We were recently asked for our advice regarding stage drapery and humidity control.  Below is essentially the text I shared.  This system is fairly typical of many auditoriums: all of the drapery is manufactured from cotton fibers and is hanging from a counterweight rigging system.

Cotton fiber, without any treatment, is flammable.  To meet building code requirements, the cotton fabric used in your drapes was run through a flame retardant solution.  This solution is a blend of metallic salts that help to prevent the fabric from sustaining flame.

We strongly recommend maintaining a humidity-controlled environment for natural-fiber fabrics for several reasons.

  1. Untreated natural fibers in a humid environment will deteriorate at a faster rate—moisture accelerates decay.
  2. The salts used to flameproof this fabric are hydroscopic and will “pull” moisture from the air.
  3. Salt, combined with moisture, is corrosive and will further accelerate decay of the fibers.
  4. Large drapes can soak up a significant amount of water from the air.  Even if the rigging system was left balanced, the water in the drapery can dramatically change the balance of the lineset.  This is, at best, an unpleasant surprise for the next operator of that lineset.  At worst, this is a recipe for a runaway lineset that can result in serious injury.
  5. When the fabric is weakened, the additional moisture (weight) has the potential to tear the drape away from its reinforcing webbing at the top.  This has a similar result to the scenario described in #4, above, except that the out of balance condition is in the other direction.  The results have the potential to be just as unpleasant.

There are other drapery fabrics that were either not available or not of an acceptable quality at the time the system was purchased.  These velour fabrics are made from polyester and are inherently flame retardant—meaning that they do not require treatment and the side effects that come with it.  These fibers will not deteriorate because of moisture.

As long as the drapery in use is fabricated from cotton, we believe that their environment must be humidity controlled.  One of the fabricators that we work with regularly states that they will not warranty cotton drapery that has been subjected to more than 65% relative humidity.  Failing to control the environment both creates a significant safety hazard (that increases that the drapery ages) and shortens the life of the fabric.