Thanks in advance for your replies.

We just put a new central A/C system (complete) as a replacement for a leaking unit. One unfortunate result was heavy condensation along one duct route that created lots of moisture in a closet and ceiling when it was very humid outside.

The dealer inspected the duct and said it was well insulated and he saw no problems. The season is about to begin again and I would like to know if there are any steps available to prevent this.

After installing our central AC I had the same problem. Luckily our ducting was all exposed so it wasn't a problem to deal with. I simply insulated the entire length of duct and have no more condensation. You may want to look at some of the foil/bubble wrap style insulation as it serves as both the insulation but also as a vapour barrier.