Double-glazed windows are usually made from pre-fabricated glass panels. Although this is a good design to reduce the cost and complexity of manufacture, there are drawbacks. For example, if a double-glazed window is broken, replacing it may be very difficult and expensive due to custom sizes, discontinued models, and companies going out of business. Most double-glazed windows are only expected to last 10 years in moderate climates, and less where there are temperature extremes due to the seasons, which is reason enough to be able to maintain a window.
The design shown below is a true DIY double-glazed window. It can be built and maintained by a person of modest skill; and at minimal cost. If someone breaks the glass, it can be replaced with readily available glass sheets from a home-supply store. If it begins to collect moisture, the seals can be replaced and new desiccant can be added.
One often-mentioned disadvantage of an air-gap (instead of noble-gas filled) double-glazed window is the convective loss due to air circulating around in between the sheets of glass. Other gases may lower the convective losses due to higher mass, which leads to slower diffusion. However, I think that most older windows have long since lost whatever special gas filling they may have once had. Air-filled windows have the advantage that a normal person can replace glass and replenish desiccant without any special equipment.
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Step 1: Making the Frame
The sash of the window is the trick to this design. It acts as the spacer between the sheets of glass; it provides a place for the O-rings that seal the glass to the frame; it provides a place to add trim, which holds the glass in place; and it provides a structure that supports the glass.
The air gap is created by using the wood of the sash. I used a 1/2-inch gap. Note that the thermal performance of this window sash is improved by not using a metal spacer between the layers of glass.
I designed my frame to accommodate 1/8" glass and 3/8" trim. The O-ring groove is 1/8", which is a poor choice because there were no available O-rings with the appropriate diameter. You should find the O-ring and let that determine the width of the groove. The O-ring should be chosen based on it's cross-sectional area. That area should match the cross-sectional area of the groove. See the selection at http://www.mcmaster-carr.com. I was surprised at how much some of these materials cost. Find something weather-resistant so the UV-radiation doesn't destroy it immediately. For my prototype, I used silicon calk, which was cheap but will make the glass very difficult to remove. DO NOT USE CALK!!
The wood that I used for this demonstration is pine from framing-grade 2"x4" lumber. When I do this for a larger project, I will use something much nicer. I've read a lot about windows and there doesn't seem to be a consensus on what wood is best for windows. I would use white oak, but most any hard or soft wood can be found in commercial use, so I guess it doesn't much matter.
When you're cutting this profile, make sure that you produce enough for your project. Measure the outside perimeter of the windows and add 10% to the total for all of the windows. If you are making a frame for a very large window, or if your stock is short pieces, you may have to be more careful about the exact amount of stock. Also, for a window larger than 3'x3', you'll want to make something more sturdy than the design presented here. Just a thought, Mr. Fox.
One of you will want a drawing of the cross section---I just know it. I've decided not to provide a technical drawing because the point of this article is to demonstrate a concept; it's not a engineering consultation. You'll know the difference because concepts are free. The other comes with a fee---and usually 10% up front.
Step 2: Assemble Frame
Assemble the sash. This design requires that you miter the corners instead of some fancier joint. (Some joints are, indeed, stronger than others. But the point here is to get something done, not argue about obscure points of jointery.) The groove for the O-ring must be continuous in order to make a good seal. You could probably get away with some other joint, but you'll have to do some whittling to get a continuous groove.
Use wood screws (coarse thread, flat head) to hold the frame together. These are counter bored because I had pan-head screws. If you have flat-head screws, a counter sunk holes would be nicer. Leave the corners a little loose until the glass is in. The frame doesn't have much strength itself, and having it rigid will just make it harder to put glass into it. The glass will probably have 2 factory edges, which should keep the whole works square.
Step 3: Cut Glass
Cutting glass is not too difficult. There are just a few important things to remember. First, you should take normal precautions---wear gloves and try not to get anything in your eye. Then, you need a work surface that is flat, clean, and really strong. Use a dab of light oil on the glass cutter before you start. Third, use a straight edge and figure out how to clamp it down at the ends of the cut. Cut using one motion, as smooth as possible. You have to press hard---there is a sound that tells you when you're doing everything right. Then grab the glass at the end and sorta twist it along the cut.
For this window design, you'll need to measure accurately within 1/16" or so. That sounds hard, but it is well within the ability of normal people. I know, because even I can do it.
Find some YouTube videos if you want to know what cutting glass sounds looks and sounds like. For example, http://www.youtube.com/watch?v=n4k7AgA3e3Y right around 3:05. Or, http://www.youtube.com/watch?v=5p8-ySk3boI.
After you cut the glass, polish the raw edges before doing anything else. You can use a file or 400-grit sand paper to take the sharp edge off. The file will make little glass chips, so don't do this part over carpet. The sand paper makes a little dust, but is altogether less messy than the file.
Step 4: Assemble Window
Dry-Fit the glass in the sash. Then, take it out, add seal (use O-ring, NOT calk), put the glass back in, and add the trim. Then, paint it with outdoor paint, add hardware, and call it done.
When you add the trim, tighten it down slowly. Begin with the screws near the middle of the strip of trim and work your way out. This avoids putting too much pressure on the corners of the window, which are quite fragile. There is probably some way to relate torque to durometer of the seal, but you can just tighten based on your intuition. If you have a lot of moisture problems, change the desiccant and tighten down the trim a bit more.
The whole point of this project is to have a window that can be fixed, if need be. But fixing usually requires taking apart and that may be a problem if the screws have been painted over. Also, most people won't like to see the screws that hold on the trim. So, try to think of a way to cover the screw heads that allows for maintenance and looks good. And you have to do it so that the next home owner realizes that there is a way to maintain the windows. I haven't hit on the perfect solution yet.
Desiccant should probably be placed between the sheets of glass. You could add a hole at the top of the sash for this, and seal it with a large screw or a conical dowel. You can buy desiccant at McMaster-Carr. You can also use dry air to fill the window if you have an air compressor. I think some moisture will come from the wood, so it may be advisable to paint the wood bits before and after assembly.
After a year, the window pictured here has worked without fogging. Granted, it was not in my kitchen or bathroom, where humidity problems normally crop up. The window is in my tuck-under garage, so the temperature extremes are less than could be. But it was the only part of my house where I was able to avoid the oversight of the Decorating Committee, so it was a good place to experiment. I did break one of the panes of glass and was able to replace it. The calk is a real hassle. I think the O-ring idea would work a lot better. And if you're making your own windows, you're saving so much on the project that buying the o-ring material should be justified.
Step 5: Addendum
If you use standard-drive screws (not Phillips, square, hex, etc) you can usually chisel paint out of the groove.
A decorative knob can be used to cover screw holes. Unlike flush plugs, the next person who works on your windows will have a clue about how to take the window apart.
Perhaps the outer pane should be held in by points and glazing. That's the traditional method and it might be air tight. Perhaps a a small bead of calk around the glass, after it is installed, would provide the seal. The weather-resistance would be provided by the window putty, which could be applied after the calk dried.
Sometimes it can be really hard to get newer window putty off. Traditional putty, I think, is easier to work with.
If you use putty, seal the wood wherever the putty touches with boiled linseed oil and leave it un-primed. Paint over the glazing a few days later, overlapping the glass 1/16 inch or so (several millimeters).