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RE: please critique my greenhouse foundation (Follow-Up #9)

posted by: zengeos on 12.19.2007 at 02:58 pm in Greenhouses & Garden Structures Forum

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.


clipped on: 03.21.2009 at 05:32 pm    last updated on: 03.21.2009 at 05:33 pm

RE: Greenhouse wall and foundation insulation (Follow-Up #6)

posted by: birdwidow on 12.25.2008 at 11:02 am in Greenhouses & Garden Structures Forum

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!


clipped on: 03.10.2009 at 06:01 pm    last updated on: 03.10.2009 at 06:01 pm

RE: Greenhouse base - novice needs help (Follow-Up #6)

posted by: lovjen on 03.20.2007 at 08:56 am in Greenhouses & Garden Structures Forum

Something I found out after I installed my greenhouse was the real need for insulation. I am in Zone 5, but even in your Zone 6, you will need to insulate the perimeter of the greenhouse ground so the earth on the inside of your greenhouse becomes a very effective "heat sink." I am sure everyone is familiar with those blue panels of foam insulation that are installed on homes prior to siding. The trench you are digging should go down below the frost line, and the insulating panels sunk into the ground (vertically) and attached to your treated foundation timbers. If you do not do this, the ground itself inside your greenhouse will leach away your hard earned solar heat. As mentioned earlier, you need to position your greenhouse so a long side has full southern exposure. This will very effectively "charge" your heat sinks. Heat sink examples are 1) your insulated ground, 2) the rock and or pavers you install for your greenhouse floor, and 3) large containers of water. 50 gallon drums, black if possible, are the best option. However, I just picked up the plastic tubs used for storage at Halloween, and filled them. It works wonderfully. It is also used as my watering water since I did not run water out to my greenhouse. (It is very convenient in the wintertime.) These barrels or tubs should be placed on the north side of your greenhouse. The slanted rays of wintertime sun will warm them up, and the water will slowly give up heat throughout the night to warm your greenhouse.


clipped on: 03.10.2009 at 02:10 pm    last updated on: 03.10.2009 at 02:10 pm

RE: Greenhouse base - novice needs help (Follow-Up #4)

posted by: sheila0 on 03.18.2007 at 10:25 am in Greenhouses & Garden Structures Forum

I thought I would add, that those who use cinder blocks usually place a "sill plate" (a piece of weatherproof and termite proof wood) on top of the cinder blocks, that is held down by bolts, that are sunk in the cinder blocks with cement. That is the foundation at that point, however, if you put the foundation in without any access to the other utilities in place, it will make a mess for you later on.
As far as where to put it, if you plan on using it in the winter, you would want it to have the most southern exposure possible, and if that is away from the house, so be it. Read, Read, Read, that is the FIRST thing to do.
Since you are doing that now, your going in the right direction for sure.


clipped on: 03.10.2009 at 02:04 pm    last updated on: 03.10.2009 at 02:04 pm

RE: Greenhouse base - novice needs help (Follow-Up #1)

posted by: birdwidow on 03.14.2007 at 11:36 pm in Greenhouses & Garden Structures Forum

A trench filled with gravel under the base timbers for drainage is always a good idea, because it's the moisture in the ground that rots the wood. Give water a place to drain off, and the timbers will last many times longer.

You could put in concrete footers and use bolts to anchor down the timbers. It all depends on how inclined your soil is to frost heave. One nice thing about footers is that they will not only secure the base, but allow you to get it absolutely dead level, regardless of the ground slope.

But you really should start with drainage, because regardless of how you construct your base, once your GH is up, the last thing you want is the need to address drainage under it.

But before anything else, a question: are you planning to heat your GH for winter use?


clipped on: 03.10.2009 at 02:03 pm    last updated on: 03.10.2009 at 02:03 pm