now...no money to build a greenhouse :/

zengeos:

There is nothing at all odd about your soil structure. If if your soil is rich and permeable at the top and "hardpan" below, it indicates a typical wetlands soil profile; permeable organic topsoils above inorganic clays.

However, if it was previously farmed, with your description of the profile, it may well have been drained by field tiles, and if so, and they retained them, old winter croplands airial photos taken by the SCS (now NRCS) will show field tile locations and allow you to have an accurate picture of what lies below, which is vital to planning drainage.

If you can't build your GH at present, you could do as we did; plan, and dig. I waited many years for my GH but as I did, we got the groundwork (no pun intended) done for the drains, water, gas and electric. There was very little cost involved; just a lot of grunt work that I'm glad we got done when we still had the energy for it.

BTW: Farming doesn't compress subsoils, but construction bulldozers wreck havoc on both natural and manmade subsurface drainages.

Ok, I thought when you dug down into the earth you would get a constant temperature, something like 55°F. This would be great in winter or summer. Why do you want to insulate that out?

"I thought when you dug down into the earth you would get a constant temperature, something like 55°F. This would be great in winter or summer. Why do you want to insulate that out?"

You don't. That's why I vote for insulating the perimeter, but not the floor, of a greenhouse. And that's why none of the GH heat calculators factor in the floor. There just isn't much of an unfavorable temp differential between 55F earth and the inside of a GH.

The part I am looking at is the summer cooling factor. If I have a hole that can deliver 55°F most of the time, I could put a fan down it and blow the cool air up into the GH. This would be great.

I know an man that blows cool air from under his house (basement) into his greenhouse and he does not have to use any type of cooling systems. He has most of it shaded cause he grows cattleyas but he still has 3000 to 4000 footcandles of light which create heat.

So would the cool floor help in the summer. It wouldnt hurt in the winter.

I can only speak for myself, but I insulated my GH floor along with the perimiter because the foundation is slightly elevated and I wanted a warm floor in winter and a cool one in summer, and got both. I know warm air rises, but cold can and does creep up through hard surfaced floors. It could be that the issue is specific to a solid, hard surfaced GH floor, but that's what I wanted and have, so insulated against the cold.

What drove me was the experience of standing in my tennies on the concrete floor in our well insulated, heated garage in winter and suffering cold feet, and if I can't be comfortable in my tennies, I am not a happy camper. So I fixed it so I could comfortably walk barefoot in my GH at 0 deg. if I felt like it, and often do. To each, their own GH use and comfort level.

Meanwhile, my GH held at a steady 72 deg. through a month of very cold weather and our gas bill hardly noticed it. Maybe Nathan knew what he was about when he designed my ground insulation?

I agree with Orchiddude a concrete foundation would be best. You never have to worry about replacing it. I dug my footings 12 inches wide and 8 inches deep. I filled the trench with rocks and pieces of brick, poured concrete over this and smoothed it out and let it dry. Then I laid a row of concrete blocks along the footings and built (wood) on top of that. I also agree agree with Orchiddude about not insulating the floor. Orchiddude and I both live in Alabama so our conditions probably vary from those in NJ. Our ground holds heat for so long it is an asset. Many nights I direct a fan to blow across my brick floor which picks up the heat and warms the greenhouse (gh) without firing-up the heater. On a 30 degree night in early to Mid-winter, I can get 45-50 degrees without a heater. Usually by Christmas the ground has begun to cool so I need the heater. I spent two winters in Philadelphia and the ground seemed to be colder than here. On Valentine's Day 1970 we had a snow that stayed on the ground for weeks. Here the snow usually melts within 2 days because the ground is too warm to sustain it.