How does it Happen
Basically warm moist air flowing through a cold pipe is going to cool quickly. This will cause moisture to condense on the side of the pipe. It then run to the bottom of the pipe and drip out of the joints in this particular pipe. If it is cold enough on the outside of the pipe the water will freeze. The ice can melt later when the weather warms up. This water can cause extensive damage to underlying floors and walls including rotting wood and mold. If it is cold enough the moisture can actually freeze in the pipe causing an ice dam inside the pipe as well as outside.
Dryer Vent Ice Dam – Solution
Change the pipe to a single continuous pipe with no joints or seams. If there is a seam along the length of the pipe, make sure you install it with the seam on the top. In addition surround this pipe with insulation so that the pipe is kept relatively warm. This will avoid freezing the moisture in the dryer exhaust.
Any time you have icicles on your roof or forming from a dryer vent, there is a problem somewhere. Homeowners need to pay attention to these kind of issues immediately to avoid much more expensive repairs in the future.