Post by georgeadams on Nov 29, 2014 19:12:17 GMT 7
I currently live in zone 8 (SE England) and am considering getting a greenhouse. Does anyone have any advice on what would be appropriate as far as features are concerned? Should I use twin or triple-wall polycarbonate? What kind of heater should I use? Should I have a paraffin (backup) heater in case the weather becomes below zero outside? What orientation should it be (N-S or E-W)?
If you're a tight budget, you may want to consider something other than polycarb. One of the best inexpensive greenhouses I have seen was built from pressure treated lumber and two layers of polyethylene greenhouse film, one layer stapled outside the frame and the other stapled inside. The dead air space between the two layers provides some insulation, and the owner has no problem heating it for tropical orchids in zone 7.
My greenhouse is twinwall polycarbonate, but that's only because I have to deal with squirrels who drop hickory nuts on it from a great height (not to mention the occasional large branch that is shed by the trees). If you don't have tall trees in your garden, the strength of polycarbonate panels may not be very important to you.
But wouldn't polycarb provide better insulation and ultimately save more money in the long-term?
Also, I live in zone 8 (apparently), just for reference.
Marginally. 8 mm twinwall polycarbonate (the glazing on my greenhouse) has an R-value of ~1.7. Two layers of polyethylene with the space between inflated seems to be about R value of 1.4-1.6 depending on what website you ask. It's the dead air space that does most of the insulation. For the design I mentioned, with the two layers stapled to the inside and outside of the frame, I bet you could increase the insulation further by sandwiching bubble wrap between the two layers. That would cut down on convection currents between the two layers. In twinwall polycarb, the bracing connecting the two layers serves the same purpose. I also staple bubblewrap to the inside of my greenhouse in winter, and it seems increase insulation further without significantly reducing light.
Twinwall polycarbonate is good stuff for impact resistance. Triple-wall is better for insulation, but neither is inexpensive.
Also, you can save heating costs by insulating the north wall and even north slope of the roof (assuming your greenhouse is oriented with a long wall facing south and the gable ends east and west). I use rigid foam covered with "reflectix" (basically bubble wrap covered in mylar that blocks radiant heat loss).
Oh, your second question: Definitely keep a backup heater that runs off different fuel than your main heater. My main heater burns LP gas, so I have a couple of inexpensive electric heaters as backup. Haven't ever needed them...knock on wood. If your main heater is electric, then a paraffin backup would be a good idea. Just make sure there is sufficient ventilation.
Insulated base, 25mm multiwall polycarbonate spaced 1-2 cm inside the glazing and very importantly a wooden frame! This will give you the best possible insulation without cutting out too much light.
Orientation is a matter of convenience and surrounding buildings etc. The amount of light entering will be about the same.
The payback is shorter with thicker polycarb, 25mm polycarb provides about 40% shading. From an insulation viewpoint 30mm polycarb would save more money in the long run but may block out too much light. Aluminium is a very good transmitter of heat and this makes aluminium frames much harder to insulate due to 'cold bridges'.
As the previous reply said you can insulate the north side with some good building insulation, such as celotex.
How much effort you want to go to may depend on how big a greenhouse your thinking of getting, a typical small greenhouse can just be bubble wrapped in winter and wont cost too much to heat.
My current 18ft wooden one costs far less to heat than my old 10ft aluminium one after paying a lot more attention to insulation.
Electrical heating is the most convenient and if you want a backup system you could consider a small backup electric generator. A problem with gas is you have to provide ventilation, which seems to defeat the point of having good insulation in the first place.
If you use a direct-vent gas heater, you can seal the greenhouse tightly (though some air exchange is desirable even in cold weather). The firebox of a direct vent heater is sealed off from the greenhouse and draws air for combustion directly from the outside.
A backup generator is definitely a good idea, even if you aren't using electric heat. In my climate (SE USA), overheating is the primary danger during power outages. We usually lose electricity during summer thunderstorms, and then the sun comes back out! Perhaps in SE England you can get away with passive venting, but I need an exhaust fan. I used a portable gasoline powered generator for a few years, but the problem was that I needed to be home to wheel it out of the shed and get it running. Now I use an automatic generator that runs off the same LP gas tank as the greenhouse heater. I sold it to my wife as a way to keep the freezer and well pump running during power outages.
Its not so easy to buy those kind of gas heaters in the UK unfortunately.
An extraction fan would be usefull here too, greenhouses can easily get to 40C or higher in summer. Most orchid/nep growers seem to end up putting excessive shading up to keep the temperatures reasonable or just leave the doors wide open (+vents obviously). An extraction fan is a bit more automateable. That said summer heat is rarely fatal here, its definately winter outages you have to worry about.
Mind you in a few years we may avoid freezing weather altogether, this year we had 15% of our native plants flowering on new years day! (and that is unsual to say the least!).
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