Thinking about a mini-greenhouse - need guidance

I'm thinking about making a very small greenhouse. We have a small collection of tropical/subtropical orchids which are a bit delicate for Sydney winters.

I'd like to have a greenhouse which will keep the internal night-time temperature just a few degrees above the ambient. It can be quite small - just maybe 1500mm high and maybe 900 wide and 500 deep. I know there are various mini-greenhouses at Bunnings etc but I'd like to make something a bit more attractive, a bit more robust, and a bit more orchid-friendly.

So I'm thinking I'll build a timber frame, then insert glass panels. Add 3 shelves and a few hanging hooks. Paint some bricks black and use them as a heat sink and the bottom shelf. Have a hinged top or side which can swing open at 9:00 am (to provide the critical disease-preventing airflow) and close at 1:00 pm (allowing the heat sink to warm up over the rest of the day).

My main question is DO I NEED TO USE GLASS ? Presumably the glass is used because it allows the energy to pass through during the day as visible light, but retards it passing back out during the night when it takes the form of radiant heat. Do plastics do the same ? If so, would thin plastic sheet suffice ? How would it compare to glass in this essential property ?

Is there a better way to do it ?

Any guidance. Any ideas ?

ps. I dont want electric heat.

cheers
Arron

We used plastic as a covering for my partners succulent shade house, and had nothing but problems, just couldn't get the warmth in to keep the plants healthy. It now has polycarbonate roofing with shade cloth sides and window panels for the air. Ended up putting a panel of 5 louvres in to help with air movement as mealybugs were a problem with her succulents.
If you want to use glass, get hold of some windows that have come from demolition sites. If you can get the complete window frames, with 2 windows, it will allow a window to come down or up to allow for air movement.
Kryn

Interesting. What sort of plastic sheet did you use. Was it dedicated horticultural sheet like this Greenhouse film, greenhouse plastic, greenhouse covers, handyscreen, Gro Tuff, hothouse covers, igloo covers .

I Googled this subject and came up with several makers of specialised greenhouse film. However, although they published the amount of light they let through none of them quantified the amount of heat they stop - which makes me think the answer is 'not much' or else they would be very quick to promote it.

Hi Arron

I used opal coloured polycarbonate sheeting on my orchid hothouse, mainly because I recycled it from the back patio roof when I extended the house. Be careful using clear sheeting or clear plastic if your orchid house gets full sun, especially in summer. It will get very very hot, maybe too hot. The opal coloured polycarbonate seems to work pretty well. With regards to ventilation, I clad the bottom 400mm of the north and east facing walls with only shadecloth to allow some cross airflow. The cross ventilation is needed moreso to prevent fungus infections. The mealy bugs are a real bugger here, I spot spray them with confidor.

Regards
Twosheds

I've seen on a few sites from the UK and US (where greenhouses are a little more popular) a number of thermally controlled hinges. You put some of the upper windows (or the roof panel) on a thermal hinge and as the heat builds up in the day, the hinge opens to allow a measure of the hot air back out. Once the ambient starts to drop off, it closes up and you get the moderation throughout the night (within the heat retention capabilities of the glazing). Also provides the advantage of some air exchange throughout the day. You'll lose out on humidity maintenance, especially in summer, but that can be remedied by other means.

Glass is a superior material for greenhouses, edging out even polycarb (just). Provided you don't have kids/pets/hail storms/drunken stumblings through the garden, you shouldn't have to worry too much about the fragility of the glass. With polycarb it's vital that you get the UV-film coated version or you're wasting your money, which is the other thing, polycarb costs more.

Glass should last longer and stay clearer throughout it's lifetime, provided it's not subject to mechanical stress. If you're really dedicated, you can polish scratches out of it once it starts to look a little scrappy.

If polycarb is cracked, you don't really have to worry so long as the crack closes back up. Chances are with glass that anything that cracks it also turns it into mess.

All of the plastics (polycarb and acrylic are some of the most popular) are generally easier to use than big sheets of glass (tablesaw/circ vs. finicky score and snap). You can of course snap poly and acrylic, but you're not obliged to like you are with glass.

My reading says 3mm glass for greenhouses (to maximise light), but that's mostly for light-hungry europe and US. In Sydney it's only the very depths of winter where we get properly short days and even so, they're still 9 hours long from official sunrise to sunset - beats the 5-7 hours they get in the UK.

If you're really spending the dough, apparently there is a double wall polycarb available that gives you all the heat retention you could ever need - I've never looked for it and I don't know how annoying it is to cut and seal.

Adding some thermal mass is a great idea, provided you can get it warm, otherwise you end up with a box of cold all winter as the rock stubbornly refuses to warm up. Good positioning and thermal hinges to account for summer are likely to do a better job, but if you do put rocks in there, they'll need to be in the sunniest part of the greenhouse. Unpainted dark coloured bricks are likely to do almost as ones with black paint on them and placed into a tray of water could even help to keep up the humidity during the day.

Lastly, glass is greener (see what I did there) because it is more readily recycled once broken.

Thanks for the responses, guys.

Thinking about it a bit further, I think I would like to use the greenhouse from Late June to end of August. The rest of the time I will disassemble it and pack away. I have found the perfect location for growing the orchids the rest of the year - so don't want to mess with that. However it is quite exposed and they will be affected by the cold in winter.

FWIW, it's just a small collection, mostly oncidium hybrids, odontoglossum hybrids, coelongyne, species dendrobium, Laelias and Brassias. Most of these are pretty cold-tolerant but some will need shelter. If the heat retention concept works then I might stock up on the cattleya alliance a bit, but i think I would need to be retaining about 4 degrees above ambient temps for most of the night, which sounds unlikely.

I should also point out that none of these species prefers direct sunlight - even winter sun - so opacity is probably a plus for whatever cladding I choose.

Cheers
Arron

A guy in town here used recycled windows, clear plastic and shade cloth.
Worked well for him

As advised previously thermal mass should not be overlooked, can get you through a cold winters night with the cheapest plastic/glass film. Containers of water have always worked a treat in this regards - cheap and easily found.

I know this is an old thread but I'd like to add my 2 cents.

I'm sorry but I totally disagree with the statement that glass is better to use than polycarbonate panels. Glass is heavy , it's breakable and it produces very uneven heat for a greenhouse . Polycarbonate sheets are lightweight, almost indestructible , are fire resistant and can be made corrugated or layered for added insulation and diffusion of light in excess heat. They are also glazed so they don't let in UV rays but one of the most important things is they offer more even distribution of heat, very unlike glass. Because poly panels are glazed means that light can pass through them but cannot be reflected back out , unlike glass , so you can trap heat in to use at night. Water bottles , rocks or anything that can hold onto this thermal mass can be used and release this stored energy for later use at night when temps drop.
Yes these panels can get too hot mid summer BUT that is why you have automatic vent openers. If your greenhouse has the manual type you can purchase this separately and is well worth the investment.
Also if you build your foundation down to the frost line , you could make a 4 season greenhouse while adding extra insulation for the hotter months. . It's definitely something to look into.
For a cheaper method of diffused light you might want to use the woven fabric sheets like they use in hoop houses .
The only disadvantage in using the fabric is that it isn't as strong or as durable as panels .
It's definitely doable.


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