Using tabletop glass in a window

I'm in the planning stages of an earthbag/ferrocement dome home.  For giggles I looked up the price of a conventional 48" round picture window.  The multi-thousand dollar price tag has me seeking other options.

My local thrift store has a 1/2" thick glass tabletop for < $100.  Could I build a round frame for this, build the earthbag walls around the frame, then use silicone caulk or something to install this tabletop into the wall as a window?  Apologies if this has been asked before but I couldn't find it by searching.

Yes, you can.
You might check a Habitat for Humanity Restore for used windows, I have seen some like what you are trying to make. They get removed out of places and sold cheaply. Worth checking.

But yes, tabletop glass will work. Be warned, it wont be insulated at all (being single pane) and round is tricky to make drapes or shades that fit tight. Also it wont have on it any of the Low E coatings etc. If this matters to you, don't use table glass. A plus is that's lovely thick glass, won't break easy. A minus is if it ever does, it won't be easy to replace.

I suggest if you do this, make the wall frame for the bags, and make a separate frame that holds the glass. So you end up with a framed window that can be installed in the framed wall. Few less potential issues that way.

Thanks, Pearl!  You brought up several things I was mulling over.   I should have mentioned that I intend to use two tabletops and have a double pane, with dessicant between the panes to catch moisture from my inevititably non-airtight construction. My plan was to leave the frame in the earthbag with a lip/silicone seal, build the windows inside the dome, and push them through against the silicone seal.  Then provide a seal on the other side and attach the window.

One thing you mentioned I've never heard of:  Low E coatings.  I'll look into that, thanks.  Also I'll check out ReStore!

edited to add:  this article makes me think seal failure in IGUs is not that dire, but maybe it's rose coloured glasses:
https://www.thespruce.com/how-to-tell-if-a-window-seal-has-failed-1822894

Well thanks again for that suggestion.  I read here that I can apply an alternative film:

Does window tint work?

If you don't want new windows, sun-blocking window film or window tint is a good alternative. Compared to low-E windows, the overall cost of installing tinted film on clear glass is less expensive and newer version of window film can also help your home retain heat in the winter.

This is a weird suggestion, out of my head, without data to back it up, so look it up before you do it...

If I were going to make a big double paned window, knowing the earthbags etc will outgas water, I'd put weep holes in the window frame, so there's a bit of airflow between the glass panes. Not a lot, but enough to let it go someplace besides fogging up those big panes. Like drill a couple of holes with a 1/8 bit that reaches between the panes.  Yes, you would lose some R value, but that big of a glass window isn't going to have a great R value to start with, even double paned.  No glass does. Attempting to keep it sealed well is probably not a realistic long term strategy, if that article says you can't expect the expensive ones to hold the seal, how can you expect your silicon to hold? And wanting to fix that window if it fogs up would be a serious PITA. So I'd assume drains would be a good thing to have in place, before it fogs up and makes a mess. I think if it were me, I'd open the holes to the inside of the house, so the air that goes in is house heated, not outside cold, and air that goes in is only ambient wet, not rain wet.
I doubt a desiccant would be useful, but that again, is just out of my head. Seems to me if that worked, they'd do that instead of sealing them, less work.

Like I said, look it up more, this is out of my weird head, opinion with no solid data to back it with, just a lot of odd experience.

My personal thing, if I were building your house and wanted a lot of glass, would be to get a collection of smaller windows from Habitat, and arrange them in a pattern I liked. Then if there's issues, it's less catastrophic, and would be more possible to replace the windows without needing to mess with wall structure. More visual drama in one big one. More resilience in several small ones. I'm into resilient. And how weird is the wall structure above a 4 foot round hole? Smaller ones don't destabilize the whole wall if there's issues.

:D

I really like your ideas.  Thanks for helping me think this through.  I might mock up a window and put it outside in a brick cube "house" to test what happens.  One with the weep hole installed and one without.

You mentioned that the hyperadobe outgasses moisture, which is definitely a consideration.  I'm planning to create the window as one discrete unit, probably lined with aluminium flashing or some similar metal ring between the panes.  Like a shallow coffee can with a window at each end.  All sealed as best I can with silicone and aluminium duct tape. Basically I'm thinking of taping the heck out of the thing to seal off any air, with the dessicant tastefully enclosed in ribs or some such.  No one will ever see the outside anyway.  The dessicant is a way to control the moisture that will exist when I build the window, and hopefully none will ever enter again.  This is my theory.  Ahh, theories.

Now what you say about the support structure is really important, and I admit I have been laissez faire about that.  My idea for that was to create an arch from the foundation, around the window, and back down, and run a couple of hyperadobe courses up and over that arch.  But that's a really half-formed idea so I welcome any thoughts.  I really like the idea of one round window focal point and I'm not ready to compromise yet. Even though I totally get what you are saying about the easily replaceable windows, and I am pondering that too.

I have used them and shower screens as well on roofs and walls

Check recycling centers and window replacement contractors (they have to pay to discard it) for old window glass.  These days double paned material shows up - some are no longer useful but others can be fine.  You may not find exactly what you are looking for but you also sound quite resourceful :>).  Good luck!

A mock up is a very good idea! Experimenting is always wise before making structural decisions.

I don't know what your house design looks like, so I might be off base here, but I'm still thinking do double framing, a frame that holds your wall stuff, and a frame that holds the window, two separate items, that way you can break the functions up easier. The wall frame would just have the function "support this wall hole and the wall around it" and the window frame would have the function "hold this window securely and hopefully dry." I think trying to make one item that does all those functions would be serious engineering, and have more potential failure points. Then your wall frame can support the arch properly, and not risk wall failure.

Occurs to me one other thing that two frames would do is let you build the wall, let it out-moisture for a while (depending on circumstances) before you add the window in, so there is less moisture to potentially get into the window.

And I agree, a 4 foot round window would be a AWESOME design detail!!

Edit: one more thought...
What I'm envisioning, I think, is basically a structural arch that the window is incidental filler for. Might be worth your time to look up arch construction, and see if there's anything useful to you in  there. I'm envisioning the cathedrals where they build the arches, then rubble, artwork, or window fill the space under it, the ARCH being the structure, not the fill.

Pearl Sutton wrote:

Edit: one more thought...
What I'm envisioning, I think, is basically a structural arch that the window is incidental filler for. Might be worth your time to look up arch construction, and see if there's anything useful to you in  there. I'm envisioning the cathedrals where they build the arches, then rubble, artwork, or window fill the space under it, the ARCH being the structure, not the fill.

I think we're on the same page there!  I'll put a design in here once I've mocked it up.  down the road a piece.

Excellent!
I look forward to seeing it :)

I use reclaimed Department store shelves for windows. There is a scrap yard near us which has 100's and charge 1.16€ / kg which is way cheap. The only drawback is that the glass is slightly trapezoid, but I just make a wider rebate to take this into account.

Storm windows -- the kind you put up in the fall, and took down in the spring -- often had several 1" holes in the bottom sash.  This was covered with a thin batten.  When the window frosted up, you opened the inner window, opened the batten, and closed the inner window.  The ventilation would defrost the window.

You could do something similar for your window.

Houses move.  In designing your window frame, you want to to allow some room for movement.  Read a book on making your own doors and windows.

If you are willing to work with just ordinary glass, you should be able to get your local glass company to cut you a couple of round pieces fairly cheaply.  

But curved glass cuts are harder to do.  A hexagon or octagon might be much cheaper.

The window cannot be perfectly airtight between the panes.  Air pressure varies by about 3% on a day by day basis.   15 psi = 2200 pounds per square foot.  3% of that is 70 pounds per square foot.  The usual way to deal with this is to use a strip of foam between the edges of the panes.  The foam compresses on high pressure days, expands on low pressure days, so the window changes sizes to accomodate the different pressures.

That said, having one side of it mounted in a way that can be lifted out and cleaned makes a lot of sense.  But making this change means making a sealed unit is impossible.  Make it removeable and you will have to remove it now and then.  

If you make storm window style vents, put them on the outside -- less humid.

chris thorpe wrote:I use reclaimed Department store shelves for windows. There is a scrap yard near us which has 100's and charge 1.16€ / kg which is way cheap. The only drawback is that the glass is slightly trapezoid, but I just make a wider rebate to take this into account.

I'm so glad to hear that you've done this successfully.  Can you tell me more about how you mount the trapezoids?  Some of mine will be the same way.

Sherwood Botsford wrote:Storm windows -- the kind you put up in the fall, and took down in the spring -- often had several 1" holes in the bottom sash.  This was covered with a thin batten.  When the window frosted up, you opened the inner window, opened the batten, and closed the inner window.  The ventilation would defrost the window.

You could do something similar for your window.

Houses move.  In designing your window frame, you want to to allow some room for movement.  Read a book on making your own doors and windows.

Great information!  I will read up on this and incorporate your guidelines into the design.

I make a three layer sandwich. 1 outside rectangular frame. 2 trapezoid glass and shaped fillers same thickness as glass with bathroom sealant to make water tight. 3 inner rectangular frame, all screwed together from inside. I use Oak on the outside and Douglass fir for the rest.
Attached two photos of windows.