Glass for greenhouse project

I may not get to build my greenhouse this year but I’m certainly putting down the sonotube foundation. It will be attached to the house, south facing, 20’ by 9’ deep so I need 4 sonotube pylons in place to hold that side.

Besides foundation I am also collecting glass. It is funny how glass is no longer used because of the expense, except if you take your time you can pick up all sorts of free glass; my favorite is somebody getting rid of old storm windows. I just picked a stash of eleven 4’ by 2’ 1/8" single pane storm windows for free, to add to my growing stash of glass for said project.

The question is what sort of glass I want to use… This is for a 3-season greenhouse. It will be warmer than ambient air by virtue of it being attached to the house (even heat radiating from the ground will have an effect) but just keeping plants off the sub zero 40mph winter winds and gaining me a few weeks in spring and fall is all I want.

I could:

  • Use 1/4" glass
  • Use 1/8" glass
  • Double pane 1/8" glass.

Which one of these makes the most sense?

I plan on standardizing my glass pane sizes so when I send a shovel through a glass I can just replace it with a spare. I figure having 5~6 extras should last me through a few accidents.

You don’t want anything that reflects back or absorbs blue wavelengths - a very common feature in picture window glass.

I had to special order my glass for my south facing windows because I wanted solar gain. Pretty much all U.S. windows come with reflective “low energy” coating. You may want to research how to tell the difference between the glass types. Maybe storm doors don’t have the coating?

I seriously doubt it. Storm windows, which is basically a sheet of 1/8" glass on an aluminum frame, are such a low tech old technology that I don’t think much else went in there.

I do plan on ‘frosting’ the top side for diffraction. There is this spray paint that is used for privacy; it gives the window a frosted look that makes it hard to see inside. The practical outcome here is that it diffracts light which in our intense Alaskan summer sun I’m sure will help.

My dad was the king of old sliding glass doors. In my farmhouse I have 4 bay windows with them. He built a crude greenhouse and a cold frame as well. I picked up a half dozen or so at habitat for humanity years ago. Thick glass maybe lasts forever… im a cheapskate though.

I’m not sure how much standardization you’ll be able to do if you’re scavenging all your windows. I’m not sure how much choice you’ll have either. I don’t think any common residential windows or doors even use one quarter inch glass, except for maybe extremely large ones. If you just want a three season greenhouse that keeps the wind off then I just go with the cheapest glass. Which will be single pane 1/8 obviously. Except that you should probably talk to someone about the snow load that 1/8 inch glass can take. I’m guessing you can have some pretty high snow loads in alaska. Also if you get lots of hail, 1/8 in might not be enough.

1 Like

As far as standardization goes, Don could design it so that the weird sizes go on the roof which should be less likely to be broken by a shovel.

I inherited 32 sheets of sliding glass doors 34 inches by 74 and 12 sheets of 46 inches by74 inches.

4 layers thick 4 feet deep 16 feet wide for the big one. I also collected about 4 double glass plastic framed windows. I don’t cut glass.

I’m stripping the glass from whatever frame it may have and cutting it down to a standard size. For instance the last stash I got a hold of is about 2’ by 4", I may standardize my custom made panes to say 2’ x 3’, if that’s the size that can give me the most panes out of my disparate stock of glass I have.

The snow load is a concern that I need to figure the math for. Hopefully it will boil down to roof angle and glass widrh; my guess (that I need to verify) is that a 12" wide 1/4" glass can withstand a load better than a 24" glass, so the roof glass can still be a 2x3 pane but using two pieces of glass on a metal frame. Everything would still be 2x3 (or whatever size I end up standardizing to) but two pane standards; one for walls and one for roofs.

Snow load on a greenhouse melts really quick. Personal experience.

You are going to love your attached ghouse!

I have similar-sized attached south-facing ghouse, a 40-year-old structure that has experienced only 1 inadvertent shovel crash! The sloping windows are 1/8" single pane and roof windows are double-paned, extending halfway from the front of the roof.

If I had to do it over I would have the roof windows cover the whole roof AND not use double pane. The initial reasoning for a double-paned roof was less heat loss, but, over the years, the double panes have failed. They now have moisture inside and are somewhat cloudy. Also, in the current half-roof configuration, in pre and post season I feel the need to use lights on the back wall to boost light level. A fully glassed, single-paned roof would have been better.

I close the ghouse off from the house in the coldest months but, because of attachment, the temp inside ghouse does not fall below 36F, even in an unusually cold year with an outdoor low of 7F.

Before I retired, I’d come home to a 95F ghouse even with the exhaust fan running for hours. But now that I’m home I can keep the ghouse/house temps in equilibrium, the house benefitting from the ghouse solar gain and the ghouse benefitting from the house as a heat sink.

You will love it.


I don’t have any useful input on glass, but consider reading some stuff by Eliot Coleman to make use of it in winter. He uses unheated (or barely heated) greenhouses to grow salad greens all winter long.