building a greenhouse

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I am going to build a greenhouse in the Spring. I have gathered many old intact windows (hand-me-downs from a neighbor who installed those white plastic tilt-in things) and a couple of doors and a door frame. I have a couple of tons of used lumber and some wonderful beams from a mortise and tenon barn.

I would appreciate some practical advise regarding the structure. I was thinking of a Southern exposure and a slanted add-on roof coming off of an old grainery building. I have concerns about snow weight and placement of the studs, since the windows will be various dimensions. I think I will use palettes for the floor, and maybe also to frame in some lengthwise planting boxes, filled knee height with dirt.

Someone else has thrown one of these together, right???

-- Rachel Kerwood (rldk@hotmail.com), February 06, 2000

Answers

If you are going to use a greenhouse, you must have good ventilation, because on a sunny day a greenhouse can heat up very hot. It does not have to be very warm outside for this to happen. At night it cools down fast, so you have to be there to open and shut doors and windows according to the weather (cloudy or sunny) and at night.

You will need a good slope to the roof or you will have to shovel and scrape a lot of snow off. (This job is a cold and hard one)

You will need a heat source for at night, and a fan for the daytime.

Are you wanting to just cold frame, or run a greenhouse, this all comes into play in how to build your structure.

Where are you located? Very cold or very hot weather?

I have helped my brother greenhouse in the past for around 10 years, if I can help let me know.

Beth

-- Beth (craig@icu2.net), February 06, 2000.


Hi, Rachel,

First of all, I strongly recommend AGAINST using old windows for the glazing on the roof. I hope that's not what you have in mind, unless they are tempered glass, or laminated. Regular glass would be very deadly, should it get broken for whatever reason, hail, windthrow, snow, etc.

They are fine to use on the walls, however. Code here is they have to be 18" off the floor, I guess to keep kids from falling through them.

You have a good idea, building it against the south side of a building. If you could do it against your house, it would be even better, as the excess heat referred to by Beth could be used to heat your house, rather than venting it. Another trick is to put LOTS of thermal mass inside the greenhouse, if you expect wide fluctuations in temperature. (This will depend on your climate) An easy way to add thermal mass is to put your greenhouse shelves on top of 55 gallon drums filled with water, and maybe a rust inhibitor. Another way is to fill five gallon buckets with water (be sure to have lids on them to avoid adding humidity to the space) Even smaller containers, placed wherever there is dead space, will help. If the water containers can be heated directly by the sun, so much the better.

I personally like having the floor level of the greenhouse below grade, with the shelves at grade level. That way, there is a lot of earth sheltering of the space, which keeps the temperature more uniform, as well. Make sure there is drainage from the floor space, though, as you don't want to end up with the thing filling up two or three feet deep with water!

A very cheap source of roof glazing is USED sliding glass door panels. Here in Oregon they can be pruchased for about ten to fifteen bucks apiece, in sizes of 34"x76" and 46"x76", and probably other sizes. All the ones made in recent history (maybe twenty five years?) are made of tempered glass. I had one inexplicably break once, and it made a mess, filling up a bunch of pots with pieces of glass, but the biggest pieces were the size of green peas, so there was no danger of getting decapitated.

Good luck!

-- jumpoff joe (jumpoff@echoweb.net), February 07, 2000.


These are great thoughts. I appreciate the consideration. I live in Minnesota, and the greenhouse will be a starter place in the Spring and a season extender on both sides of Summer. I am hoping to get two more crops of radishes, spinach and lettuce at least. Realistically, my growing season is June to August.

I like the idea of building the floor below grade. Since I have no tractor, this means a lot of manual labor! I will have to enlist my 9 year old to help. I was going to use regular windows on the roof, so thanks for the heads-up (no pun intended) about tempered glass.

Sliding glass door panels would certainly expedite the roofing process! What, five or six panels and I'm done! Where do you find these in Oregon?

No problem with venting the structure. I was going to put several windows on hinges to tilt out, and I can run electricity from a nearby pole for fan and lights.

I will try to remember to let you know how this goes! Getting anxious for Spring already, and it's only February.

-- Rachel (rldk@hotmail.com), February 07, 2000.


Don't forget to check out what your greenhouse is going up against. If it is your house and that house is wood be sure to protect your house against moisture. Greenhouses put off a lot of moisture especially on a sunny day after watering. You don't want to rot your house. If you are using wood for any structures be sure to protect the wood as much as you can by treatment or outdoor paint of somekind.

Happy greenhousing....:>)

Beth

-- Beth (craig@icu2.net), February 07, 2000.


Rachel, I for one would love to hear from you on your progress.

A couple of other thoughts. I don't need a fan at all in mine. I have fairly large opening windows all along the south wall, and a small window on the west wall. But I still had an overheating problem the first spring. The temp got up higher than my thermometer could read (130+ degrees). To solve this problem, I glued (silicon caulking, actually, after cleaning the glass with rubbing alcohol) 1"x6" strips of velcro every couple of feet along the edges of the tempered glass on the roof. My wife sewed the matching pieces of velcro onto the edge of 84& shade cloth. All I have to do is roll up the shade cloth, stick the velcro together along the bottom, then give it a push with a broomhandle to roll it out onto the glass, where it is stuck down. Easy to get off, too. I only put it up once in the spring, then take it down once, in the fall. Too much trouble to do it every time the weather changes.

After putting the velcro on, the temp inside the greenhouse stays more or less the same as outside air temp.

Moisture has never been a problem in mine, but I have not had it totally stocked with plants. Only a few so far. But if it's too moist in the winter, and it's sunny, I open the windows and door into the house anyway, to let the heat in, and the moisture inside is welcome in the winter, as the relative humidity always is a bit low, when the wood stove is fired up, or the heat pump running.

-- jumpoff joe (jumpoff@echoweb.net), February 07, 2000.



One other thing, Rachel. Since my greenhouse floor is below grade, the south wall is only about five feet high, which makes it easier to put the velcro on and off.

I also forgot to answer your question about my source here in Oregon. I get the used sliding door glass from my favorite window and autoglass store (happens to be Advanced Glass), but I know other glass stores have them too. I think their source is probably when someone breaks one piece of glass in a thermopane door, the piece that is left gets sold as second hand, when the whole panel is replaced.

I used to buy "factory seconds" for about the same price, but Advanced tells me that they are no longer available. But you could check.

I actually like the idea of second hand better than factory seconds. They've been field tested. One time, my son and my father in law were carrying one of the factory seconds, and it went "poof", and disintegrated. They were standing there holding air, basically, feeling kind of silly. There were tiny pea sized pieces of glass all over the place. I guess there was a bad enough flaw in the glass that it was just waiting for an excuse to break. They didn't even bump it up against anything.

-- jumpoff joe (jumpoff@echoweb.net), February 08, 2000.


Rachel, We bought "thermopane" patio door sized glass for our greenhouse at a window outlet place (Arctic Glass) in western Wisconsin. Depending on where you are in Minnesota, it could be close enough to drive to. We got ours really reasonably priced, bought twice what we needed so we have spares to replace broken ones for many years to come.

-- Jim (jiminwis@yahoo.com), February 08, 2000.

Hey, Jim, what's 'reasonably priced'? I am probably two hours from the Wisconsin border.

You people had me drafting plans yesterday. How do I drain the floor so I don't end up with a wading pool in heavy rain? I was casing out that grainery. I'm going to have to work out a trade with one of my farming neighbors to get that area dug out for me.

Do I need electricity in there if it is 2' below grade, has planting boxes (full of straw and soil) running down both sides and I have barrels full of water to retain temp? Sure would be nice not to dig that trench!

How about using palettes as the retaining wall (2' below + 18" above)? I can get those free. Are they impossible to deal with, or can I put 2x4's through the spaces and nail them into a wall?

-- Rachel (rldk@hotmail.com), February 08, 2000.


If you are using your greenhouse in the summer, and have shade and sun loving plants you can use shade cloth to shade half of the house for the shade loving plants.

Also there is a paint that you can paint on the greenhouse (we always used it on plastic) and it may not look great on glass, but when sunny it is white, and when it rains it turns clear so light can come through. Thus cutting down on the heat.

If you can not afford shade cloth, you can use a white sheet, or any color sheet for that matter.

The shade cloth is put up next to the glass.

Sometimes you may want to build a small bench with florescent lights about 20 inches above this bench. You can get heat cables and run on top of this bench with a small 1 inch ledge build on to it. Lay down the heat cable, (do not cross heat cables over each other) in a close pattern put purlite over heat cable just to top of 1 inch ledge. On this you can start flats of seeds. To start seeds in flats use soil from store for starting seeds, Not just potting soil. ( if used flats wash with clorox first to kill bactaria.) pre moisten soil before planting seeds (mix in a clean bucket with water let sit, check to see if moist all the way through, but not soppy wet)Place soil in flats and use a flat piece of wood to level off flat to 1/4 inch from top of flat. Place seeds in flat. Cover with plastic or glass. check each day to see if the seeds have sprouted, as soon as seeds sprout remove plastic or glass. Let remain under lights for a week or more until they are transplanting size. You may transplant to a peat cup, or other small container. Just remember not to transplant to too large a container or the soil will spoil because there are no roots to keep it fresh.

When transplanting from a peat cup to the ground, you can put this directly into the ground, but you MUST peal the top of the peat cup down to the soil level in the cup, if not and this sticks up the peat cup will act as a wick and pull all the moisture away from your young plant.

If you do not plan to have a propagation light, at least use flats with pieces of glass or plastic over them to give that little bit of extra moisture and heat till seeds have started.

Lots of luck

Beth

-- Beth (craig@icu2.net), February 09, 2000.


Regarding thermopane windows on roof; I opted not to here, in my mild climate, because I've had trouble with doing so in the past. I put some up for a skylight in a studio. After a year or two (?) the top piece of glass had slid down some six inches, thus breaking the seal between the two pieces of glass, and causing lots of condensation between the sheets of glass until I replaced the whole thing. Since this was a heated space, I replaced the skylight with another thermopane. But I made some brackets, attached to the curb of the skylight, which held the pieces in place so the slipping could not occur again.

Be aware that thermopane glass does not transmit quite as much light as single pane. According to Edward Mazria's "The Passive Solar Energy Book", a "On a clear winter day, a vertical single plate glass window transits about 85% of the solar energy striking its surface, double glass about 75%.

The closer to perpendicular the light source is to the glass, the less loss you get from reflection. So, in your climate, if you can find thermopane glass you can afford, I'd recommend that you get it.Although you will get slightly more solar gain during a clear day with single pane glass, you will approximately twice as much heat during the night.

Mazria recommends, and I agree, that the best solution to glazing a greenhouse is to use SINGLE pane glass, and insulated shutters. Unfortunately, this can add a lot of expense, and the necessity to close the shutters at night.

I don't recommend pallets for this application. They will rot out too soon, and they need to be strong enough to support the load of the wall and the roof. Concrete, six inches thick with at least 3/8 inch horizantal rebar three inches above and below the bottom and top of the wall, respectively,and every four feet vertically, is best, but you can also use 8x8x16" concrete blocks if you place steel in the same pattern as I mentioned for concrete, and fill all the cells with concrete. Use "bond beam blocks for the bottom and top courses, to facilitate placement of the steel. Code actually calls for 1/2" or even larger, rebar, but for a small greenhouse, you're probably ok using 3/8, assuming the soil around the greenhouse never gets saturated with water, causing hydrostatic pressure.

Concrete is cheaper than the concrete blocks, but the labor is more expensive, usually, if you have to hire it out.

As far as drainage for your floor, you are wise to be concerned. If you have extremely well drained soil, and put rain gutters on the roof with the water draining away from the greenhouse, you may not have to worry about it. But it would be a drag to have to put on hip waders every time you wanted to play in the greenhouse, not to mention the humidity that would exist if the floor were under water all the time!

The best way to ascertain that there is no water problem is to put down about three inches of pea gravel, with some perforated pipe which will drain any water which finds its way there out and away from the greenhouse. This means maintaining a positive slope of about 1/4" per foot away from the building until the pipe "daylights". (1/8" per foot is the bare minimum slope)

Obviously, you need a place for it to daylight to. Around here, it's not a problem, because everything is built on a hillside. If your ground is very flat, you would probably do better to build the greenhouse on top of the ground, then add backfill against the building to make it similar to being below grade. Backfill can consist of sand, topsoil, manure, sawdust, etc. The main thing is to protect the bottom few feet of the wall from the cold air. You might be able to utilize any trench you dig to put in water lines or electric lines for the drainage pipe . 1/4"/foot is the equivalent of a 1% slope, so the pipe will be dropping one foot per hundred feet. So if you are on a hill, you won't have to go very far with the drainpipe.

As far as freezing, that's very difficult to determine. There are so many variables, especially temperature, sunshine and insulation. I can't even hazard a guess without a LOT of information. Why not just put in the wire, if you have to dig a trench anyway, then you can hook up power if you need to, to prevent freezing. Besides, it would be nice to have a fan and a growlight or two, right?

If your neighbor digs the excavation, can he also dig the trench? If not, check out renting a trencher. They dig very rapidly, without producing huge amount of backfill.

-- jumpoff joe (jumpoff@echoweb.net), February 09, 2000.



Boy, I am getting such thoughtful responses I can't believe it!

I've never built a permanent structure before (we'll see how long this thing hold up!). But I have been gardening for years and years. My outside garden last year was about 1/2 acre. So, I'm pretty sure I will be able to get enough use out of the greenhouse once it's done.

I probably didn't mention that I have no capital to put into this greenhouse. Okay, I can scrape up a little here and there, but there will be no contracted workers. Not to mention, I live in the sticks.

Now, I can get 8x16 blocks and rebar cheaply enough. I have seen farmers selling blocks used and cheap. Can't wait to haul those! The spot is completely level, and the ground pretty hard there. As a matter of fact, I can see for miles in every direction from my property, and not a hill in sight. So backfilling would be easier than digging out and draining. (Like the classic German inspired Wisconsin dairy barn)

Now, can anyone tell me (I live in Minnesota) if I have to anchor this structure below frostline (four feet under) if I use concrete blocks for the foundation?

Okay, okay, I will run electical. It needs to be functional when it's done. I will call around about the used thermopane glass.

I have no expectations that this little greenhouse will be growing lettuce in January. I don't want to pay to heat the thing anyway. I'm just tired of starting seed flats all over my kitchen counters (ALL over) and running out of growing time in the fall.

Thanks to all of you who have taken the time to contribute your info.

-- Rachel (rldk@hotmail.com), February 10, 2000.


Good news that you can find rebar and used blocks, Rachel. As far as frost heave, I am totally out of my element. We had a freeze, I'm told, back in 1972, three years before I moved to these parts, which froze the ground to about 12" deep. The deepest I have personally seen it freeze here was only three or four inches. We are required, when building buildings to Code, to have the bottom of the footing (called "footer" in some parts) a minimum of 12" deep. But I have built lots of "non code" structures over the years, often only backfilling a couple of inches. Never had a problem. But again, no four foot deep freezes, either.

Why not ask a reputable builder, engineer, or, gulp, County Mounty (building inspector type) what would happen if your footing weren't buried verry deeply? It might be that it would cause the building to shift enough to make doors stick? Or windows break? Or nothing but cosmetics? Let us know if you find out, ok?

By the way, I recommend a footing for what you are doing of 6" thick (vertically) by a foot wide, with at least one piece of 1/2 " rebar. This will help if you do get minor movement, to prevent the block wall from breaking apart, especially if you use the other steel I mentioned earlier.

-- jumpoff joe (jumpoff@echoweb.net), February 10, 2000.


Rachel: As far as patio doors are concerned you might be able to find some free ones by talking to contractors who do remodeling jobs. Alot of people just junk them and theres usually nothing wrong with the glass which would have to be removed from the frame for the roof. For the sidewalls I suspect they'd work fine still in the frames. Also you might try putting an ad in the local buyers guide or paper specifying FREE used patio doors. I have a little hauling business and people have paid me to haul them away.

-- john leake (natlivent@pcpros.net), February 13, 2000.

I have built a greenhouse-tool shed-entryway-solar collector using recycled patio door glass. Glaziers who remove door panels which have suffered an air leak are glad to get rid of the glass. I simply take a utility knife and cut the insulated panes apart, clean them up, and set them in a cedar frame. I have an animated gif showing the construction of this simple but very usefull structure. I use moveable board insulation combined with good ventilation to regulate the temperature. URL for animated gif: http://hometown.aol.com/dometruss

-- Robert Conroy (Robert_Conroy@compuserve.com), April 23, 2000.

The unit I built is on the west side of my garage. Starting with a foundation of pressure-treated 4x6 and 6x6 (found free)large pannels of single glass (again free), untreated 2x4 painted on ALL sides (before nailing) , and regular greenhouse plastic for the roof (again recycled), the 2d layer of glaze is vinyl stapled to the 2x4s, Heat sink is 55 gal and 30 gal. water barrels (rc). The garage wall and the north green house walls are insulated and painted white. a 12 foot stainless seel counter (OH yea $1200 worth free !) and a grow bed, last the south wall a 10 foot double argon filled slider (my favorite word-free). What would I do differently..It faces west and over heats, south would be better. And a real foundation, However without a foundation I can call it a walk-in cold-frame, greenhouses are subjected to property taxes and cold-frames are--FREE. I tryed the pvc pipe ..don't its cheap but doesn't stay up...I lost 1000 three inch potted plants...that wasn't cheap or free.

-- Greg Sanderson (stonycft@worldpath.net), April 26, 2000.


Thanks, Robert and Greg. I'm getting ready to get started (as soon as I finish the grainery roof) and it's encouraging to hear from people who have already expended similar effort.

-- R (rldk@hotmail.com), April 26, 2000.

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