That's a great point about fire-rating, Jennifer. Very important.
Building codes and fire codes do seem to vary a lot, but using fire-resistant (or even better: fire retardant) materials in your studio is just common sense. Especially with fabrics: if the fabric you like is not fire-resistant (a.k.a. "flame resistant"), then there are chemical treatments you can apply to make it so.
From this point of view, mineral wool insulation (often made from slag) is preferable to glass wool insulation ("fiberglass"), which is made from sand. You'd think that, since both of them are basically just rock, and therefore inert, that it wouldn't matter, but it does. In general, mineral wool insulation is more fire resistant that glass wool, and it is also deals with water much better. Fiberglass insulation that gets wet usually ends up as a useless soggy mess, but mineral wool handles it better.
Anyway, getting back to fire code: you often see that fire code does allow inserting a strip of mineral wool between framing sections to comply with fire ratings. It does help prevent flame spreading across gaps. In most places, it should probably be acceptable to just have mineral in the gap between door frames, covered with flame-resistant fabric to make it look pretty.... But don't take my word for it! Check your local building codes, and if there is still any doubt at all, then call your local building inspector and ask him: After all, he's the guy who will have to sign off on your inspections! Do what he says, not what I say.
Now, for the gaps around windows, that's a little different, since the windows are sealed in place and will never be opened: from that point of view, they are just extensions of the wall surface. But that exposed gap is actually rather important, acoustically, for isolation! A substantial port of the isolation of a studio wall is due to the insulation between the leaves.... but for obvious reasons, you can't have insulation between the two panes of glass! The best you can do is have a large surface area of insulation exposed around the edges of the window. It does help. You can also compensate for the lack of insulation by increasing the mass (surface density) of the glass, so it is a bit higher than that of the wall itself, and also by setting the two panes as far apart as possible, thus having a larger air gap between the glass panels, than between the sheathing panels.
One more thing with windows: condensation. The air trapped in the wall cavity will have some moisture in it (humidity), and that can condense on the inner surfaces of the glass... where it is impossible to get at it! It will fog up the glass, so you can't see through it, and it will also leave marks on the glass when it eventually dries out. Not good. The solution is: desiccant. You need to place some desiccant in that gap between the glass panes, to adsorb the moisture out of the air. Silica gel is good for this: that's the same stuff you sometimes see in small white envelopes inside the packaging for new electronic equipment, cameras, medications, shoes, and suchlike. Silica gel is not the only desiccant that you can use, but it is inexpensive, easy to get, and very effective. There are equations that tell you how much of that you need inside your window cavity, based on the volume of air you have trapped between them, but a simple rule of thumb is: You need at least 200 grams of silica gel beads per cubic meter of air space in the cavity. That is not a lot, because the surface area of the granules is quite high. You do need it! If you look around the edges of sealed double-glazes window units in your house, you will probably see a perforated metal strip around the edge: inside that, there is probably some desiccant too! Even though they take great care in the factory to keep moisture out of that cavity, they still do put in desiccant, just in case. So you should too! But using perforated metal strips is a pain... so...
A simple method is to make a sort of "tray" from thin metal or plastic, that partly covers the gap between the two window frames (but only touches ONE of the frames!), and scatter your desiccant all long that, then put your breathable fabric over that, so it is hidden from sight but also exposed to the air between the panes. Usually it is enough to just do that along the bottom of the window, but for large windows or damp climates it's a good idea to do a second tray across the top, and drill small holes all over it, so air can circulate better. In this photo, you can see that an area has been routed out of the window frames to receive the desiccant tray:
Another note about silica gel: If it is the type that changes color, make sure it is the right color before you put it it! Either blue or orange, depending on the type. But never pink or green! If it is the wrong color, or if it is not colored at all and you don't know if it is dry or saturated, then heat it in an oven set to about 125 °C (about 250°F) for several hours, then put it in the cavity and seal the glass in place as soon as possible afterwards. Silica Gel is pretty inert: it's actually similar to quartz or sand, chemically, so you can put it on or in pretty much anything. Those labels that say "Do not eat! You will die!" or whatever, that you see on the packs of silica gel in cameras, shoes, pills, handbags, etc. are because of the additives that they put in so that it changes color when dry / damp: That color-change chemical stuff is poisonous, but not the actual silica gel itself. It's no more dangerous than sand. If you get the stuff that does not change color, then you could use pretty much anything you want to make the "tray" to hold it. If you get the color-change stuff, then maybe plastic would be the best choice: you can find plastic and metal "profiles" of various shapes and sizes in hardware stores, such as Home Depot, and use that to make your trays, then glue and staple it to one side of the frame gap. Don't use wood for the tray, as that can warp with humidity, and there might also be unwanted reactions with the color-change chemicals. Spread out the silica gel as much as you can in the "trays", to get maximum surface area exposed to the cavity air.
And do use flame-resistant fabric over it!
- Stuart -