Double walled polycarbonate vs triple walled?

Hello,

I am building a Walipini in zone 7A (NW Arizona) and am trying to determine best glazing material. I want this to be a 4 season greenhouse with minimal to no additional heating. The dimensions are estimated at 45'x55'. It can get down to -10f in the winter (although warms back up fast during the day) and windy in the spring.
I am currently looking at the double and triple wall poly material. They state 75+% sun penetration with UV protection. Has anyone tried both? Thoughts?
Any other suggestions?
I also would accept advice on what materials to use for support. 45' from ground to peak is rather long.

Thanks
Jim

I've been living in high desert in solely passive solar heated buildings for 20 years. But our attached greenhouses are removed for the summer, and I'm a big fan of that. Overheating in summer is a real issue with a permanently attached greenhouse. But an attached greenhouse is wonderful in winter.

I have some opinions on the following materials that we have used:

1) Flexible UV resistant plastic film. This is available at a subsidised price here because the govt supports the production of vegetables. We've had two types. They tend to last about 5 - 8 years if we attach and detach them, and actually longer if left in place with a firmer attachment system. They flap in the wind, which can be annoying or even alarming, and one flew off in an unusual gust, taking its wooden frame with it and breaking some windows on the building above (after having sat in place for about 10 years). But generally this material is the one we use on all our removable greenhouses. After the corners rip and it's no good for another winter on the building, it makes a very useful tarp for a few more years.

2) Single UV-resistant polycarbonate or single glass. I like single glazing best for places where you want to see out the window. We use glass for our windows, and polycarbonate for doors and skylights. Personally i like single rather than double or multiple for the reasons below. I like insulated curtains on winter nights, and clean single glazing during the days.

3) Five-walled polycarbonate. The advantage of this is obviously the increased insulation. We have it on some of our skylights. The disadvantage is that it's hard to seal the ends perfectly airtight, so it starts to get clouded up with dust that blows in during gusts, or bugs that get in.

4) Homemade double glass. Similar issues. Dust gets in and sticks to the humidity that also gets in, and anyway glass does eventually accumulate a cloudy layer. And with double glass, you just can't get in and clean it without disassembling it. And let's not even describe the times that it became the fashion in a nearby hive of paperwasps or something, to crawl inside, get stuck in there, and desiccate one's little body as an art exhibit for posterity. Bleah!

I like used patio door glass because it is strong, safe and free. To me, greenhouses are season extenders that are useful in spring and fall. In summer, they are a great place to heat a wading pool for evening use and in winter, they are the perfect kiln for stored firewood. Even in winter, a greenhouse that isn't watered will get very dry.

We grow stuff in most of our greenhouses, so they don't get too dry. Our greenhouses are both season extenders and the sole source of heating in winters that go down to -25C on January nights but have mostly sunny days.

In the greenhouses where the planted area is similar in square footage to the floor space of the attached building, they do get pretty humid. On a cold winter morning the day after watering, you get rained on if the plastic jiggles when you're inside (due to wind or a prankster). In the attached greenhouses with limited growing beds, there is not enough humidity to seem like an issue. Even in the building whose attached greenhouse has nothing growing in, it, I never get that parched feeling I get in heated houses in the NE US, but I don't know why not.

The garden beds under winter greenhouses tend to be VERY productive and use MUCH less water than the same beds in the open over the summer.

Thank you for your feedback. The estimate sq FT of the roof is 2500sqft. Way too big for me at least to consider glass. Although the plastic film is probably the cheapest and easiest to install, I would be afraid of the heat loss. Also, not concerned on crystal clear views out. I will be doing Aquaponics along with traditional growing. Thanks for the heads up on humidity. I still would like to consider double or triple walled polycarbonate.
Lately I have been considering also using a RMH for heat. Maybe this can offset any R value lost from other options for glazing. .

Roof? Please don't put glazing horizontal or close to it.

South facing vertical glazing has this wonderful almost magical solar geometry: In summer the sun is high overhead and hardly shines into the south facing glass. In Winter, the sun travels lower across the southern sky and shines deep into the south facing windows. Horizontal glazing or, in our case, an unplanned dining hall with a low-sloped roof, are really problematic: overheating in summer and only losing heat but not gaining in winter. I love those pictures of earthships, but the sloping south facing glazing looks like it would baaaaadly overheat in summer. Please do consider it. Overheating is really no good at all.

You do know what a Walipini is, yes?

I had been trying to source old windows etc, but was unable to get a quantity, so decided to bite the bullet and go with the triple wall polycarbonate. I've just recently finished building it, I'm trying to get a few winter salad crops in at the moment!

http://youtu.be/sZlue3qf-sk

Hope you enjoy the video.