Waterproof Fabrics for Clothes and Tents
Both waterproof clothes and tents use similar types of fabrics, so it is best to introduce them all together. The basic problem with waterproofing is that, in sweating and breathing, we emit large quantities of water vapour. If a fabric is totally impermeable to water, and it is cold outside the tent or jacket which is made from it, this water vapour will condense on the inside. During an active day you can get almost as wet as if you weren't wearing a waterproof.
Until quite recently, most waterproofs and tents were made from special close-woven cotton. As the fabric gets wet, the cotton threads swell and pack together, keeping out the water. There are small holes still left in the fabric and water vapour can pass through without condensing on the inside. Most cottons for tents and jackets also have some form of treatment which helps them to shed water without absorbing too much. They need re-spraying from time to time with this. In general, cottons are comfortable to wear and hardwearing, but they tend to be heavy, especially when wet, their waterproof qualities are limited, they take a long time to dry out and they are liable to rot. With a tent, you have to be very careful not to touch a cotton flysheet, otherwise the rain starts coming in.
Most waterproofs and tents are nowadays made of nylon (or to some extent polyester). Nylon fabrics are very light and strong, packing away into a small bundle. They can be coated in a variety of ways to render them either totally waterproof or breathable-waterproof. For very light fabrics, thicker threads are scattered through the finer weave to strengthen it - "ripstop" fabric. Nylon doesn't absorb much water and so takes very much less time to dry out than cotton. It is also resistant to rot. All in all, nylon is more practical for fieldwork. The problem for manufacturers with nylon fabrics is that they deteriorate when exposed to sunlight, and are weakened when coated with the various waterproofing treatments, but these things have been largely sorted out.
Fully waterproof coatings
Wax. This is the waterproofing used on the Barbour thornproof jackets and their clones. Heavy, slightly sticky and with a faint smell. The wax wears off and the fabric must be re-treated regularly to maintain its waterproof qualities. Barbour originally used it to make heavy oilskins, souwesters and suchlike for seafarers.
Polyvinyl chloride (PVC). This is the traditional coating that replaced oilskins in boats. It is heavy to wear and quite tough (although surprisingly easy to tear if caught on something). Fine if you don't mind carrying it about.
Neoprene. They make diving suits out of this synthetic rubber. It's tough and very resistant to abrasion so, when coated onto a heavy duty nylon fabric it is almost indestructible. The best tents use a neoprene coated nylon for their groundsheets.
Polyurethanes (PU). Very commonly used as a waterproof coating for a wide variety of weights of nylon. Not as hardwearing as neoprene, but can be applied to lighter fabric.
Silicone elastomer. Used for the lightest tents, because it does not weaken the fabric as much as PU. So a lighter silicone coated fabric can be used to do the job of a heavier PU coated fabric.
Breathable-waterproof coatings and membranes.
Goretex. This is the original breathable membrane, made of microporous PTFE (polytetrafluoroethylene) plastic which will not let cold rainwater droplets in but allows warm water vapour to pass out through the tiny pores. It can be laminated (ie. stuck on) to a variety of fabrics, or can also be sewn in as a separate drop lining between a non-waterproof outer and inner. There are several imitators of Goretex. Microporous membranes need to be kept clean, because the pores can clog up and prevent the water vapour passing through - leading to condensation.
Sympatex. The next development of a breathable membrane, this is a non-porous polyester which absorbs warm water vapour inside and passes it through to the cooler outside of the membrane. Sympatex membranes are completely water and windproof and do not need to be kept clean - a major advantage on an archaeological site. They can be laminated to a variety of fabrics, but are normally found as a "drop liner".
Breathable coatings. Nowadays there are also many breathable coatings which can be applied directly to the inside of a fabric. They are not generally thought to be as good as the membranes, but they are a lot cheaper. A wide variety of fabrics can be coated, including cottons and synthetics. Better makes of jackets etc. have an inner lining layer to protect the coating and make the jacket more comfortable.
One thing to check on waterproof clothes and tents is the seams. Even a good strong seam will still leave little holes through which the thread passes and through which water can enter. On a good quality product these should be sealed. This is usually done with an appropriately coated tape covering the seam inside the garment or tent. Fully waterproof PU coated or neoprene tape for the non-breathable fabrics and breathable tape to match the breathables. The very best tents have hot-melt tape sealed seams on their PU coated flysheets. If you have problems with a PU coated fabric which is not seam-sealed, you can do it yourself with a seam sealing compound (Bostick contact adhesive also does quite well, I gather). Silicone coated fabrics cannot be seam-sealed using tape or the proprietary compounds (or Bostick), because these simply don't stick to them. If desperate, try silicone bath sealant (the colourless variety).