I'd get cheap polyester shade cloth (made from recycled plastic apparently) in 50% and see how that goes before spending more on aluminet.

I used a simple roll up mechanism using some PVC pipe with the shade cloth stiched on so that the cloth rolls up the side of the greenhouse, with some string looping down to keep the whole thing in tension. The fly idea is a good, but when I tried to make it I found it rather hard to support the wire tightly enough. Depends on your roof/walls design I guess. Scratching was less of a problem than cloudiness due to condensation.

50% shade cloth and a thermostatically operated evap cooler kept it cool even in 45C heat.

I'm close to posting a whole series of construction pics that would illustrate the roof/wall design but, I think a pair of (essentially custom) metal brackets bolted to the ridge ends and a pair on each upper end wall would give support to either a plastic coated wire (or perhaps spectra yacht line) that would support the shade cloth. I've put enough $ into this thing so far that six stainless brackets would not be a big deal.

I DO like the idea of trying cheap(er) 50% standard cloth first...I could alway use it in other locations after seeing its effect in the GH.

I also do have an evap cooler in the "master plan" but I thought it see how we get through a season first without it.

Some sort of active ventilation is a must though. (An autovent is fine)

Aluminet only comes in 50% or less. The most effective method is to have a air gap between the roof and the shade cloth.
Next would be to lay it on the roof and the least effective is to put it on the inside.
If you don't have high winds your idea of suspending it over the GH would be more efficient.
I would go with the aluminet it never made any sense to me to put black on something that you don't want the sun to heat. If you don't want to pay the extra for the aluminet buy a white shade cloth it will outperform the black.
I just bought a new shade cloth got it from IGC but they Had it custom made and direct shipped to me from the company that made it. The name was Green Tek and it is a very well built one. For what they sell them for I didn't expect it to be such high quality but it really looks good and is thick strong well built.

gardenwantabe: Have you done the numbers on those claims? I bet that the difference between black, white and aluminet 50% shade cloth, if suspended on the outside, is not measureable. Remember that they all let 50% of the light through.

The main difference is that what heat gets through the cloth is going to be blown away if the cloth is on the outside with air gap. But if you don't stop the heat until it is on the inside. Well that speaks for itself.

As to the color sit in a black car on a hot day. Test have been done on a closed car one was white and the other was black and in 30 minutes time the black car was over 20 degrees hotter

It not just the light color of the Aluminet appeals to me but its reflective qualities as well. While I'm hoping that it does not produce an intolerable glare, I can see the effectiveness of reflecting some of the sunlight, as well as blocking it through the mesh.

I do believe that I have adequate exhuast cfm and make up air via the shutters to handle "normal" highs for much of the year.

Both book info and this forum have led me to accept out of hand the need for shade cloth in the hotter summer months. I could easily see how an air gap between the cloth and the outside of the twinwall would help, hence my request for experiences/suggesting about hanging it.

I never serioulsy considered hanging the shade cloth inside. I know others do and it seems to work for them but I have no roof vents to have to work around and I'd rather have the cloth outside rather than have to deal with it inside around and over the plants.

I do also (personally) don't understand why you would hang it inside to help reduce the heat form the sun. Seems to me even the smallest gap between the inside of twinwall (glass or whatever) and the cloth would allow the heat be generated inside the structure rather than shade the stucture itself to begin with.

But, hey, whadda I know! This is a first GH for us, and has only been closed up (as a trial without plants) for only five days so far. It's only been in the high 60s low 70s here in CA for the last two weeks or so....roughly 20-35 degrees lower than we can expect in the summer months.

I have set a milkhouse heater up (just using its own thermostadt for the moment)and set the cooling themo to roughly 75. The high/low records for outside nights of 25 lowest and days of 72 highest have been 44 and 77 inside the GH.

Seems close enough for the first shot at control for the moment. Obvioulsy the shade cloth (and potential evap cooler) will be necessary for the summer.

CLAVRO
Don't matter if this is your first GH you are on the right track. A 50% black shade cloth will reduce the GH temperature because it blocks 50% of the sun but black absorbs heat white or silver reflects it so it will outperform black.
When did you ever see a electric radiant heater with a black reflector. They are always chrome or polished aluminum because it will reflect the heat. All foil backed foam board insulation is coated with aluminum foil not black. It is designed to reflect not absorb.
You don't need to own a greenhouse to understand this your just talking simple physics.
Granted the difference in the shade cloths would be small but if you have to buy one and where you live you need all the help you can get so buy what will perform the best
Regardless of what you use for a shade cloth you will need something to cool because with a good shade cloth and a good ventilation system the best you could ever achieve is to keep it at outside temp.

The point is, if black is only 0.1% worse than aluminet, why not buy the cheapest, and why not choose a colour that looks nice? A rust brown shade cloth might allow through more plant usable radiation than a green one.

I'm quite aware of reflectivity, emissivity, absorbtion and
alpha-epsilon ratios. Foil is used on insulation due to low IR emissivity, not because of reflection. Chrome can get pretty damned hot in the sun.

http://www.electro-optical.com/bb_rad/emissivity/emisivty.htm

I'm dubious that there is a difference between black and white, because I use both beige and dark green 50% cloth and they both feel the same temperature to touch in open air in full sun (just went out and did the experiment) - the difference in absorbtion is insignificant compared to the losses to the air.

A good ventilation system and layout can make the greenhouse cooler than outside just by moving air. Two reasons off the top of my head: a) plants evaporate water, using up 2.2kJ/g of the heat - everyone knows that grass is cooler than asphalt in the sun. b) the greenhouse can be blocking the sun, but radiating to the sky - the sky is like a giant frozen wall, even in 45C heat!

That's simple physics for ya.

Since I'm not the one that needs a shade cloth I will leave it up to Clavero to decide who is right.
I see no point in arguing about it.
You can continue to put your dark colored shade cloth on the inside and I will continue to suspend a white one over my GH and we can both be happy.

Gardenerwantabe, I have been preaching the same thing for years. I have aluminet on the exterior of my GH.

Gardenerwantabe, I have been preaching the same thing for years. I have aluminet on the exterior of my GH.

yea it makes me think of and old saying.
( You can lead a horse to water but you can't make it drink. )
As far as the argument that the aluminet cost more that also reminds me of an old saying that goes like this.

IF YOU WANT NICE FRESH OATS YOU MUST PAY A FAIR PRICE
BUT IF YOU CAN BE SATISFIED WITH THEM AFTER THEY HAVE BEEN THROUGH THE HORSE THEN THAT WILL BE A LITTLE CHEAPER

But it is no big deal as long as I'm free to do what i want I don't really care what others do.

weebus,

Do you have the shade cloth draped over the outside or suspended? I'd be curious to know your experience with the Aluminet material itself. If draped, does it scratch the gh covering material?

If suspended, is it particularly heavy or difficult to handle?

As to the "difference of opinion" in the exchanges re: gets hotter or not...I used to be an "owner" (ie. organizer/moderator/janitor) of a an Internet listserver group focused on hand percussion. I've learned it best to sit back, let the chatter fly, keep my own council and, well, do whatever seems best to me from the summary of the input.

Thanks for the perspectives folks. I appreciate it greatly.

Steve

You can continue to put your dark colored shade cloth on the inside and I will continue to suspend a white one over my GH and we can both be happy.

Do you always put words in other people's mouths? I never said such a thing, and I don't see anyone else saying the same thing?

You can lead a horse to water but you can't make it drink. I prefer: You can lead a horticulture, but you can't make her think.

My point was simply that at least here, aluminet is about ten times more expensive than shadecloth, and there has been no compelling evidence that it is better. (no, neither SHOUTING nor 'I've always done it that way' are compelling evidence) I ask 'why bother?'.

Does aluminet last longer than shade cloth? scratch less? weigh less? Are there any good reasons to use it besides being shiny? I would guess that it is more likely to scratch as aluminium is harder than polycarbonate, whereas shadecloth is made from polyester or PET.

(From memory, aluminet does scrunch up well removing the need for a rolling mechanism, but then, even time I've seen it used, it was used on the inside - maybe it degrades faster in UV or something)

Clavero: good idea. be persuaded by facts, not by who says what :)

Aluminet is MUCH MUCH lighter than shade cloth, has never scratched my poly, stores much easier, and I have mine draped.

weebus: excellent. I shall consider aluminet for my new greenhouse. If you have it on the outside, how do you stop it blowing away?

We put thin roping on ours and tied it to rebar hammered into the ground. Works great. BTW, I keep the shadecloth on yr.round here in Texas. Works great, very lightweight.
hilary

I use clips and bungee cords with eye bolts that are screwed into the frame.

I have the aluminet draped over the top of mind. Held in place with some clips on the aluminum frame. I only paid 22 bucks for it (6x8 greenhouse) but it came unfinished, I did that part myself. I am very pleased with it!

clavero,
professional fern grower in the deep south here....

anyway, my preference for greenhouse growing has been two layers of shade.
sounds strange, but it's worked best for me.
Situation:
needed 70-80% shade in full summer to grow good plants.

happened to have two 30% shade clothes on hand, so I use them, I pull one off around October and put it back on around May. This has worked best, and the house stays fairly cool despite being in a very warm climate.

this of course assumes you cannot move the plants outdoors for the summer, which is obviously preferable for a number of reasons.

Drew

I'm just getting into greenhouse growing and admittedly don't know doodly-squat. But I sure am looking forward to finding out the does and don'ts.

I got this reply from Rion when I asked about their Shade Net. Now that I have the info how do I analize/make use of it? Too, any drawbacks during winter to put a clear pool cover over the Rion Net? Thanks for your help and hopefully I'll be a quick study.

"The Rion ShadeNet controls air movement by allowing hot
greenhouse air to exhaust through the manually or
automatically-controlled Rion roof vents. Rion ShadeNet provides 62-64% shade percentage. It allows 55% of diffused light and offers energy savings of 36% during the winter too."