I suppose the aluminet might reflect heat back into a GH, but it will also reflect sunlight away from it, and during the winter months when whatever light and heat the sun brings is a benefit, that might be counterproductive.

On the other hand, what applies here, where winter temps drop into the teens and overcast days can hang on for weeks on end, may not in a zone where deep freezes are a brief and somewhat rare event.

It could be that the advice I might receive on the subject from the folks at the U of I extension service may not be what you would, from GT. It would be interesting to find out.

70% Aluminet has 30% transmittance. It will reduce the PAR wavelengths that enter your GH by 70%, thereby reducing the light energy available for plants and heat by 70%.

I suppose the only way it could work in reverse is by working as a solar curtain. That's a lot of work, because, by my understanding, a solar curtain goes up at night and down during the day.

I take my Aluminet down right about now and replace it with an extra layer of poly (80% PAR transmittance).

SB

SB, I can tell you without hesitation that my Aluminet will NOT be going on and off daily, no way no how. I will have to do some further research because I do recall reading something about it that proclaimed it would work to keep heat in...seems lllogical, but I"ll get back to you all if I can find what I read about it.

Howdy rjinga, this might be one of the threads you're remembering.
Greenhouse shadecloth

I have a post in that thread with a link to a website that caused me to be confused on the same question. The website does mention Aluminet being used to hold in heat, but cuestaroble was kind enough to explain that the website was likely referring to the practice of installing the Aluminet on the inside of the greenhouses like a big ceiling drape or curtain -- as some large commercial greenhouses do. Some big greenhouses have systems that allow the "solar curtain" (that Stressbaby mentioned) to be pulled open during the day, so light can enter and warm things up, and then pulled closed at night, so the Aluminet can reflect the heat back down into the structure. Of course, this kind of application doesn't apply to our little HFGH hobby greenhouses, where most of us go through a lot of effort just to attach it to the outside in a sturdy way.

I think this is one of those confusing issues that varies a lot depending on where we are, how intense our sun is, and what kind of plants we grow in the GH during the winter. Last winter, I had my 40% Aluminet on the inside of my greenhouse (trying to avoid wind problems) and the light level was just right for my plants. However, we have intense sun here, even in winter, and I'll bet in other climates it would block too much light.

Right now I have 50% Aluminet on the outside of my GH roof, trying to better protect my HFGH panels from UV deterioration problems. During the hottest part of our summer, I also had 40% Aluminet draped on the inside of the roof as well. (As you can tell I am still experimenting and learning.) Now that the sun is less horrific, I have the interior 40% layer down, and I'm still deciding if the remaining 50% outside layer will block too much light for me during winter. I think it might, but I'm not sure yet. I'm not looking forward to messing with my shadecloth either...but I'll probably be out there on a ladder saying bad words and making adjustments.

I haven't found an easy answer to this one yet...the sun intensity changes so much here from summer to winter that I am pretty sure I can't escape making shadecloth changes to accomodate the seasons (unfortunately.) I'm hoping after I get another year or two under my belt I'll have some kind of a system figured out.

mudhouse writes: "However, we have intense sun here, even in winter, and I'll bet in other climates it would block too much light."

Exactly. My 60% aluminet shade cloth is coming down this week and the GH will be uncovered long enough for us to get it power washed before it gets too cold for us to get soaked without overly chilling our old bones, and may stay uncovered for a few weeks- or not.

But the first day we have a frost prediction, up will go the solar pool cover that along with the inner lining of heavy bubble, is more than enough to retain heat, yet allow light into the GH. I wouldn't get enough light in there this far north if I left the shadecloth up all through our dark, dreary days of winter, when we may not see the sun for weeks on end.

So it does seem that the only rational way to determine how you use a shadcloth really does depend on what intensity of weave you need and when you need it, to accomdate both what you grow in your GH and- where you live.