The following are just my opinions...

I don't think fiberglass batts would be a problem with a proper moisture barrier.

The difference between R-13 and R-19 will not be that noticeable or that important given that the VAST majority of your heat loss will be through the other walls (twinwall PC is R-1.6 or something like that). Every little bit helps, of course, but you may get more bang for your buck by upgrading some other component of your GH operation.

6x8 is a small GH. For larger GHs, perimeter insulation is all that you need. The temperature differential, which will determine your loss of heat, is not that great between the typical GH air temp and the temp of ground straight down. But as you shrink the size of the GH, then the floor becomes more important. I did some calculations on a previous thread (search if you want to find it) that showed, IIRC, that heat loss through the floor of a 6x8 could account for a significant portion of your total heat losses. Perimeter insulation? Definitely. Insulation under the slab? In Montana, with a 6x8, yes if you can do it.

Fantastic- thank you for the advice and suggestions. As far as insulating under the slab, what thickness of rigid insulation would you recommend under the slab, and is this just laid down over a packed gravel or road mix base under the slab? Do you have references you'd suggest for insulating the slab?
I realize it is a small greenhouse, and I am debating on increasing the size. Space is an issue, but I could go as large as 8 x 10' if necessary. My initial thought was to build something small and well made with good quality components, rather going for a larger size.
You mentioned upgrading other components of the greenhouse as a way to get more return- what components were you thinking of?
Thanks again, I really appreciate your advice.

Being ignorant of zone 3 and 4, I have a question. does the ground under a concrete slab freeze? On cold nights when the temps stay above freezing, I blow a fan across the brick floor and it raises my greenhouse temp about 5 degrees.

Birdwidow will be more help than anyone when it comes to insulating a floor. She is in northern IL. Hopefully she will chime in. My recollection is that she has 2" foamboard under crushed 3/4" white rock.

I'm not so sure about putting foamboard under a slab.

If you are just interested in growing some veggies and seedlings, have you considered an indoor light setup? In your climate, the cost of keeping even a small greenhouse above freezing is going to be prohibitive, especially if you get a lot of wind. I wouldn't be surprised if those veggies cost you over $100 per pound. On the other hand, if you really want a greenhouse, that's different.

As I have pointed out before, with an indoor light setup, all of the waste heat helps to heat your house, so your efficiency is 100%. Also, you don't have to go outside to get to the plants. And there are much fewer problems with pests.

I'm here Stress, in sub freezing No. Il, with a fairly warm floor in my heated 12 X 16 GH.

We used Type 250 insulating foam board under the foundation as it had to bear the weight of the finished floor, which is glazed quarry tile over cement board, which in turn, is laid over 3/4 exterior OSD, over packed limestone screening over 3/4 gravel.

Type 250 foamboard is exactly what is used to insulate under poured concrete, primarially in basements and garages. It's designed for that purpose. I learned about it watching TOH, as they poured concrete over it for a basement and garage, both of which were then fitted with radiant heat coils. I drooled with envy, but while we couldn't retrofit our basement or garage, I could at least take advantage of the material for my GH, and did.

The proof for me is both a reasonably warm floor in sub zero temps, and the fact that none of the tiles has cracked under the legs of the stands holding big tanks, each of which weighs full, over 1,000 lbs., so the base under them is as firm as we had hoped it would be.

The outer walls are insulated with 2" thick Type 150, which is not quite as dense. That foam runs around the perimiter of the foundation, from the top of the kneewall down to the frost line and more is laid out on the sides, as a flange to 2 ft. out. The inner edge of the flange, that slopes down a bit for drainage as it goes out, is laid directly against the outer perimiter foam and once it was all in place, we used expanding foam to fill any gaps.

The outer flange of foam is hidden under decorative river rock, held in place with timbers all around the outside edge. I couldn't grow in that space, so filled it with the stone and use the space in the growing season to hold big pots of flowers and some smaller tomatoes and cukes.

However, I freely admit that my elaborate foundation insulating was designed to suit a GH that had to hold no less than 72 deg in winter, in an area that experiences severe, deep cold, so what I needed might not be necessary for other's uses and climate.

Also, it had to have easy drainage, which is accomodated with a channel drain laid across the middle of the floor, side to side, so the floor slopes down from front to back about 1/4 inch. Not much slope, but enough. The center of the channel is connected to a pipe that connects to a sump pit under the sink set against the back wall and it drains out into a long, 3" gray water line, set well under our frost line. I think it's about 4 ft. down. I know my DH dropped the blade on the DitchWitch all the way down when he trenched.

The scheme worked for me and my particular needs, but in any cold climate, I still believe that no amount of insulation is too much and no amount of time and effort expended on getting the foundation right before erecting the structure above it is wasted.

After all, we can do a lot of retrofitting after the GH is up and as we use them and learn of new materials or different techniques to keep them warm or cool, often do, but the last thing we ever want to have to face is tearing it all down to redo the foundation.

The best of the holiday season to you all. May your seeds germinate, your seedlings flourish and grow into big, lush plants.

And to those of you down south who have the nerve to come here and grouse about an overnght frost, I say: Bah and Humbug. Come on up north and learn the true meaning of the word "cold." LOL!

Thank you birdwidow and stressbaby for all your advice. I have been reading up more on rigid insulation under slabs and it really does seem like the way to go. Your advice is really helpful and it gives me a lot to think about. Fortuneatly, there is plenty of time for thinking and planning because it will be a while before the snow and frozen ground goes away!
Stressbaby- I really enjoyed seeing the pictures of your garden and GH on your page- how did you add pictures to your page?
Thanks again, happy holidays.

wildlife,

Thanks. It has been a while since I added those pics, but my recollection is that you upload the pics to a host like photobucket, then copy the image tag into the text box on your page.

SB

I have leaned alot about insulating foundations and knee wall from this site and some extensive googling. I have a question that I can't find the answer too. How do I insulate the floor of a greenhouse that is going to have crushed stone for the floor? Can I use rigid foam and cover it with stone or di I have to take another approach?

boxcar,

The foam is waterproof, so if you lay it under crushed stone with no place to drain; it won't. However, if you have the patience to fiddle with the crushed stone, grading and packing it firmly to the center or one end and can dig out a drywell, you could lay the foam over it, leaving the top of the drywell open from the foam, then more stone on top, and the water would drain to the drywell.

But it would result in a much nicer and easier to keep clean floor, not to mention tracking the stuff, to just overlay the foam with concrete patio blocks or bricks. As long as the crushed stone is well and truly packed, the foam really will support the bricks or blocks, that will lay easily and could be butted tightly to avoid the need to fill between them. You could even broom mortar into any cracks, wet it and when it dried, would have a really solid floor.

BTW, just in case you aren't familiar with drywells- They are just holes dug down to below frost line, lined in the old days with mortared brick or stones and now, plastic liners that can be bought for the purpose. Or, you can use a length of fairly wide plastic culvert pipe.

You could also use the same heavy plastic channel drain as I did, except that instead of the section with the drain drop leading to a sump pit, it would go directly into a drywell.

There are a number of options, but for any drywell, always- the deeper the better, then fill it with large stones to the top of the liner and either set a made for the purpose cap or channel drain drop section over it, or top it off with with 1/2 inch hardward cloth and lay a piece of window screening over that, to prevent anything other than water from seeping into it.

I drain my GH with a channel drain that connects to a sump pit, fitted with a sump pump, that empties the pit to a 150 ft. long line, but I fitted out my GH out to serve as a tropical fish hatchery, so often need to dump hundreds of gallons of water at a time, and have a large utility sink out there too, with H & C water, so it's a different GH use and set-up than most.

I also laid OSB over the foam, then topped that with 3/4 inch concrete board, but that was because I wanted a glazed quarry tile floor and it requires far more elaborate under pinnings than do patio blocks or bricks. The tile also lines the kneewall, but there too, my needs were different.

All you really need for a GH in which you only grow plants, is a means to drain off excess water from the plants and avoid too much dampness that might cause fungus on them as well as the interior walls of the GH, although the ability to hose down a concrete block or brick floor with cold water is a nice extra, for keeping the GH clean and help cool it on hot days.

It really all depends on HOW you want to use your GH, and WHEN in relation to your climate, and how much time, effort and money you are willing/able to spend to fit it out for your own maximum use and enjoyment.

After all, a garden GH is really only for pleasure, because unless you are feeding an army of kids, for what it costs to build and maintain one, it's cheaper to shop for veggies at Whole Foods and buy started plants at any nursery.

So by all means, put a lot of thought and effort into your foundation and if, after you have the GH you need to back off on the fittings until your wallet recoups; do so and in the end, you will have what you really want.

We spent an entire season preparing the foundation for my GH and by the time we actually got to erecting it, all we had time for before winter set in was to install the heater, but as anxious as I was to use it, patience paid off. (Of course, we are also a pair of arthritic old poops, so everything takes us longer now.)

I am glad to see the change in attitude in floor insulation. The new thinking is room envelope for determining heat loss. Anything inside of the envelope with a lower temperature than the ambient temperature will act as a heat sink.

John

In following this thread, I'm not certain I've found quite what I'm looking for in planning my own greenhouse foundation. Would those of you posting here mind taking a look and sharing your suggestions, please?

The GH will be 8x15, interior sq ft. I'm building onto the south barn wall which is covered with white aluminum roofing and sits on a six inch concrete foundation. The site slopes toward the barn so I'm planning to have it backhoed out.

Here's my first question: should I just have it dug out to level with the exposed barn foundation (about 12") or should I have it dug out down to the frost line (about three feet).

The plan is to lay about three inches of small gravel around the three sides of the perimeter, then stack a cement block wall to just above ground level, and then backfill. Next question: do I line the interior face of the block wall with the rigid insulation? Do I backfill over it?

The floor is planned to be sanded wood pallets over another three inches of gravel. Is there a way to insulate such a floor or will I need to if the block wall is insulated down to the frost line? I'm in agreement with Birdwidow that there's no such thing as too much insulation, especially with the winters we've been having in the Idaho panhandle. However, I need to put it in the right places and I'm not sure where that would be.

Thank you.