How large is the concrete slab in relation to your greenhouse?
Will the walls of the greenhouse coincide with the slab? or will the slab extend out significantly from the sides of the greenhouse?
The reason I ask is that if the slab will be flush with the greenhouse walls, one option would be to insulate the outside of the concrete slab. Set up the insulation material so that it goes beyond the frost line all around the slab.
The result is that the exposed concrete inside the GH will accumulate heat from the sun during the day and release that heat back during the night.
This would provide some thermal buffer during day and night and lighten the load of the artificial heaters.

Just a thought.

Christian-
I agree...perimeter insulation should be better energy-wise, and may be easier, too. I'm guessing your frost line is probably around 3-1/2' below the surface, and the ground temperature at 6' below the surface is probably around 50 deg F, even in winter. If you insulate the perimeter, the temperature differential between the floor and the greenhouse temperature will be a lot less than the differential between the air and the glazing. I'm not going to claim that I've done an energy analysis, but my gut tells me that you'll be losing more heat through the above surface structure than you will be through the floor if you insulate the perimeter well. The other thing you can do to save energy is to buy one of the bubblewrap materials available that you can adhere to the glass during winter to reduce heat loss.

While I agree that perimeter insulation is better than insulating the floor, I don't know that it would be worth the trouble.

Information from studies cited by Cuestaroble show at most 3% of heat lost through the perimeter. This, presumably, is for the reason Kudzu mentions, that the temperature differential between soil under floor and GH is far exceeded by the temp differential between outside air and GH. If you alredy have the perimeter dug up, by all means, go ahead and put in the insulation. But if not, you would be far better off to spend the time and money on the roof glazing, through which over 50% of the heat is lost.

I have come to believe that in this setting, the concrete will not have a great impact as thermal mass. The reason is that its heat will not be given up at 3am when you need it to keep the temps in the house at 58F and reduce your energy consuption. It will be given up from ~4pm to 9pm as the temps drop from the daytime high to the low of 58F. During that time there will be a temp differential favoring loss of heat from the concrete. By the time the heaters would kick on to keep the house at 58F, the concrete will have lost most of its heat already. I kept my GH at 57F last winter. This is what my water thermal mass experiments suggested last winter, anyway. This winter I'll confirm that by checking temps in the floor.

While perimeter insulation is going to help I would insulate the floor.
You don't want to pay for keeping the first three feet of earth warm. If you keep the GH at near 60 degrees you will be putting a lot of heat into the ground.
I insulated then put 4 inches of sand then laid concrete paver's for a floor but like it has already been pointed out the idea of thermal heat storage is overblown in real world situations it just don't work out that well.
After trying the water storage I decided to remove them they took up to much space and I build my GH to grow plants in not to store water.
I saw no measurable change in my heating bill when removing the water.

Thank you all so much for the ideas.

The existing slab is 10x20 and one side is attached to the house. The greenhouse will be 8x12. From these figures insulting the permiter would be fruitless (I think).

Of course, it will be attached to 2x4's to anchor it.

decisions, decisions, decisions.

Any recommendations on the type of insulation to use?

Much thanks once again,

Christian

Carrilloenglish, your slab foundation will be way too big to make perimeter insulation effective.
You best bet in your scenario would be to just insulate the floor inside the GH, and insulate all surfaces of the GH.

Bubblewrap material seems to be a favorite material for insulating the GH walls and ceiling. It is relatively inexpensive and you can set up multiple layers without sacrificing light transmission into the GH.

Christian:

If you want to grow orchids in sub-zero temps in a GH, you really do need to insulate as much as you can, or spend more on heating the GH than you do your home.

No, it's not practical to use perimeter insulation with what you are planning- presuming you are set on putting the GH directly onto an unheated concrete slab, with only 2 x 4s.

But- if you are willing to give up just a foot of light at the base, you could construct a 16 inch high insulated "kneewall" frame around it and set the GH into it, atop a 6 inch high, raised, insulated floor. It would mean stepping up to get into the GH, but just 6 inches up shouldn't be too steep. Or, if you need handicapped access; a short ramp.

You would need to spend about $600- 800.00 in additional materials, but I would bet that expense would pay back in just a few winters, and at the rate energy costs are rising, maybe less.

Insulating material: 2 inch thick, Type 250 foam. It's the dense, rigid pink stuff sold in any home center. About $25.00 per 4 X 8 ft. panel. Check for sales. It's construction season, so foundation insulating materials are frequently on sale now.

BTW: raising the floor would also allow you to install a floor drain, and finish the floor it with a waterproof and easy to clean surface, such as commercial dry back tile.

But then you would need an outlet for the drain, and for that, a drywell set just beyound the edge of the slab would allow drainage, prevent cold air infiltration and keep drain water from seeping out and becomming ice in winter.

The easiest for a raised floor, would be a channel drain along one edge, tied to a PVC pipe leading directly to the drywell, so you could give the floor an even slope. Just 1/2 inch would do it.

Channel drains; PVC; drywell liners: add about another $500.00. But you would be able to mist with abandon, and never have soaked floors, or ice outside of them.

Wrap a cable heater around the exposed drain pipe and over wrap with flexible foam, aluminum faced insulating and your drains will run free all winter. Laying some of the Type 250 over the cover of the drywell wouldn't hurt either.

Just some thoughts, but do give your foundation great attention, because it will be the one part of your GH your can't improve on afterwards, without taking the whole thing down.

I would do it just about exactly the way Birdwidow has described.

This link might just help you out, if you can see all of the 47 posts/replies.

Here is a link that might be useful: GH installed on deck, insulated and sealed floor w/ drains

It's a pay site.

Christian-
Your first post omitted one of the most important pieces of info...that your slab is much bigger than your greenhouse. This makes the subject of insulating the perimeter meaningless. You're right: doing perimeter insulation in this situation does nothing as all the heat is lost through the top surface of the slab that surrounds your much smaller greenhouse...there is no measurable heat reaching the perimeter to save.

DG has always been a pay site. Some of the forums are open to general viewing, but I couldn't remember whether the GH forum was or not.

Anyway, too bad you can't see the thread without joining...it is a photoessay on this topic. Basically he uses 2" rigid foamboard insulation, covered by plywood. Then he cuts the drain holes and then seals the whole thing by applying a paint-type product from a company called Acrolabs, into which a polyester fabric is embedded. It roughly amounts to a roof on the floor.

Very interesting and informative:

So if I understand most of the comments here, I should build a "deck" to place the greenhouse on -- a deck of plywood. Underneath the deck should be a few inches of foam insulation. Is this the general concept.

Much thanks yet again,

Christian

Christian:

Yes, you have the general concept, but you would still need drainage. Do NOT neglect drainage. You really can't use gravel over concrete. It won't drain off and will just sit there, rotting out your base.

If you use 4 x 6 timbers set upright for the perimeter base, you could then use 2 x 4's for your floor joists and with 2 x 4 hangars, could easily get a slight slope to one outside wall, where you would install a channel drain, placing the joist hangars just 1/2 inch lower on that wall.

You would need to cut a hole at the bottom of the 4 X 6 to accomodate the drainpipe, but just a 1-1/2" pipe would be enough and would not compromise the integrity of the timber.

The channel drain would run along the drain outlet wall. Just make sure you run the joists parallel to the channel drain.

You could lay the 2 inch thick Type 250 under the joists, for a solid insulation base, then, if you set the joists on no more than 16 inch centers, pack unfaced fiberglass between the joists and with ply or OSB, end up with a very sturdy, well insulated floor that with a few coats of oil primer to seal, would accept a waterproof floor covering.

Hint: countersink the nails or screws you use to hold the wood floor down, then fill and sand, for a smooth surface.

Also: A vapor barrier under/over wood is good too.

Ohh man,

I appreciate your advice and suggestions but I have to admit, it sounds like I am out of my league.

Maybe this is a good moment to admit I need to hire someone to build this base.

I wondered if setting up the greenhous eon the slab without a drain would be a major problem. I honestly knew it would be, but it is different when someone confirms this.

Anyone know a good handiman in central New Jersey for hire?

Christian

Christian:

If a framed base is beyond your carpentry skills, then yes, you should hire someone to build it for you. Without that base and a means of draining water out and off of your concrete patio, you would eventually lose not only the base of the GH, but the concrete patio as well.

If you have the space in your yard, could you site the GH off the concrete? If so, you would still need drainage, but could use the ground to accomdate it.

You could also then use the ground to help insulate, by doing some digging and sinking those foam panels down and out from a simple treated base.

Trenching takes no skill; just a strong back.

I don't know much about ground temps and frost lines in NJ. It has been almost 40 years since I spent 2 winters in eastern PA. The concrete floor should pick up heat from the ground and be helpful with maintaining temps. On nights when the outside temps are going down to between 32-35 degrees, I direct a fan across the brick floor in my greenhouse and it keeps the temps up by several degrees. I doubt the ground under the slab freezes. Why not build a concrete block footing on the slab by mortaring a single row of 8"x16" blocks to the slab at the perimeter. Leave space for doors. Anchor a 2"x8" timber to the blocks using mortar anchor bolts. When you tighten the boards to the top of the block, you can put a plastic foam insulator on top of the blocks. It comes in rolls for about $20. When the bolts are tightened down, this seals the board to the blocks to stop leaks. Your sills are now 8" off the slab, will not rot due to sitting in water and the frame sills are air tight. At the door openings, you can put down 2" cap blocks at the door sills and install 2"x8" on top of that. Hopefully the mortar seal under the blocks will stop most water intrusion.