I have a High Eff. Furnace and we have an insulated pipe for the intake of fresh air from outside. The insulated pipe is dripping (or sweating) all along the outside from the outside wall almost all the way to the furnace. We noticed the dripping due the brown stains on our newly installed t-bar ceiling tiles. What could be causing this and how might we fix it? Thanks.

Your newly installed ceiling is the problem. When you covered the ceiling you created a place for moisture to migrate. The cold pipe from outside is causing the moisture to be attracted to the pipe causing condensation in the furnace on cycle. The pipe gets cold and water or ice collects when the furnace sucks in cold air from outside. When the furnace turns off and stops bringing in cold air the frost/ice or condensation drips off the pipe causing brown stains.