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?

birdwidow. i haven't even gotten that far yet (heat).

i wanted to put it right behind the house. the man tha tis going to build it suggested otherwise. so now i have two choices. one, directly behind the garage (not secured to it), or the other option, directly across from the garage in an open space. my biggest problem with behind the garage is the mess that the neighbors call hedges, all their junk grows over the property line and no matter how much chopping i do on my side, it doesn't come close to being under control. they refuse to chop it out on their side. not sure why they want the dead hedges, weeds, honeysuckle and poison ivy and the green ivy, but it must be attractive to them on their end.

behind the house behind the garage both give me water and electric opens without an unnecessary inconvenience. both of these places would still afford me the opportunity to heat, if i choose to do so. if i put it in the open space, i have other issues to deal with.

i was just hoping to give the guys a start. i guess i'll really have to decide the best place for it with the least amount of headache.

m

maryanne;

I asked about your winter GH use plans because if you do plan to heat it, how you prepare your foundation will be completely different than if you use it only in the warmer months.

I had some pretty foolish notions about GH bases when I first came here to the GH forum, but was quickly disasbused of them after some of the real experts here explained the facts of life, and science to me.

Once I understood, I changed my foundation plans and while it added to the cost and delayed completion of my GH, this past winter proved them right.

It's a decision only you can make but you should do so before you even start, because, as I wrote before; it's not one you want to have to address AFTER the GH is up.

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.

thanks for the help. i guess i still have some thinking to do. i know once the person that is doing the real assembly shows up, he'll know where it should go and all the other stuff. i will have printed out all the comments and suggestions that everyone prior has posted and i'm sure if there are still questions, i'll get them answered real quick.

thanks again for your help
maryanne

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.

I will be forever grateful for the lectures on thermal mass and energy I read here, and instructions from Nathan for using dense, rigid foam as insulating. It was that new knowledge that delayed the completion of my GH foundation last year, but oh- what a difference it made!

I can't imagine trying to do it AFTER the GH was erected.

I hope it's ok to jump in with another question on that same track. We're going to be building the foundation of my 6 x 8 HFGH this weekend.

We plan to build a platform, like a mini deck and then mount that on concrete anchors. We plan to dig a perimeter trench around the base for insulation. My question is regarding insulation UNDER the wood floor. Should we limit the insulation to the perimeter or place a sheet directly under the floor as well? I think I remembered reading that you shouldn't, but I can't think of why. I guess it might inhibit drainage and actually block out heat from the earth??

I'm in Zone 5, this will be an unheated greenhouse mainly for extending the growing season. Thanks!

i'm going to have to track down nathan's posts and lovjen, you are giving me so much more to think about and appreciate reading before this project starts!

maryanne

Maryanne and Turquoise:

Again: everything depends on intended use. In any climate subjected to sub-freezing temps, let alone sub-zero, if a GH is to be heated and used year round, ground insulation is vital, presuming you don't want to spend more to heat the GH than you do your home.

But for only seasonal use, and it's not fall season extention that's the challenge, as by the time the leaves are falling after a hot midwestern summer, the ground inside a GH is nice and toasty and will stay warm for quite a while, even after after the first frost. It's bringing it back up after months of hard freeze, when you would presumably want to jump start seeding plants.

If I were planning only seasonal use and had no drains, I would still use foam insulating panels, but limit them to the perimiter and rent a Ditchwitch, to cut a deep, narrow trench all around the GH base and set them in upright.

Also, I would install a drywell into the center of the GH, as a simple yet effective way to get rid of excess water, especially when the surface of the ground outside of the GH is still either frozen, or already saturated from spring thaws.

For that, either rent a post hole digger with the largest auger, or get onto the working end of an idiot stick and start digging. To line a drywell, just cut out the bottom of a very large, tall plastic garbage can, set it into the hole, fill it with rocks- 4-6" road base stone is good, then lay a piece of heavy vinyl or aluminum screening over it, to prevent small stones and dirt from draining into it and eventually cloging the spaces beteween the rocks. That also allows the use of small stone for the GH floor, such as pea gravel. The screening will prevent the pea gravel from falling into the drywell.

It also means a bit of grading inside of the GH foundation, but not much; just enough to aim excess water to the center. Then, lay heavy weed blocker fabric over floor, level with the top of the drywell, check the slope with a garden hose, and when it's all where you want it, fill in the entire space with small gravel. Road mix is easy to rake, and will pack down to a nice firm surface, but I prefer pea gravel myself. It's costlier, but also drains a lot better and is comfortable to walk on.

Either way; no weeds or puddles. You can then use patio blocks or brick for your walking paths if you want, or just lay more fabric down, to prevent disturbing the small gravel.

But for any drywell to be effective in cold country, it must be below frost line at the bottom. The deeper the better. If it's necessary to extend the length of the garbage can, just use two. Cut the bottoms out of both, then set the top one into into the bottom can to the desired height and run in a few bolts to hold them together. Once it's all in the ground and the outside has been packed in well and the center filled with rocks, it won't move or separate.

Also: I admit that I keep harping on it, and maybe it's just me, but I will NOT have mice in my GH... If you lay 2 ft wide, 1/2" mesh hardware cloth just under your timber base and out, then secure it down, no mouse will ever burrow under the base and into your GH.

Thank you very much birdwidow, lots of good information!

I know that Birdwidow holds firm her believe in the importance of insulating the foundation. I do too, really, as I did the same thing myself when I built my GH.

In the interest of science and discussion, however, I will toss out Cuestarobles figures, studies which show virtually no heat lost through the floor of a GH and only 2-3% of the heat lost through the perimeter.

If these figures are true, one must consider the payback on insulation of the perimeter as we have been doing. With 50% or more of the heat lost through the roof, maybe we are better off upgrading the glazing there...or maybe our time would be better spent installing permanent insulation on the north wall.

SB

Here is a link that might be useful: Thread with heat loss figures and reference.

SB: I remember reading that thread several months ago, and the links, but went back to read them again and wonder of we are comparing apples to oranges regarding the value of perimeter and/or floor insulation for a small, garden GH, as opposed to a huge commercial sized one that in total cube, would swallow 100 of my 12 x 16 X 10.

Wouldn't the total cu. ft of air in any enclosed space have some effect on it's ability to hold heat? And one of the projects was a GH with heating coils in the floor. The value of perimiter insulating didn't seem to be questioned.

However, as one poster on the other thread commented: they can work in their heated and floor insulated GH in winter and their feet stay warm. I prefer the comfort of my Keds, and would get no pleasure from the need to wear mucklucks, just to putter in my GH in winter without getting my feet chilled to the bone.

So aren't we really back to the most important question of all for a HOBBY GH? What do you want to be to do in your GH, and WHEN? Spring and fall only? Spring through fall? All year?

Do you want to be able to putter in your GH when you want, in comfort, and thereby derive pleasure from it, or have it become such an uncomfortable experience that your GH ends up yet another PU only listing on eBay?

BW, I also worry about whether these figures apply to hobby greenhouses, I don't know. The perimeter temp might affect a disproportionately large fraction of a smaller GH. Also, Cuestaroble lives in CA...Do these figures apply to a zone 5 Midwest GH? I don't know, good questions.

"So aren't we really back to the most important question of all for a HOBBY GH? What do you want to be to do in your GH, and WHEN? Spring and fall only? Spring through fall? All year?"

Yep. This gets back to the original poster's question. Step one is always, "Decide what you want to do with the GH."

I have read many opinions on line, researched in books and on line and actually put some of the suggestions in practice over the past two years. I run my greenhouse year round in Northwest Indiana. The heating cost significantly reduced with the simple practice of using packing bubble wrap on the North wall, door and vent shutters. I started adding large plastic tubs of water on the North wall with the addition of about 20 milk jugs scattered around. My greenhouse is 6mm twinwall acrylic and is 7'9" by 15'. I have read the thread about heat sinks and understand how thermal mass is not enough to run your greenhouse without supplemental heat. However, thermal mass and insulation WILL reduce the amount of supplemental heat required. I could not afford it earlier, but Farmtek sells reflective greenhouse insulation that will also help. I plan on purchasing this and using it for the next winter season. I am moving my greenhouse in April to my new home, and I am hopeful that with the added ground insulation, thermal mass, and reflective North wall insulation, I will get my supplemental heat down from $100 per month in January and February. With just the thermal mass and bubble wrap, I dropped it nearly $50 per month for winter heating.

I should add that during the coldest months, I keep the heat around 50 just to keep the plants from freezing. Then when I start seeds in early March, I move it up to 60.

Another good suggestion I got from the Greenhouse Book: When you are starting seeds, you can place the pallet on top of the lid on the tub of water for the added heat needed for germination. This really works and is "killing two birds with one stone" since you will not need a heating mat or cable to start the seeds, and you have thermal mass for your greenhouse. The whole "heat sink" thread goes on about practicality of the use of all of these tons of water needed, so use the thermal mass as additional bench space to start your seeds.

Hopefully, "food for thought" (pun intended).

Lovjen: You make a good point. Anything we can do to retain heat to reduce heating costs is good, as long as it dosen't blow out so much usable space the GH no longer functions for its desired purpose. Gallon jugs set in otherwise unused spaces may not do much individidally, but collectively they probably did. Every extra bit surely helps.

If you are in NW Indiana, we are likely not very far away from each other. I'm in Illinois just west of Dyer, so I'm guessing we experience pretty much the same weather and may have some advantage. All that lake effect snow seems to add insulation to GH's, at least it did mine. It piled up around it like rolls of fiberglass batts and one day, with the outdoor temp in the 20's and the GH thermostat set at 50, the sun came out and we had to prop the door open, just to vent the heat that rose to 90 deg. inside.

But the snow wasn't on the roof; it was piled around the BASE. So there is no question in my mind that all that snow was serving as additional and very effective lower wall/foundation insulation.

Logic would seem to dictate that the better we can warm, and hold warmth at the floor, the warmer it will be at the ceiling, because the one thing we can all agree on, is that warm air rises.

I'm clipping from this thread for my own greenhouse this summer and wonder, Mary Anne, how did your greenhouse turn out? You asked all the same questions I've had about the greenhouse I want to build so your thread has been very valuable to me. Thank you so much.

Hi mizwilly. I see that you havnt had a response from Mary anne! How did your project work out? My 12'8x 16'9 5 wall poly greenhouse has just arrived. Right now i am working on insulating my 4 1/2 ft trench underground with xps board. I made my trench a foot wider than my gh dimmensions which was a mistake since now it will be tricky to install the xps against air! lol oh well it will all work out! Hope all is going well for you!