Every August we receive a few calls from customers complaining of water stains on their ceilings. We arrive with a short rake and sometimes a leaf blower and find that their ductwork has been covered with blown-in insulation. We remove the insulation from the ductwork and the problem goes away.
Seems like it should work. Why have those cool ducts baking in a hot 120 degree attic when a little blown-in insulation will keep them cool. Sounded good to me. Until one day while chasing the cause of a water stain in Daphne and I put my hand down into 2 feet of insulation and felt soaking wet insulation around a perfectly sealed and insulated duct. I pulled all the insulation away and the problem was solved.
Here is a pictorial drawing of the thermal properties involved in the problem. In this duct buried in insulation, it is wrapped with 1.5″ foil faced wrap and covered with 4″ of blown insulation. In a ventilated attic, humid air of 91 degree dewpoint travels through the blown insulation and condenses on the 78 degree foil facing of the duct insulation.
The blown-in insulation is causing the foil faced vapor barrier of the ductwork to cool down. This doesn’t occur when the ductwork is in the open attic. The solutions to hot ducts is outside the scope of this article. But always remember to instruct anyone installing insulation to install the insulation so as not to contact the ductwork.