Perhaps I have found enough answers to use radiant barrier reflective aluminum insulation, at least in places. Because my first priority is extending the growing season by primarily passive thermal mass water containers for heat, sunlight from the south glazing must shine through to them so they will absorb heat. If I place aluminum reflective foil on the north wall behind the dull black drums, then will this reflection when it hits the back of the drums, be absorbed as heat?

The aluminum foil shields that are commonly inserted behind radiators in older houses are radiant barriers, blocking radiant heat transfer from the radiator to the exterior wall. In other words, the heat is reflected back into the house.

In my house, I have a window ledge that I placed seedlings before. If I didn't close my white backed thermal curtain, the seedlings will bend toward the outside where the light comes from. If I close that curtain, the seedlings grow straight up, with not a tilt of their leaf direction anywhere.

Here is more useful info that I found:
Radiant barrier foil insulation systems BLOCK radiant heat energy instead of absorbing it like fiberglass insulation. A pure aluminum radiant barrier reflective foil insulation is unaffected by humidity and will continue to perform at a consistent level no matter how humid it may be. A radiant barrier foil insulation system is a layer of foil facing an airspace and is installed in the envelope of a building.

No, I don't sell the stuff. I am wondering, if it sounds good on the north wall behind the drums (and that is 'if'), would I want normal fiberglass insulation everywhere else, to absorb heat? Or radiant aluminum sheets everywhere else to reflect back heat? These two insulations (and yes, aluminum is not a true insulation but a reflector) seem to be opposed to each other in purpose.

To go completely off the wall (or on it, he he) with thinking, if I placed radiant barrier foils in every place where there is not glazing, would I have reflection from one to the other and increase light but not heat?

~Tom

To go completely off the wall (or on it, he he) with thinking, if I placed radiant barrier foils in every place where there is not glazing, would I have reflection from one to the other and increase light but not heat?

The amount of light and heat entering a greenhouse is purely a function of area of glazing and its properties. Reflecting light from a wall increases the chance of the light hitting a plant rather being absorbed by a non plant surface and is thus a good idea. But the amount of heat in a greenhouse is going to be roughly the same with our without the reflection.

Most of the heat in a greenhouse is from light being converted to heat.

Thank you, Nathan. So, placing a reflective radiant barrier behind the black drums on the wooden north wall and / or northern facing roof will do nothing for increasing, decreasing, or even retaining heat. It sounds like only R rated normal fiberglass insulation would make sense if my primary condern is retaining heat in fall and spring.
~Tom

Didn't I see a recent post where snow on the ground increased the reflected light into the greenhouse thus spiking the temps? I noticed my temps went higher than normal when we had a freak snowfall last week.

Wouldn't an aluminum or sheetmetal wall on the North side of the GH contribute to increased temps as well?

Nathan is correct in that aluminum 'insulation' will block IR by reflecton but it does not emit hardly any heat. He can explain it with his background and education better, but my question really was if this reflection was being hit by dull black drums, would it be absorbed as heat just as if the IR was direct through the windows hitting it? I think he said it all: "the amount of heat in a greenhouse is going to be roughly the same with our without the reflection." In other words, the amount of glazing I have isn't being increased!
~Tom

Yeah, your heat is mostly coming from visible (not heat) light coming in through the glazing. Increase that and you'll increase the temperature. Once it is inside, it will tend to bounce around until it gets absorbed by something. Of course, if you reflect the visible light back out of the glazing you'll lose that heating capacity.

Outstanding point that escaped me: Escaping light from the reflection back outside!!! It probably makes more sense that if any shiny surface were used, that it reflected the light in the right places. For example, if the glazing on the south wall were at 45 degrees, some light would be reflected out the glazing from a 90 degree north wall reflection. It would be like playing billards on a rotating service trying to keep up with the sun's movements. If I want the warming rays, I cannot reflect any of it out. Because I want passive heating and not concerned with cooling, a radiant barrier makes no sense.
Thank you all for helping.
~Tom

Don't give up on radiant barriers - they are the cheapest way to get an extra R1 (US R6). Just put something to absorb the light inside the greenhouse.

Smoke and Mirrors! ok, I've found these solar guards at Wal-mart that go inside car windshield that reflect the sun back out. They had a large size that was by far cheaper than others for $2.88 U.S. dollars. The size is 27 inches by 67 inches and they only have aluminum reflection on one side. So, I bought 3 of them and I'll afix them in places that will reflect back on the drums, and missing those, light will reflect toward the shelves. I'll make them removable so that I can indeed reflect light back out the glazing in the summer and reflect inside the solar shed during the winter. If this experiment goes well, I'll spring for the price of some brand named, like Reflective insulation and do it up right. I'll post back soon my early results and if I can figure out how to post a picture, I'll do that too.
~Tom

Regarding foil, reflective barriers/insulation, this is counter-intuitive to me, so I thought I'd best post this:

Face reflective surfaces downwards or keep them vertical. The 'anti-glare' surface of single sided foil sarking should always face up or towards the incoming IR.

I wonder how many people have done that backwards with the shiny material facing the sun? I would have if I hadn't been stuborn and kept reading about radiant barriers. It seems to be overlooked or an afterthought on many pages.

~Tom

I am so confused. Here is another website with exactly the opposite information. This copy is talking about laying it on top of existing insulation in the attic, but the instructions are oppossite of some other sites in which side,shiny or dull, do you place the reflective material.

'Care must be taken during installation to insure that the reflective side is facing up, and that the radiant barrier does not actually touch the insulation.'

facing up? this would mean that the shiny side faces the IR incoming waves.

Totally confused, totally inept~

~Tom

Here is a link that might be useful: Do It Yourself

Tom,

"...this would mean that the shiny side faces the IR incoming waves."

That is a good thing if you are trying to keep your house cool during the summer. And a shiny metallic surface is the opposite of a black body, so it doesn't emit much heat as radiation either, which is also a good thing if you are trying to keep your home warm during the winter. A house is not a greenhouse.

For the same reason, a vacuum bottle is shiny on the inside to keep heat from radiating across the thermally non-conductive vacuum gap.

MM

You just want the foil to point into an air gap. Even better if it is small enough that air doesn't convect inside. This means that given a choice between foil towards a big space and foil towards a small space, the towards small space is the better option.