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Polycarbonate roofing

Posted by barbara_b Vic Australia (My Page) on
Tue, Apr 18, 06 at 4:13

Hi, this roofing lets through very little UV rays. We roofed our dog run with it and it also stops a lot of the heat coming through. We used one called Opal which is a translucent white. I'd say much less heat would come through than if we used a glass roof or corrugated iron. Can someone comment on its suitability for a greenhouse roof please? If we used the translucent one, would we still need to cover it or paint it for the summer. Barbara B


Follow-Up Postings:

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RE: Polycarbonate roofing

I've been using clear PC without trouble. You will need to roll on some shadecloth over summer (and make sure it's well attached after last night's storm :). The single wall stuff is about $10/m^2 from bunnings, the double wall stuff is $30/m^2. I think single wall is good enough for most unheated plants. I've kept Kava and Pepper in a greenhouse with single wall in Melbourne without drama.

I wouldn't use the white stuff because in winter you want all the light you can get, and shade cloth is much cheaper than two roofs or extra lights! Corrugated iron will let much less heat in than any PC, and light coloured colourbond even less.

Whereabouts are you? Do you get frost? What do you want to grow?


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RE: Polycarbonate roofing

Hi Nathanhurst, we live in Somerville, Victoria. I was thinking of using Laserlite sheets for the roof instead of the double walled polycarbonate, but I take your point about as much light as possible in winter. I will have a shadecloth cover over it. My "better half" will make provision for that when he builds it. We get an occasional frost. I'd really like an automatic vent in the top of it too, but I haven't found out where to buy one yet. Do you know? Barbara B


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RE: Polycarbonate roofing

Barbara

The amount of light you need for the particular plants you want to grow will determine what roofing choice you make. I grow gesneriads which need only 500 foot-candles. To get some idea of what this means - this is the brightness at approx 15cm from twin fluoro tubes. Full sun is about 5000fc

I don't have a proper light meter but I find that I can use my SLR camera to measure the brightness as follows. I set the lens to f/11 and set the film speed to 100ASA. If I look at foliage, I find that I can use fc=approx 70/E where E is the exposure.

I currently grow under a roof which has alternating clear and colorbond sheeting (1 clear, 2 colorbond) - which has some good and bad aspects. In summer it is the correct brightness but gets too hot, mainly because of the colorbond. I use an exhaust fan to keep things cool. In winter it is too dark and loses too much heat. I am about to re-roof in all polycarbonate with 25% transmission.

The thermal conductivity of polycarbonate is k=0.17W/m.C
By comparison it is 46 for steel!

Heat loss through a material is k.A.T/t where A is the area, T is the temp difference and t is the thickness of the sheet. The problem with single sheet polycarbonate is the small value for t (only 0.8mm) which makes the heat loss large. 3mm glass sheet is 4 times thicker than 0.8mm polycarb and thus 4x more lossy for this reason alone. This is where twinwall really wins.

In summer you also need to consider the shading coefficient (amount of solar heat transmitted compared with glass) as well as the light transmission. Ideally you want max light but the smallest shading coefficient (fraction of heat transmitted). Opal is 56% light transmission and 0.53 shading coefficient. Clear is 93% transmission and the shading coefficient is 1. Thus Opal will be half as bright and twice as cool as clear. There is a polycarb product called Solar Control which lets through less light (25%) but also a lot less heat (only 21%). I am about to cover my growing area with this material as it wil give me an excellent compromise between summer and winter - I don't want to mess around with shade cloth. However due to the heat losses I will still have to heat at night, in winter to ensure I maintain 15C. I find that a 1kW electric heater is enough to raise the temperature by around 10C.

Hope this helps......


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RE: Polycarbonate roofing

You can use the roofing, just get the clear stuff!

There are a few shops in Melbourne that stock autovents, although I'm wondering whether it would be about the same price to buy direct from overseas.

Thermalconductivity is fairly irrelevant, most of the insulation is provided by the boundary air layer.

We need to know what sort of plants you want to grow before we can make a suggestion - domeman's light levels would be useless for my plants, which are mostly alpine and desert species requiring maximum light.


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RE: Polycarbonate roofing

Hi,
Domeman, thank you for your detailled and full explanations. I can see the double walled polycarbonate material is probably going to be the choice. Actually Tony has plenty of sheets of toughened glass out of aluminium sliding doors but we can't work out a way to use them in a roof.
Nathanhurst, I want to raise vegetable seedlings for my garden, grow tommies out of season, strike cuttings of common garden plants. I have a few orchids but have found they're pretty tough although they may like the luxury of a hothouse.
As you can both tell, I'm on a pretty steep learning curve just now.
Barbara B


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RE: Polycarbonate roofing

Double wall is a lot more expensive, only comes in large sheets (5mx2.5m) (requiring special delivery) and gives dubious improvement in insulation. I originally planned my greenhouse to be in double wall (see other thread, read to start greenhouse) but after doing the sums I concluded it wasn't worth it in victoria (except perhaps on mt bogong :).

I start veggie seedlings in a poorly sealed single wall greenhouse with polyethylene film walls without trauma.

Do you have much land? You might think about buying a steel tube and plastic film type greenhouse instead - they don't look as nice, but you'll want the space if you want to grow toms!


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RE: Polycarbonate roofing

Hi Nathanhurst, we have 2 acres under lawn and garden but I don't particularly want to grow the tomatoes in the hothouse, just start them off in there. Tony doesn't much believe in buying when he can build, and he likes the idea of building a hothouse with a shadehouse on the south side of it. Barbara B


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RE: Polycarbonate roofing

Barbara

Some useful advice from Nathan. I use a cheap plastic covered greenhouse to start tomatoes which I move outside to mature. I use the polycarb area to grow specialised plants which require controlled conditions (>15C).

BTW See my blog for views of plants in my polycarb 'greenhouse'.

http://www.whitepage.com.au/domeman/

BTW Thermal conductivity IS not important? Then why does it become hot under a metal roof in the daytime and cold at night?

Ask for catalogs from Laserlite (www.laserlte.com.au) and from Suntuf (www.suntuf.com.au) - lots of good info included.

Here is a link that might be useful: domeman's blog


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RE: Polycarbonate roofing

Hi Domeman, I've never even seen these plants before. I guess a hothouse opens all sorts of possibilities. We have a small local paper that we can advertise locally in and in this week's edition there is a 2400 X 1800 aluminium frame and glass hothouse for only $150. My dearly beloved said it's not good enough - he wants a bigger, better one - but my fingers are itching and the telephone is right near me. :-( It would be up in a week if I got it. (sigh)
Barbara B


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RE: Polycarbonate roofing

Barbara

There is a useful book called "The Greenhouse Expert". ISBN 0-903505-40-1.

I purchased my copy from the Canberra Botanic Gardens Bookshop. Dicusses greenhouse design as well as greenhouse plant culture.


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RE: Polycarbonate roofing

domeman: the R-value of a 0.7mm piece of steel is 0.000015m^2 K/W, the R-value of a 1mm piece of polycarbonate is 0.006m^2 K/W. The R-value of the layer of air on the inside of the greenhouse wall is 0.5m^2 K/W.

To work out the total resistance we just add elements in series, so an iron roof has a resistance of 1.000015, compared with a polycarbonate roof with 1.006...

iron roofs are hot in the sun for a number of reasons: iron is a good selective surface - it absorbs visible light, but doesn't radiate back. It has a high thermal inertia, so it stores plenty of heat. It has a good conductivity (as you point out), so touching it draws heat from a large area.

Compare with polycarbonate: it absorbs less light (particularly the clear stuff), it readily radiates heat, it has a low thermal conductivity and a low thermal mass.

But we aren't particularly interested in the temperature of the roof, instead we are interested in the temperature inside. And a clear pc roof is going to heat up inside much more than an iron one. (because the insides are being heated directly with light rather than by reradiation)

I'm not sure why (or whether) an iron roof is 'cooler' at night. Both have low emissitivity and about the same total insulation. Perhaps the ground inside isn't as warm under iron so it cools down faster?

Blog is good.


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RE: Polycarbonate roofing

OK, I've ordered the book from Amazon this morning. I've stifled my disappointment at not having the aluminium and glass hothouse tomorrow, and will wait for Tony to build his super-dooper glass hothouse. He's now looking at about 6M X 4M after reading all the advice. Would it be alright to build a shadehouse against the south (long) wall with the entrance through the south side?


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RE: Polycarbonate roofing

A simple example of why pc is better than pc/iron to achieve say 50% light levels.

If a gardener wishes to create 50% shade, it is possible to do this in two ways
in two ways.
1. Alternating iron/clear pc.
2. All sheets 50% pc (approx 50% light and heat transmission)

Assume each sheet is illuminated by H (heat) and L (light) and that metal sheeting absorbs and then radiates all the heat falling on it. Consider two sheets:

Case 1. Alternating metal/pc
Total light input to greenhuse = L + 0 = L
Total Heat input = H/2 + H = 1.5 H (H/2 because metal radiates equally out and in)

Case 2 All pc
Total light input = L/2+L/2 = L
Total heat input = H/2+H/2 = H


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RE: Polycarbonate roofing

North wall would be a zillion times better, as you'll get more winter sun, when you need it most. Good to hear you're going for a big greenhouse. If you can afford it you'll be thankful for going bigger. A door directly into the greenhouse is fantastic as you can go out there and read a book in the middle of winter!

You will have 24m^2 space. People have lived comfortably off veggies grown in 50m^2 without any extra food in temperate climates. You might be able to do this, with the bonus of some of the more hardy sub tropical crops (perhaps grow some ginger?).

domeman: analysis looks good. Depending on what your goal is, you might even use 93% transmission PC (clear) and 50% shade cloth inside. This will give the lower light, but collect more heat. In summer you would either vent, or move the shade cloth outside. PC is also half the price of colourbond (but only lasts 1/3rd as long).

If you put the shade cloth on the inside, two things to note:
a) it would be good to have a light clear film of something like PE (say drop sheets) directly under the PC (like double glazing) to protect the PC from hot humid conditions (apparently PC breaks down quickly in hot humid conditions, but not when just hot)
b) keep the shade cloth say 20cm from the surface to allow good air movement. A large diameter fan such as a ceiling fan at the top of the greenhouse could then move heated air down when required (thermostat).


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RE: Polycarbonate roofing

Nathan

You wrote......"apparently PC breaks down quickly in hot humid conditions, but not when just hot".
This is the first I have heard of this. Do you have a reference?


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RE: Polycarbonate roofing

Unfortunately, no. I know this second hand from a friend who tracked the data down from DuPont. He can't find the data now (I asked him today). I believe him, though I would also like to know exactly what constitutes hot and humid vs just hot or just humid...

I spent a good half hour trying to track this down using google, to no avail (I just don't know the right question for the oracle it seems).

Perhaps you could ring dupont and get us some real data?


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RE: Polycarbonate roofing

I'm looking for a shade cloth to reduce the coming summer heat; can anyone tell me if I can put the cloth inside. In Idaho the wind usually blows for 3 or 4 wks every spring at 30 to 40 mph so it would either blow it off or beat it to pieces.

Does anyone know if a cloth inside will cause heat between the cloth and the PC to be extreme and damage the PC?


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RE: Polycarbonate roofing

  • Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
    Tue, Feb 4, 14 at 23:14

I can't answer the question about inside. I think outside would be better. The woven black polypropylene with sown edges and grommets is extremely tough. The wind won't phase it. We have 40-60 mph winds 20-30 days a year. It does scratch poly film but I don't think it will scratch polycarbonate, but not sure about that.


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RE: Polycarbonate roofing

As fruitnut mentioned, the best location for shade cloth is outside the greenhouse. It absorbs/reflects heat, which is dissipated away from the GH. If placed inside, the heat is already in the GH, so the effectiveness of shade cloth there for temperature control is greatly reduced. My 10 yr old woven polypropylene still works fine and when secured with ties to the grommets, has never been affected by wind, as also noted by fruitnut. It has not caused any noticeable damage to the polycarbonate panels.


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RE: Polycarbonate roofing

Put shade cloth on the outside. Dark shadecloth absobs heat, and in summer putting it inside will create a trapped heat zone. I have black prolyproylene shadecloths that are twenty eight years old and still using them. I special ordered them sized to go bench height over the arches, with grommets and secure them each season by ropes attached to the kick boards. They don't go anywhere in wind situations unless the ties fail. You can adjust them so that they go lower to the ground on the exposures needing the most protection from high light intensity, or for the crops needing the most shade. The indirect light coming from the area under the bench does not harm the plant material in any way or add much to the solar gain and actually allows the cloths to be on for a longer period before removal. If you have a poly film sided greenhouse they also seem to double the life on your films by protecting them from UV degradation.


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