I made my first GH with old church basement windows about 30" X 36". The glass was the kind with chicken wire imbedded and about ¼" thick. It was not frosted but it was the kind you cannot see through with detail. All the plants did excellent and it was easy to heat. I am sure the polycarb has a higher R-value but the payback time might not be worth it. If you choose to go with glass, I would at least use tempered glass on the roof. There is probably more glass GHs in use than polycarb or plastic film put together.

John

Here is a link that might be useful: Johns Journal

Thanks for the advice John.
Tempered glass is a good idea.
I think this will end up costing more initially, but in the long run this thing will be around 50 years after every HFGH has bit the dust.

"last consideration is R-value. It may not be as good as 4mm poly."

Single pane glass, no but thermopane- no contest.

Check out the prices on non-operable thermos. You will find them a lot less costly than you may think, especially as you don't need any fancy trimwork. Then think about how many you really need to be operable.

One of the best windows available for lower wall venting are the awning style. Buy them and your non-operables in solid vinyl and you will never need to paint.

You forgot another advantage of glass: Whitewash sunshade. Spray it on a white stained, wood framed framed, vinyl framed glass in late spring, and power wash it off in fall.

Also, with glass, you will be free to power wash your GH with the most effective yet cheap cleaner to remove both dirt and mildew: bleach, with no harm. All you will end up with is a sparkling clean GH, inside and out.

Glass is a great material, and is a good choice for longevity, clarity, versatility and structural strength. If you are building something "in town" or you want to attach to the house and blend in well from an architectural standpoint, it seems that glass is without a doubt the best choice.

For my particular situation, glass isn't the best choice. I'm using single layer woven poly on my three greenhouse structures for several reasons:

-Inconsistent framing on a salvaged building turned into a greenhouse is easy to secure with a poly cover.

-Curved surfaces on one greenhouse won't accept glass.

-Poly isn't heavy nor dangerous to install by myself.

-It isn't prone to failure with hail. We have more hail here in Cheyenne than anywhere else in the country. Cars get dented, roofs get damaged and windows get broken.

-A single piece covers an entire building without any need to frame each pane.

-Much lower initial install price, and about the same cost over the life of the buildings, even if I consider the cost of replacing it several times.

-The cost of the entire structure and poly glazing is about what glass alone would cost.

We have one more greenhouse to build and it will be a wood structure attached to the barn. I'm thinking of using poly carbonate either in rigid sheets or rolls.

Clair

Here is a link that might be useful: My homemade greenhouses

We are using an old Janco with single pane glass. We had to install a wiggle wire track and poly inside because the heating cost was so high. It was just like heating an open space.
Since we added the plastic on the inside though, causing a dead air space, it's helped TREMENDOUSLY. Most of the winter we can heat it with a couple of little electric radiator type heaters, kicking on the gas one only when it goes below zero.
If you have any tree limbs, etc. Glass can be very scary. We have already replaced 2 pieces because of storms/flying limbs. The trees aren't all that close to the g/h but they really fly in the wind.
You need a really good foundation for glass. As opposed to poly that will bend and flex, glass isn't very forgiving.

John, I have to disagree with a point you made (in a friendly fashion, of course): There are far more polyfilm (Mostly polyetylene) structures in the world than glass structures. For agribusiness, glass greenhouse are more or less relics of the past, by the middle of the 80s most greenhouse acrage was under polyethylene, due to it's low costs of production and improvements in it's light transmission/cold tolerance/brittleness. Also, it imparts negligible weight to a greenhouse superstructure, allowing buildings to utilize thinner structural members, and thus cast less shade on the contained crop.