Generally, treated timbers work fine for a base foundation, and, if you're connecting the frame to the timbers and the timbers to the concrete piers, you should be plenty secure. However, I'm a little unclear about your plan for sinking it in the ground. Are you talking about bringing the dirt up against the sides (which wouldn't be a good idea)? And if you're going to have a space between the dirt bank and the side of the greenhouse, I'm not certain that all that excavation will add much value. There are much simple ways to insulate the greenhouse. Also, I'd be very concerned about drainage problems with a greenhouse structure sunk into the ground.

I was thinking of digging down about 2-3 feet, and putting the bottom edge of the greenhouse structure on top of the surrounding ground -- none of the structure will actually be buried or touching the soil. I want to use the greenhouse in winter, and from my studies I have learned that going down just a few feet can increase the heat inside a great deal.

The area where the greenhouse will be is raised above the surrounding ground, so the water table is about two feet below where the lowest inside area will be. It's also on the edge of a cliff and the entire property slopes gently toward the cliff, so I don't think we'll have any problem with standing water. The soil around the greenhouse has a *lot* of sand in it too.

It will look like 1/5th of a block house, with a greenhouse sitting on top of it.

:)d

You can probably get much the same effect by putting a line of 8inch thick polystyrene foam sealed together in a trench around the base. Maybe to a depth of 4'. And you won't have so much to dig, trouble supporting the banks, or drainage problems.

"You can probably get much the same effect by putting a line of 8inch thick polystyrene foam sealed together in a trench around the base. Maybe to a depth of 4'"

Really, you'd go eight inches?

I've been reading about this stuff:

Insul-Shield (http://www.specjm.com/)
Formaldehyde-Free series of semi-rigid or rigid thermal and acoustical fiber glass insulating boards. They are made from inorganic glass fibers bonded with a thermosetting binder. Product is available coated black facing.
Footings and foundation should be insulated with 1- to 2-inch sheets of rigid insulation or with a 4-inch-wide trench filled with pumice stone that extends to the bottom of the footings. Use Bild-r Tape construction tape to connect the foam pieces. This insulation is placed vertically and extend down to a minimum of 2 feet. Place the vertical insulation in a trench around your greenhouse perimeter.

It recommends using the foam, *OR* pumice stone. I want to find out which is cheaper, and will pumice stone really work as well?

So you'd change it from "4 inches thick" to "8 inches thick" regardless of what is chosen?

I can't get my mind around how it would look if I used the rigid foam pieces. I'd dig a trench 4' deep around the outside -- how close to the wall? -- right up against the perimeter blocks and wood?

Then, take the foam pieces and slide them into the trench long-ways until I have 8 inches.

Then what? Bury them?

I'm thinking a four foot deep, 10 inch wide trench is going to be pretty hard to dig, and then hard to keep intact while putting in the foam/pumice.

what are your further thoughts on intense insulation methods?

thank you for your help! :)

Diana

pumice will be worse because it doesn't stop water. Most of the heat is transferred by water movement.

I was suggesting 8 inches for completely different reasons - stiffness and easy of working. 8inches will resist soil movement much better (even more so if you put fibreglass + cement stucco on both sides). It is also easier to lay in a straight line.

I don't know what your climate is like, but 4inchs is probably enough to make your thermal problems move elsewhere. Think of the foam as a barrier to heat. You must make sure that there are no other paths for the heat to go through. basically this means no gap between the foam and shell of the building. Perhaps you pour your foundations directly back to back with the foam.

To dig your trench hire a ditchwitch (trenching machine). You'll do the job in maybe half an hour and you can trench for water and power while you're at it. The depth really depends on your conditions. I think the general principle is to do to the deepest frost line, then add a few inches.

If this is too hard then your other options are insulation under the green house and insulation skirting around the greenhouse.

yes, I am planning on laying insulation under the pebbles inside the greenhouse.

I am not going to have a full pad foundation or footing - it's just going to be pier blocks with 6x6" wood. I'm thinking that if I can find 6x6" fake ABS decking, that would be even better since it would virtually never rot.

Where would the 8" insul. go in that case?

thanks!

underneath?

Nathan,

so you are saying that the 8" of insulation that are to go 'outside' the greenhouse should be directly underneath the pier blocks and wood footing?

just want to make sure that's what you mean.

OR - were you saying the insulation 'inside' the greenhouse goes underneath the 4" of pebbles? I did grasp that. But, I think I should sink it under the soil a few inches and put the pebbles on top of the soil, as I don't think it will withstand 4" of shifting pebbles -- walking on it all the time.

:)