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Topic # 101368 30-Apr-2012 15:38 Send private message

I'm thinking of having a thermal greenhouse built, mostly so I can grow veges successfully through most of the year - brocolli, carrots, strawberries and tomatos in suitable seasons, etc. I've been reading a bit about it mostly on USA websites, but local knowledge and materials would be useful.

It'll be out the back of my shed, one wall will be the back of my shed, which is made of those big 30cm long grey concrete bricks. The rest will probably be a 4x2 timber frame, unless I find a better/cheaper method. The side opposite the shed is a sudden drop into gorse over a motorway, so it'll probably get a bit of wind too, not much shelter there. The long side is pointing west, with the door pointing north, which isn't ideal for collecting winter sun, but it'll be a lot better than no greenhouse. I'm having a tree cut down so more light gets into it too.

The brick wall will be painted part black to absorb the heat of the day, which will then release it back over the night to keep the place warm. I may place some other thermal mass in there too, black painted water tanks, if there's space. On the flipside painting the wall white would reflect light back to help the plants grow. I may go with grey, or alternate black and white, not sure yet.

As the sun's high in the sky in summer when you don't really want full sun I'll probably have darker material in the roof, or some way of blocking the light in summer. The walls will be clear so in the winter as much light gets in as possible. Because light goes through best when it's perpendicular to the glass etc I'll probably angle things so in summer much of the light bounces off, but in winter most goes in... in theory.

I'm mostly interested in materials for the roof and walls. It needs to be clear and double skinned so it insulates well in winter, I need to seal it so there are no air leaks too. Can anyone recommend a suitable locally available material? The two google has found for me are

http://www.psp.co.nz/Building-Brands/Roofing-Systems/Plexiglas-Acrylic-Multiwall (or one of their other products)
http://ampelite.co.nz/Thermoclear_Domestic.php

Alternately I could use some kind of standard plastic, two sheets with an air gap, though I doubt "home made" double glazing would be hugely effective.

Relatively cheap is important, I'm not spending a fortune to do this, but there's no point saving $500 if it reduces the effectiveness of the structure.

I wonder if I need any planning permission for this type of thing - does anyone know? It's in a position quite visible from a motorway off-ramp.

Any other thoughts or suggestions are most welcome.




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  Reply # 617548 30-Apr-2012 18:07 Send private message

Bubble wrap makes a great insulation material for the south wall, very cheap too.

For the roof I would go for the clear plastic roofing sheets, like (rnd wide site)
Plexiglas Acrylic Multiwall | PSP Limited or
Suntuf Corrugated Polycarbonate | PSP Limited
or Twin walled for insulation. Twinwall Polycarbonate Roofing | PSP Limited
Trademe is a good source for budget shopping TradeMe.co.nz - roofing clear for sale, New Zealand

The walls could be plastic film TradeMe.co.nz - greenhouse film for sale, New Zealand using two layers would help, an inside and outside. Some damprid between these would help with it condensation and growing mould.

I would also add an arduino to monitor temperature and open vents if too hot and also to do the watering,
but I'm geeky like that.



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  Reply # 617564 30-Apr-2012 18:41 Send private message

I'm not going super cheap, I want something permanent I don't have to mess with too much. That counts out bubble wrap and the cheap glad wrap type stuff.

It's hard to choose between plexiglas acrylic multiwall and twinwall without seeing them. Whatever's cheap that insulates better than standard glass is all I need. A single sheet isn't enough though, not if I want to grow in winter.

I'd probably rather buy an established solution for opening vents and watering, rather than building my own. It just sounds too time consuming, and I just want something that works. Or in winter leave the lot closed, in summer leave the lot open. Or something like that.




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  Reply # 617607 30-Apr-2012 19:49 Send private message

You don't need building consent for a moderate size greenhouse - it's like a garden shed so no worries there.

My green house is about 15years old and made of corflute type acrylic, a bit like your twin wall acylic. It works well. We're still getting a few tomatoes, the odd strawberry and LOTS of chillis now. Lots of capsicums too. The only problem is that mine is pretty brittle now due to UV.

I wouldn't bother putting in dark ceiling etc. Mine gets up above 40 degrees in summer but it doesn't matter so long as you have plenty of water. The best thing I did was run irrigation into mine, makes it a breeze with the circuit on a timer. I'd also recommend building tubs down each side, it's much easier not having to bend down to work in it.

You need openings all year round if you have flowering crops so bees can get in. The biggest problem we get is pests, especially white butterfly.

This is a photo of mine earlier this summer


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  Reply # 617612 30-Apr-2012 19:58 Send private message

I had heard of putting black plastic barrels of water in there to store the heat.



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  Reply # 617678 30-Apr-2012 21:40 Send private message

Thanks Handle, lots of good info there. That pipe structure looks like it may be a better idea than wood. Any thoughts there?

I could manage to run irrigation in easily enough, and put a timer on easily too. I hope I don't need power out there, that would be more difficult. Even getting a permanent underground water connection would be tricky, my builder suggested a rainwater tank, which I'd probably have to top up in summer. Do you have any problems with mildew or anything with the moisture? Do you have opening vents?

My area isn't that large, around 4m long by 1.5m wide. To maximise growing space I figured I'd do ground level, and on each side have another two shelves, though I have to plan what I what I want to grow and allow for decent heights.




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  Reply # 617686 30-Apr-2012 21:50 Send private message

To be consent exempt:
- the distance to any boundary needs to be more than the height of the greenhouse
- the distance to any other building needs to be more than the height of the building
- no toilet and/or kitchen (can't remember which)
- if it is used for sleeping then it cannot be larger than 10 sqm (I think)
- there are limitations on placing it in front of your house, I think it needs to be behind the house front face
The idea is that if it falls over it cannot damage another building or fall onto the neighbour's property. Also it cannot be rented out as accommodation, but still be used say by your kids as a sleep-out. Character areas may have by-laws in addition to the above general rule.

Based on the regulations I have no idea how those sheds that fit under the eves of a house can ever be sold in NZ. Except if it gets some inappropriate classification if it is screwed to the side of the house and considered to be a conservatory (like a closed in porch) rather than a shed.

If you put it on wheels then you can call it a caravan or trailer and there are no restrictions on how close it can be to the boundary or other buildings...




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  Reply # 617693 30-Apr-2012 21:58 Send private message

Can't see why you'd need power, tomatoes aren't too hungry on power :)

Mine is a commercially made kitset greenhouse. It's pretty stuffed to be honest, the white plastic is breaking down because of UV but it was installed in 1997 so can't really complain. It's got a sliding window on the door and a roof vent which has broken off as the hinges got really brittle with age so plenty of air circulation.

We used to get a bit of rot on the strawberries from over watering, we only water about twice a week now in the morning which has stopped the rot. The biggest problem is the feast or famine issue you get with growing your own veges - either no crop or lots! Last season we were a bit better at planting as we go but it's a bit harder with a 13 week old baby in the house :)

The only other thing I'd really recommend is think carefully about what you want to grow. Some stuff is easy but for you to grow but you don't get a great crop for the amount of space you use. Also Tomatoes are good, so are chillis (4 chilli plants = about 300 chillis a year), capsicums and cucumbers aren't bad either. Beans are a bit of a waste of time as are peas. Lettuces are great if you pick a few leaves a day off a 8 or 10 plants, they last about 8-10 weeks with lettuce every day. Herbs are really good but only ever grow mint in a self contained pot or you can't every get rid of the bloody stuff. I wouldn't bother with strawberries but the chief gardener and holder of the purse strings likes growing them so meh...



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  Reply # 617801 1-May-2012 09:27 Send private message

Thanks guys. It would be on the boundary and adjacent to a the shed out back, but not near anyone elses property. Sounds like I need to look into consent.

I'd expect a greenhouse to last more than ten years, so maybe I'm best building it out of 4x2s as a permanent structure. If the panels need to be replaced every 10 years that's not really a problem.

I've read the book "square foot gardening", it goes into how to grow things to make things useful, so you don't have 20 brocolli ready this week, 50 tomatoes, then nothing. Basically plant per square foot, staggered, shouldn't be too difficult :) I like strawberries, that's one of the driving reasons I want a greenhouse :)




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  Reply # 629158 22-May-2012 20:37 Send private message

This thread is a bit old now but a few suggestions.

Twin walled polycarbonate is the most used double skinned material for greenhouses.  But it is inflexible, which can be a problem in windy conditions.  My former father in law makes greenhouses from a curved aluminium frame, and plastic film, which are extremely resistant to wind.

Moisture and then algae can develop in the cavity in any double skinned system.

As well as the water barrels suggested above using a concrete path down the middle of the greenhouse provides more thermal mass to absorb heat, as well as a clean surface for walking on.

In summer an easy way to shade the greenhouse is with a section of shade cloth that fits over the outside.  Using eyelets on the shade cloth and boat canopy fittings on the frame makes fitting and removing easy.

Provide irrigation at ground level, to avoid wetting foliage, will reduce fungus/blight problems, especially on tomatoes.

Remember to dsign in some sort of overhead rail you can tie tomatoes/beans/cucumbers etc up onto.

Hope this helps.






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  Reply # 629169 22-May-2012 20:52 Send private message

Thanks Mike, it's still in the planning stages. I just ordered a book on greenhouse design so I'm waiting until that arrives before I finalise anything.

The double skinned system seems like it'll be fine, it'll be sealed at each end to stop moisture or dirt getting in, and if it does I may be able to clean it out. Interestingly enough one reason I'm doing the greenhouse is because it can get windy, and frosty, so it will give the plants a better chance to grow.

I'll do shelves along a lot of it, but I'll make sure there's space for tomatoes to grow, with rails. The "glass" will probably be over a wooden frame, glass outside, so there should be plenty of spaces to add climby things.

I'm thinking about a complete concrete floor which I will insulate underneath, and growing in big long tubs. That means the thermal mass of the concrete over the whole area will help keep it warm at night, cool it a little during the day, and make it easier to clean. I also like the idea of growing in a growing medium, not in dirt in the ground. It'll probably be better insulated than my house! Shade design will definitely be considered, maybe even something semi permanent that pulls out.

All ideas welcome :)




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