We've just been discussing this in the last couple of posts to the "Power Lost to Greenhouse" thread.
Agardenstateofmind and I have both agreed that the clear pool cover hardly blocks any light at all....everything continues to bloom, fruit etc. with no internodal stretch. There are lots of other folks on this forum that have pool covers also. In particular, look up Cactasfreaks My Page where there are great pictures of their greenhouse with the cover on and the plants they have growing inside.
Whatever small loss of light there is, is more than offset by the amount of extra insulation and retention of heat you get.
My greenhouse is glass so there is no loss of light from the first layer at all. Because you already have two layers of poly which block more light to begin with, you may lose more light altogether than I would. However, since you already have the poly, that might be the way to go.

themaniam1

Actually, that's exactly how our GH is set up. Double poly inflated with 3 layers of 1/2" bubble wrap sandwiched between the poly.

Works a treat.

Sure, there's some light loss, but like wyndyacre stated, the tradeoff is worth it.

We are able to keep the GH nice and toasty on the coldest nights and as for light, well, the plants sure don't seem to mind a little light loss.

The banana trees are growing, the palms putting on new fronds, plants are flowering, etc.

But, only for simplicity, I think we're going to the clear pool covers for next year.

A local supplier can supply 12'X24' sheets for only $55 CAN$ and the plastic they use is 11 mil. We'll either buy 2 sheets or perhaps 3, depending on light loss and insulating value.

We tried 5 layers last year, but there was too much light loss, to the point where the GH wouldn't heat up during the day very much. We had 3 the year before and have gone back to 3 this year.

It's working out well.

Thanks for the replies! That is what I needed to know. I am glad that the light lose is not enough for the plants to care. Thanks again, my heating bill will be less because of your replies!!!!

Oh, did I mention that there's a 10% surcharge for this info for saving on your heating bill? LOL ;)

According to Cactusfreak, the person who got us all started with solar pool covers, the pool cover transmits 80% of the light. SB

if light is really important, perhaps you could use a thinner material on the south side? say single layer polyethylene?

The other thing to think about is that snow reflects a lot of light, as does water or ice at low angles. Perhaps you can increase the light with a flat patch of snow or a pool on the south side? A pool will also provide some extra buffering capacity and some cooling in summer.

South side? are you kidding me! LOL ;)

The stupid design of your yard makes it impossible to have the GH positioned in such a way as to have the side of the GH facing south. Unfortunately, the rear end of the GH is facing south. It has foam insulation, butts up against a fence and on the other side of the fence are tall pine trees! :(

The sun hits the GH in the morning and later afternoon. That's all I can get. If I even do decide to tear down the garage extention and put a permanent greenhouse there, then it would get morning to late afternoon sun. But then I'd also have to move the palm trees, that are in the ground where the GH is now. THAT is more than I can do. This is due to a physical limitation that I have had for over a year now. :(

Does anyone think that putting some bubble wrap on the inside of my greenhouse would help? Not that it is too bad, and i am in the south - but still, every $$$ helps.

My GH is built of polycarbonate - one layer though - all the way around -

I actually need something on the roof too - to cut down so i don't get QUITE as much light in there - it burns some of my plants in late fall and early spring (when too cool to be out of greenhouse for good yet).

And, I need EASY ideas - sorry, but getting a divorce - and it is "on-going" - so it is just me to do stuff - per my D attorney - no men at the place - even though STBXH lives with his G/F.

Here is a link that might be useful: pic of greenhouse

Rosey-lots of people on this forum have used bubble wrap on the inside of their greenhouses for insulation. You could easily cut sections or use a few long pieces-it doesn't weigh very much. You would just need a ladder as I see you have a fairly large greenhouse.
I have to put up some kind of shade cloth in my greenhouse also even tho I'm much farther north than you. There is an "official" shade cloth material but I just used a large piece of very sheer, spun polyester material that is used for crop row covers and tacked it to the inside of my ceiling. Very easy to pull down for the winter and put back up in the spring and cheap too. I used double thickness to get the amount of shade I wanted, it is quite sheer.

Here is a link that might be useful: Lee Valley Row Cover

THanks bunches wyndyacre. Yes, it is very large - i got all of my mom's plants after she died, used some of the life ins benefits to build it - my STBHX did it, and the one thing he did wrong (on this anyway! :) ) is had the roof trusses built - and there is a LOT of wasted space up there - wish i had just done a slanted roof (one side) - hindsight, on a lot of things.

I will definitely get me some bubblewrap and put up on the walls for insulation. guess it is time to post on freecycle! :)
How long does the bubblewrap "last" up there -

1 more question - suggestions on how to put the bubblewrap up - and keep it up? Gray tape? That is MY answer to everything!