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overview

I'm looking into building a cabinet sized greenhouse for my balcony. I have a very limited income so its a low budget project. I have looked at ready-mades but none fit my budget or space.

I'm a bit unsure if the materials I have selected are ideal:

  • I was considering building the frame out of 45 x 45mm (1.7") pine.
  • For the back I was planing on a sheet of plywood painted black.
  • For the bottom i'm considering a galvanized steel plate.
  • For the glazing I considering using channeled polycarbonate (about 8mm).

Access will be by sliding the front panels.

Are there better choices for this project?

Aluminium could be an option but I don't have any experience with metalworking really.

What is the best way to fasten the polycarbonate sheets to the frame?

I only have basic hand tools to I'm looking to avoid tongue and groove.

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I live in an apartment building so I cannot fasten it to the wall. I can fasten it to railing on one side to prevent tipping. The balcony is south facing and on the second floor of 4. There is a balcony above it so does get wet in a storm but not soaked.

I live in northern Sweden. The Köppen climate zone is Dfc (Cool Continental / Subarctic). We often have snow from november to the beginning of may. Winter temperatures often drop down to -25°c (-14F)

The reason I want a greenhouse is that June, July and August are the only guaranteed frost free months and nighttime temperatures plummet down to 4°c (39F) during may and september.

I intend to bring it inside when winter comes.

What do I want to grow?

I want to be able to grow tomatoes, peppers and maybe physalis and kiwi berries and maybe cool down my indoor citrus plants in the fall.

My tomatoes, peppers grow ok at there now, but I have to haul them inside every night.

Site today

Size

The overall size is 320 * 1100 * 2000mm (12"*43"*78").

Wireframe Wireframe Wireframe

  • aluminum will not insulate the 'greenhouse' well. It will dispense heat rapidly. Think of picking up aluminum in our oven when it's 350F. Using the materials you have mentioned is much better if you want to retain heat. – Citizen May 28 '16 at 20:12
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    Aluminum cuts very easy with a hacksaw. And thanks for the additional info. It makes sense now. – OrganicLawnDIY May 28 '16 at 21:01
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    More then likely you will not be able to raise the temperature sufficiently to grow those tropical plants with the materials you've mentioned without an additional source of heating as well. – Graham Chiu May 28 '16 at 21:27
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    See if you can find polycarbonate twinwall panels or equivalent in your area for the clear panels. They're insulated so should help retain heat. You'll probably still need some sort of small heat source. Then look for framing members that work with the twinwall panels. In Europe Keter greenhouse seem to be popular. Made of a bubble wrap for greenhouses. Don't forget ventilation. – OrganicLawnDIY May 29 '16 at 0:02
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    It may be sufficient to use water to keep it warm enough at night so the plants aren't damaged. Either but some hot water in a couple (or more) old milk bottles and place them in the greenhouse at night. Or you can paint them black and let them heat up from the sun during he day. If you have enough sun and enough water it might work. – OrganicLawnDIY May 29 '16 at 12:54
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That's a pretty short season and your main issue is going to keep the greenhouse warm at night. During the day if it gets enough sunlight it can get quite hot on it's own. So much so you will probably need to provide ventilation.

I took a shot at coming up with some modifications to your design that I think would work well in your circumstances. Forgive me but I'm going to be mixing mm and inches because I'm more familiar with materials in inches.

45x45 (2x2 in USA) for the main framing should be sufficient. 22.5x45 (1x2 here in USA) for the door frames, top angle members and shelf slats Polycarbonate twinwall panels for the clear panels 2" rigid foam insulation with a reflective foil face for the back and bottom 1/2" plywood for the bottom to provide more strength instead of just the foam

Since it will be sheltered by the balcony above and because most of the wood will be covered by poly panels I don't think you need to be too concerned about a rot resistant wood unless water pools on the floor of your balcony in which case you might want to consider using something like cedar for at least some of the framing. The framing will also be getting wet when you water your plants so if you can I would suggest a naturally rot resistant type of wood. You'll also want to try to anchor it to the side of the building just to provide more security so it won't tip over. Think of the type of safety straps they have for tall furniture to prevent kids and pets from knocking them over.

If you want to have two rows of gallon milk jugs you may need to make it slightly deeper than 320mm depending what the actual dimensions of the equivalent of a gallon of milk is over there or whatever containers you can get cheap to use. The milk jugs can store hot water to heat it up at night. If you paint them black they can heat up during the day if they get enough direct sunlight or you can put hot tap water in them in the evening.

How many you need I don't know. I chose to make 2 sets of doors so you can replace jugs without releasing as much of the heat inside. There's a lot of information on heating greenhouses with water that is heated by the sun. I wish I could find the source but there was a guy with a cool hoop house in Massachusetts or near there that was growing in his hoop house year round and heat was provided by storing heat in large water containers. It's all about having the right amount of water for the volume of greenhouse space.

Foam weatherstripping around the door frames to help keep air in.

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For eggplants, tomatoes, and, I believe, Cucumbers you will need heating - if the plants get frost its immediately "game over" - and they really need to be kept above 14 degrees C (although a single night at 4 degrees won't kill them)

As far as construction goes, more light = more growth - there is no magic to the construction - you can quite easily get away with wooden frames - it will slow growth down very slightly because of less light, but probably a better choice because aluminium will conduct the cold through it.

Polycarb is a good choice as it is, effectively, 2 skin insulation - although the research I've done indicates that you may be better off with thinner=cheaper polycarb and put the extra money into a heat/light source.

You may want to reconsider painting the wall black - I've not yet done any tests, but you would probably be better off with a highly reflective coating so the light gets reflected back off the plants - I'm guessing amount of light is going to be a major issue. A black floor would probably be a good idea though. There is also talk on the Internet about lining the inside with bubble-wrap to provide more insulation. Also, if you could make the unit bigger it might retain a bit more heat.

The water bottles referred to by another poster would help with heat buffering, but are unlikely to be sufficient - and if you are going to boil water for them (which will be of most help when its least needed - ie when it starts getting cold, rather then at the coldest), you are probably better off spending the money on incandescent lights to provide heating and a bit more light.

  • Thanks, I wish I could provide heating but I live in a coop and there is no mains outside. I would have to get board approval and hire an electrician which is a rather expensive proposition. Kerosine/gas heaters are banned due to the risk of fire. – max May 30 '16 at 9:26
  • Fortunately there is a lot of light here from may to september, around 20h a day. I might try experimenting with building a solar heater or see if vermicompost can provide a bit of heat. – max May 30 '16 at 9:32
  • Not something I've looked into too much yet, and the volume you have may be too small for it to be viable, but it is, in theory, possible to use a compost heap to create a good amount of heat - google "hot composting" - if you want to explore this you probably want to raise your hothouse and build the hot compost heap directly below this. (I mention this option as it does not require electricity or gas heating) – davidgo May 30 '16 at 21:39
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  • NOT pine. It will only last 1-2 years outdoors, if that. Avoid wood for this. Pine is considered a soft wood and rots quickly.
  • PVC works better. Get the thickwall type and it will be more ridgid. I made a 6'x4' A-frame trellis for only $15usd/125 krona. (Not including 1/4" plastic grid.)

Aluminium could be an option but I don't have any experience with metalworking really.

For aluminum you really just need a drill, a hacksaw, and file to remove rough edges. Just don't cut the aluminum indoors as the shavings will get all over. And it might just work better than PVC for holding the polycarbonate panels.

I see your 4 shelf wooden shelf thing. You could wrap 3 sides in clear plastic, then make a frame for a door for the 4th side, and wrap the door with plastic, then put the door on hinges. Get the thicker plastic, it will last longer and is worth the price.

Now it's easy to get to your plants. I try to find ready-made shelves for this type of thing and modify it as needed.

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I have a couple of suggestions to consider. One thing to think about is that 320mm. That's not as wide as it seems. Some of those plants, such as the tomatoes and peppers can get large. I suppose you can keep them pruned back, but they'll probably still touch the sides of the container and there can be a significant difference in temp between the interior and right up against the outer walls. I think you could easily grow herbs in this size container, but you might need to make it larger for some of the plants you want.

I don't think it really matters what material you build it out of, because you should coat it in some kind of sealant. I don't know if you have similar approximations over there, but I'd seal the frame and back with something like pond sealant or a garage floor finish. It'll make it last much longer as well as giving you a darker color to help absorb heat. To help combat temp loss, I'd suggest having all the walls insulated. You can do this with high density foam in the rear wall and the side walls if they aren't going to be clear to let light in. You could also just stuff it with newspaper and get some decent insulation value.

I would probably build it out of 2"x2" (50mm x 50mm), which is just about perfect for adding the HDF and putting panels on either side. Our local supply store even sells the textured plastic 4'x8' (1.2m x 2.4m) sheets of textured plastic. Typically you see it used as the lower walls in fast food restaurants, because it's easy to clean. You could use this inside the green house to so it doesn't matter if it gets wet or not. I would use the double walled poly carbonate where you want light to come through. It adds extra insulation value.

Based on an idea another user suggested, I do have a modification that might help. They suggested using PVC to build the frame. This isn't a bad idea. The only issue is that it can degrade and become brittle in sunlight, but a coat of paint will help with that. My idea was to paint the frame black and fill it with sand. The black pipe will absorb more heat and hopefully transfer it to the sand as a heat sink through the night. I've never attempted that, but it popped into my head and seemed like a good idea, worth trying.

Other alternatives to help heat a greenhouse like this without power are to make a solar heater. You can look up instructions on youtube.com, but basically you build a shallow box with a clear front. You stack soda cans you painted black and punched holes in the bottom of, in rows. The theory is that the light comes through the glass and heats the cans, that heats the air in the cans, and you can rig a computer fan and thermostat to pull that air through into the greenhouse when the temp drops. You will be able to easily overheat this greenhouse as well, so be careful. You'd be surprised how hot one can get even on a really cold day.

Another alternative that is relatively cheap is a clay pot and saucer with a tea candle. Again, I've never used it, but I understand it can put out a significant amount of heat. People set the saucer down and light a tea candle on it. Then they put the pot upside down over it. The candle heats the pot and the pot radiates the heat. You could put more candles in one or find some way to rig it to light a new candle when the old one burns out. I'm sure you could devise a way to have one candle light the next as it burns low. I hear they're super cheap and you can use them to heat a small greenhouse through the winter. Enough to keep vegetables from dying. Probably wouldn't work for orchids or anything.

Lastly, you need to consider air flow. Your plants will mildew and rot without air flow. You should install a couple of small computer or desktop fans and wire them to a solar panel. We can get small ones here, fairly cheaply, from discount tool stores like Harbor Freight. They would probably power a couple of these fans and you can have them orientated to circulate the air.

Good luck and I hope this gave you a few ideas. Sorry about what's probably a horrible metric conversion on my part. :))

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