I would guess that flat white is going to work best. The diffusion effect of the white paint is beneficial to plants, and that is a separate issue from reflectivity. For example, a mirror has a higher reflectivity than white paint, but nearly zero diffusion. And yet plants like the white paint better due to the diffusion.

Thanks Cole. I appreciate your giving this question some attention, This is for keeping pipe coolest outdoors, so the degree of diffusivity or gloss isn't as important as the total solar reflection.

I'm thinking titanium dioxide pigmented paints are just as good as ordinary mirrors, all capable of being made to reflect a bit over 90%. The trouble is something that is 88% will have like twice the internal temperature rise as something that's 94% so even when they sound good it does make a big difference.

Krylon tech service got back to me this afternoon and confirmed that for whatever reason this is secret info and he just ecommended the gloss, but I'm not convinced whoever handled the question understand the important distinction you also made about specular reflection vs diffusive and so far there is nothing obvious that gloss, semigloss, satin and flat are much different to internal temperature rise if all contain similar reflective pigment levels.

I wish someone had tried a few by painting pipe tests pieces in the same sun and filling the same and measuring the temperatuve rise in each. Another thing to do one of these years.

The link below, and especially the paper it links to, contain a lot of math that I can't pretend to understand. But I can read their conclusion :)

http://rack1.ul.cs.cmu.edu/hotcars/

Clearly, everybody who wants to have cool cars, and other cool things in the Sun, should be happiest with dull-finish bright white.

This post was edited by Cole_Robbie on Mon, Sep 23, 13 at 2:20

Nice discussion/link. It doesn't sound like the guy who wrote the conclusion has any info on the difference between gloss and flat -- or even if there is a dependence) -- and I think he is using the word dull more to describe his opinion of white paint in general vs say a lustrous red Corvette (or in his test cases bronze and gray colors). Otherwise, a "dull bright white" is contradictory ;-)

I do see something he glosses over in his work, which starts out by being an excellent comment, but turns into a glaring error when extended to our situation:

He says:

It is important to be reflective in the invisible infrared part of the spectrum, but most materials that are reflective in visible are also strongly reflective in infrared.

This correspondence he suggests simply does not work for in semiopaque plastics, what you see is not necessarily what you get in IR if the plastic itself absorbs. Additionally for titanium dioxide white pigment in the violet end of the spectrum and going into UV, it is almost like a half open window. Whether that is good or bad I don't know?

This post was edited by PupillaCharites on Mon, Sep 23, 13 at 13:36

At $5 a can from Walmart, the 12 oz can the way I sprayed it does a single coat on 24 feet of 2x4 nominal channels. I probably lost less than 15% to the newspapers.It says covers 25 squ feet, I get about 2/3 the amount, usefully anyway. The sprayer valves are cheapo and drop gobs if actuated more than intermittently. I just lined them upand let the drops fall on the one beside the one getting sprayed.

It comes right off skin, but if it sticks to PVC like it is to my fingernails, it will do ok and maybe I'll see if Professor X needs a candidate with hydroponic polymeric titanium fingernails

This post was edited by PupillaCharites on Mon, Sep 23, 13 at 18:12

Another reason I espouse aluminum foil. it has excellent reflective properties for radiant heat.

My conclusion by touching the burning stove one time too many, now, is that so far Aluminum foil (Grizz brand of course) is unbeatable for blocking all types of Visible, IR and UV.

I've got two coats of Gloss White Krylon Fusion now on these 2"x4" nominal (in fencing; they are sold this way but measure actually 2.0" x 3.5").

Hard earned conclusion: Two coats of white is not sufficient to block light. Fugetaboutit! Unless you are simply using it to prevent PVC degradation in which case by all means use it knowing you plant roots will be bathed in muted Sunlight and have algal partners.

For near sure I think the bottom coat of black and then white would have worked just great. I wanted to try it without black, since the black although hidden will absorb all the light passed by the white paint exterior --and warm up because of this--- even though it can't be seen, it will do what you expect of it.

So for light blocking ability Grizz' experience is not surprisingly right again. This is not to say aluminum foil is superior to a pipe=>black=>white coating order. It will create that highly desirable tomb of darkness inside.

However, back to Cole's point about the Stefan Boltzmann law, the way I interpret it; it is not clear Aluminum foil will keep the interior cooler to me, not yet anyway. Important to note that Grizz uses two coats as well - his system is not simply aluminium foil. The glue may actually play an integral part in his success when analyzed from a physical point of view, as it may be a strong thermal insulator.

Whether pipe/glue/foil is better than pipe/black/white for combined darkness and thermal insulation is still an open question for me anyway. This probably means me wasting more money which is in short supply, but it is something I can't help doing ;-( Little things come out of it, like perhaps looking for a highly thermal insulating glue, possibly as importantly as the foil itself, when using foil, which honestly I wouldn't've thought of checking!

Well plastic, in general, has very good insulating characteristics as does air. So while Aluminum may conduct heat more easily than paint those two guys will limit conductive heat transmission. (remember the water is on the bottom of the trough while the heat primarily strikes from the top)
Also, conductive heating is minimal since nothing (or not much) is touching the aluminum to conduct to it.
A black then white painting certainly does an excellent job of blocking light. But dollar for dollar, nothing is less expensive and as effective as aluminum foil glued to the troughs, at least that I've found yet.
Looking into radiant heat control in attics is what really made me aware of Aluminum's benefit in reducing radiant heat. That coupled with cost and simplicity of application (toi cover the container) have me sold on it. I suspect there are better/more effective coatings. I just quit looking because of the success I've had with the foil.

Thanks Grizz. No question that you have a system that you are satisfied with and generally works. As you point out considering the very attractive economy makes it good enough to quit looking.

There are a lot of considerations that make it easy to feel ok, like your comment of air being above the solution as well.

The open question we agree on is if there is a very affordable alternative for lower thermal heatinf withing the channels. A black paint bottom coat is likely not much better (maybe even worse), and certainly more costly, considering it functions primarily by absorbing light passed through the exterior coat and gets the same sort of temperature increase in this case by radiant heating...which it then conducts.

There is a counter point of view that would describe wrapping in aluminum foil as wrapping the channels with a heating pad. Approximately 10% of the heat is absorbed by the aluminum and converted into heat - and 90% reflected back out.

Here's my reason for pursuing this. What is happening to that 10% of absorbed solar energy? I believe that has an easy answer. As heat, 1/8 of it is being absorbed and removed by the outside ambient air and 7/8 of it is being absorbed by the exterior surface of the PVC if there were no glue between them, since PVC is about 7x more thermally conductive than air. It is the same as wrapping the pipe in a heating pad.

Inside the PVC, the flowing water thankfully creates a lower temperature. How much heat conducted from the aluminum from the warm (hot) outer surface of the PVC to the inner surface of the PVC will determine how much lower the temperature can be maintained inside the channel. Once heat makes it to the inner surface of the PVC, the heat is effectively added to the system since that is the direction of heat flow, from hot to cold, and the evaporating water inside the channel makes good thermal contact with the entire inside whether bottom or top, because the air inside is saturated with water.

So one solution is to buy thicker PVC and we will have better insulation ability from the PVC itself - which you rely on and as is have a system meeting your needs. Another is to look for a better insulator to put around the PVC. Thick insulation is a bit hard to handle so, for example, a can of spray polyurethane (if the Aluminum and PVC adhere to it), might be far better than some other glue and make dramatic differences to the point of tipping that ratio of only 1/8 of the conductive heat being removed by the air to nearly half. That small adjustment likely will drop temps inside the channel a couple of degrees. Nothing to sneeze at from warm growing areas and a $5 can of PU, topped by Aluminum or if opaque enough, better yet, white paint.

I suppose the counter to that is: just toss a frozen bottle into the res and be done with it more economically. No problem for aluminum since it is 100% dark, if one doesn't forget their ice-chores I don't see how that system could be beat, but if maintaining frozen ice in the res is not convenient, it is worthwhile to throw a few bucks more at this in my case.

Probably a polymeric roofing paint integrates bother low thermal conductivity and high aluminum reflectivity in one coat. I don't know how economical or safe it would be, but it is definitely worth checking out as the rich man's aluminum coating blocking thermal conductivity as well (by suspending aluminum in an effective insulator all in one).

other options if 10% radiant heat is too much to absorb would be the poor mans heat pump. (run nutrient through a hose coiled inside a small frig/freezer) Or you could use fans to force the humid air within the troughs to be removed (though this would likely increase evaporation loss) thus taking out some of the heat. or you could limit contact between the troughs and aluminum. place popsicle sticks every 8" to 12" perhaps on the top so there is no direct contact between the foil and troughs. (a tear nightmare if you ask me) or simply don't touch the foil to the troughs at all. stretch it across a seperate frame so its sitting slightly higher than the trough tops. (i see all manner of problems getting plants placed as well as said tear issue, still)
You could potentially glue on styrofoam insulation between the media points (if pots are at 8" centers, you glue down 5" strips) this would restrict tear points and provide good insultation, but i don't think it would be pretty.
You may find if you analyze the heat gain over a day , the gain is minimal and balances out overnight except on the hottest of days. them maybe using frozen water isn't such a bad option.
Or you simply plan your cycles so your vacation is during that time.

These other options of finding a way to put airspace between the aluminum and outer plastic surface occured to me but the tearing nightmare you suggest likely would make them not practical. The total cover also, but the problem there is impeding air flow besides the reservations you had.

Maybe a simpler, closer design to the so far de facto best practical one (your glued aluminum) is simply switching to buy one of those "space blankets" which is silvered aluminum deposited over mylar foil I believe and cutting that into strips. Come to think of it, they are probably called space blankets due to their superior insulation properties for something so thin. I wonder if they are nearly as opaque as aluminum foil (which is 100% opaque) and if they can be artfully adapted to glue on the channels like you have for the metal foil.

The fan ideas are good. There is already a commercial system that does that, I think it is a new design innovation vs. their original - the American Hydroponics lettuce and basil HDPE channels. If you read carefully you can see the original design was for a capped irrigation end. Now, the new end is widely open on the top half and only a barrier where it is important for water contact. The explanation is for 'better aeration'. It is really principally to have free wind currents traveling from one end to the other, which is basically as effective as using a fan and utilizes evaporation from the nutrient solution to keep the channels cooler. One of the reason they prefer to live with algae in that system is to a large extent caused by the new open ended design. Another commercial system that does that actually does use a fan over a DWC, it is called Turbo-something if I recall and really rips across the water (seems too vigorous to me in their design).

Another good idea to add to our lists which goes along with the styrofoam inserts, is that of simply putting a channel cover, maybe in two halves with half pot-hole each if ease of removal is important, perhaps including the sides over the inner channel, and even a bottom too (a jacketed channel) in an extreme case. No problem for that to mount the net pots in the top one like normal and there could be a few spacers to keep a layer of air between. The intermediate air channel could be open to airflow freely.

Back to your original point about attic insulation. I think it would be worth trying to swap the glue you use for something like this, if it can adhere and is sufficiently water resistant.

PU spray can foam

Thanks for the discussion and please do add any ideas if they come to you. On balance your glued foil seems to me like the system to beat, and it is looking harder and harder to keep it simple. Here's my fun entry: Use your method plus an intermittent mister above during the heat of the day. Water absorbs heat much better than air or PVC for that matter. Putting that evaporative cooling on the surface of the aluminum will turn the channels themselves into evaporative coolers without touching the nutrient solution water and keepiing the plants happy. I wonder if anyone has patented a mister ;-) ;-) ;-) The efficiency outside the greenhousein a situation like this is much greater. If there is a problem with aluminum, maybe a UV reflecting clear coat over the aluminum which will give it a hardened exterior as well, or just using strips of space blanket instead on the poor man's version. Large space blankets are very cheap on eBay, I guess it just needs to be thick and 'aluminized' enough.

Space blankets are basically crinkled mylar. I would say it is as opaque as aluminum and a lot more tear resistant, too.
The space blankets work so well because they don't make smooth contact to your body and thus reflect the radiant heat your body emits back onto yourself.
I had some plans to build an "A" frame and wanted to stretch aluminum below the troughs to block the radiant heat from underneath the frame (where the rez would be located)
I had scrapped the idea because of the ease of tearing aluminum. Maybe now I'll revisit it considering mylar.

I don't like to tolerate algae myself. if it grows big enough it will block up your pumps (or clog a filter leading to said pump) If I were going to "aerate" the inside of the troughs, I would probably add a 6" to 8" extension off the ends to restrict how much light gets to the water. Of course I don't see it as being reasonable, at least economically. I like the idea of evaporative cooling much better.
Since you mentioned mylar (It had not occurred to me) if you find a comparison of reflective values and conductive values against aluminum foil, let me know. It may be simply using mylar in lieu of aluminum isn't a bad option.

Unfortunately as we are all well aware, there is theory and there is practice. I didn't want to muck this up with details LOL more than I already did. When I mention 90% reflectivity for the best white paints and kitchen aluminum foil, that's theory. That 10% of conductive heat that comes in from the the aluminum foil that bothers me is thus also theory. Let's talk mylar and foil and departures from theory since that discussion is now necessary to compare the two.

In practice, the aluminum is probably providing only about 70% reflectivity and 30% of the heat is going in. Aluminium deposited on mylar new also gives about 90% radiant reflectivity in the best case too.

On the surface it would seem a builder's option. First, Grizz you are talking about quality space blankets. This is not really quality, but considering economics it might scrape by:
HD Medical Space Blanket

Many of the cheap Chinese mfgrd space blankets are quite transparent although they don't appear so from a distance.

Aside: Perhaps double layering will work, the outher layer will take the brunt of UV and random tear damage and could be glued only along the bottom which would effectively add airspace between them. Or for that matter the bottom layer could be foil or paint.

The mylar will last about three months in the bright sun before it starts yellowing significantly. This is why I have been looking for info on whether anyone has used a UV protective clear coat hard shell finish on top - would likely solve the problem. Aluminum foil on the other hand until damaged will continue to block light completely... although polished 90% aluminum very quiclkly formas a microscopic oxide layer which cuts this to about 80%, and then it can corrode usually into a white, not very reflective oxide which is only 1/7 as heat conductive as aluminum.

As far as conductive values (not speaking of reflection for anyone else following this) of mylar in just single layer, it is 1/4 as conductive as PVC but twice as much as air. It still suffers from the same problem as the aluminum in alumimum foil from the reflective surface (put for us on the outside instead of using like a space blanket to keep heat in where it faces inside) in that the aluminum will heat up and conduct the same for all the energy that goes unreflected. Where mylar beats that though is that the incredibly fine layer has no foil backing to pass the heat since the same radiance is hitting it, there is very little capacity to hold heat for transfer which is a slower process, the deposited Al simply reaches temperature quicker and transfers it out more quickly. It would be a ratio of 2/3 of the non-reflected head going into the pipe and 1/3 going into the air but the chemical microthin deposition will dump the heat back out in the convective air currently much more quickly. On a microscopic level what is happening is say the mylar goes up say 10 degrees and even though the plastic backing interface feels this, the actual heat flow is stymied because you get an even gradient on the plastic side while on the air side the full 10 degree difference is maintained (heat flow rate is in direct proportion to the temperature difference, so conductivity is obviously not the only thing to consider in a microscopic deposited layer like this).

In summary the mylar plastic will suck up 2 X the heat in dead still air (=1X) (Vs thicker foil which does 7 X), but the slightest infinitesimal breeze will double or triple the interface temperature difference between aluminum and air vs. what is being experience on the plastic side. Having triple the temperature difference in this example will triple the heat flow into the air.

A good point to remember was your advice to Rio when he suggested using Heavy duty foil, and you said for yourself the cheapest stuff was best. You also have the same physics supporting you. HD Al would significantly add to the internal temperature of the pipes as in full metal jacket. Imagine the temp if the channels were made from aluminum with a PE liner... Here, I could cook eggs on them in Summer.

I mentioned all the above to sort of show IMO how it will be hard to get hard numbers to compare for these reasons. However it is absolutely clear from this that new mylar will cut down the transmitted heat by a factor of anything from 1/3 of foil to for our purposes all of it. 3-month old mylar ... I have the suspicion that as we reupolster we will be wondering why we didn't KIS with the foil.

That's why I think a two layer mylar with a sacrificial exterior might help. Clearly there are engineering issues with all of these things but until someone actually has done a little enthusiastic tinkering I think that is about as far a ledge I can walk out on !

Aluminum is not without its problems. By the time it is well broken in (but not torn up), that 30% turns completely into heat, so this is the drawback of aging foil cald channels. The mylar will drop down to 70% or even less, for argument's sake as well. There is no clear winner unless you don't like having to do frequent reupholstering.

This post was edited by PupillaCharites on Thu, Sep 26, 13 at 16:47