The very best fabric for SCA tents would look just like a cotton or hemp canvas. It would be light (4 oz maximum) but strong and very opaque to light. It would repel water indefinitely, like a fabric laminated on one side with vinyl or urethane, but breathe to allow transmission of water vapor, like Gore-tex. And it would cost twenty-five cents a yard.
Unfortunately, many materials fit at least one of these requirements, but none fits all of them. Every fabric, then represents a sort of compromise between the varying materials. There is no best fabric for all purposes, so what you use will depend on more on the kind of camping you do, how period you want to look, and how much weight you want to attach to safety factors such as fire-retardence and UV transparency.
When I started making tents commercially, I've had to conform to the legal requirement that all tents made in California, or sold in California, be made of flame-retardent material. I used to chafe at this requirement, because it severely limited me in the choice of fabrics I could use and the price I had to pay for them. Over the years, my thinking has changed somewhat. What has changed my mind more than anything else is seeing very large SCA encampments, sometimes stretching for miles, with tents pitched extremely close together. So fare, there hasn't been a major fire, but I think now that it's only a question of time. A few Pennsics back, a fire broke out near the camp, and only the fact that the wind was blowing the flames away from the camp prevented the fire from spreading through the camp. And those of you who were at the Thirty-Year Celebration will remember what a fire-trap that was, with closely-pitched tents surrounded by freshly cut hay. I'm now convinced that it is a Good Thing to have sleeping areas be flame-retardent.
The reason I'm going on about this is that while most of the major manufacturers are pretty good about this, there are smaller ones, even in California, whose tents are not flame-retardent. If you buy one of these tents, you are not protected. Inquire of the manufacturer whether the tent is truly made of flame-retardent fabric as required by law. They should be able to show some documentation that the tent conforms to California's CPAI-84 specification (which is the most stringent and therefore the one that everybody else uses, too). Also, be aware that most Renaissance Faires have a requirement that the tents used there be flame-retardent, of if you plan on participating in these events, you may need to show some documentation.
OK, now I'll get off the soapbox and talk about fabrics.
There are a few important differences between tent fabric and garment fabric. First, remember that tents are (usually) tension structures. Therefore, shrinkage is usually not as important as stretch.
Second, tent fabrics need a certain degree of impermeability to water. This quality is achieved in various ways. To some extent, the cotton content of the fabric helps, because cotton expands as it absorbs moisture, helping to tighten the weave. Waterfastness can also be increased by treating the fabric with either a water-proofing compound (which also makes it more flammable, unfortunately) or a silicon-based water-repelling compound. (An example of the latter is Scotch-gard, although there are other products developed specifically for camping tents.)
Among all-cotton fabrics, the most common is a canvas called "Sunforger" which is available in both fire-retardent and non-fire retardent finishes. (The fire-retardent finish is a bit heavier.) It is fairly stretchy under tension, so it fares better with tents that use centerpoles and side poles. It repels small amounts of moisture very well, and only driving rains will force water through the material. However, if the fabric is touched on the "dry side," the water will capillate through the material and drip inside. Its mildew resistence is pretty good, although it tends to absorb a lot of water and must be dried thoroughly before storage. When dry, it breathes very well.
Another material used by commercial tentmakers is a cotton/polyester canvas. It also comes in fire-retardent and non-fire-retardent flavors. Dragonwing uses the former exclusively. The stuff we use has an acrylic "top coat" which looks like a light coat of paint applied to the canvas. This helps seal the fabric even more against moisture, while still letting it breathe a little bit. The coating drastically reduces seep-through from capillation and driving rain. The cloth's polyester content reduces any tendency to shrink or stretch, making it a better cloth for tents with rigid frames (or any other arrangement where a close fit is critical). Because it does not readily absorb water, it usually dries in a matter of hours rather than days. Its mildew resistence is very good, although it should not be packed up wet.
(Even synthetics can mildew, by the way. The mildew organisms can survive and thrive by eating the organic dirt on the tent.)
These cotton-polyester canvases were originally designed as awning material, and are expected to withstand five years of constant exposure to the elements. Tents made from them have superior longevity, which is why Dragonwing likes this material so much.
Dragonwing uses a cotton-polyester twill for its sunshades. While not treated for water repellence, it has enough cotton content to allow some swelling of the threads to reduce seepage. But it's not something I'd want to rely on in a driving rain.
100%-polyester cloths are the least susceptible to stretch, and age very well. They usually have to be treated for water repellence before they can be used for canopies (roofs) of tents, but they may fill the bill for tent sides.
Nylon cloths are undesirable for tent material, unless they have been specifically designed for tentage. There are several reasons for this. First, nylon is fairly UV-transparent; you can get a pretty good sunburn sitting in a nylon tent. Second, nylon has a negative coefficient of expansion. That is, it shrinks when warm and expends when cold. So if your tent relies on a constant tension on ropes and fabric to keep it stable and pretty-looking, nylon will not do the job for you. Finally, most nylon pack cloth is coated with a urethane coat on one side to make it moisture-proof. This coating ruins whatever "breathibility" the cloth may once have had. Nylon's sole virtue is its strength, which makes it easier for you to make a tent that's light and packs into a small space, and easier to man-handle through a sewing machine.
Most of the nylon camping tents I've seen have a roof section made of polyester, to reduce the UV transparency of the tent. The nylon and polyester are also treated with other UV blockers to extend the life of the fabric.
In my career as a tentmaker, I've used all of the above materials, plus a few others. (One of my favorites was a four-ounce polyester material called "Camplite" which I used extensively for sidewalls and even a few canopies with good results. Unfortunately, it's no longer made.) All these tents had their good and bad points, but generally, the limitations of the lighter materials outweighed their advantages. Tentmaking uses a lot of time, and tent buying uses a lot of money. Since most of us have a finite amount of both, it makes sense to spend a little extra time and money for a lot of extra durability. In the long run, you'll be happier with your tent.
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