A regular reader named Ed has a gas furnace air intake pipe that sweats condensation during cold weather. It gets bad enough that water drips onto the basement floor. “What can I do to stop this condensation from happening?”, wonders Ed.
The plastic gas furnace intake pipe can be insulated, but any insulation strategy must include a vapour barrier on the outside. Without this, indoor air will permeate the insulation and condense internally against the pipe. Bad news.
Plumbing supply outlets all offer high-quality foam rubber insulation sleeves for use over various sizes of pipe – even 4″ or more. These are like the foam insulating sleeves sold for domestic hot water pipes coming from your water heater, but larger and better. They usually include an adhesive strip to seal the slit after installation.
Insulation of this kind would work very well on your air intake pipe. After installation you’d need to wrap the whole thing neatly with black electrical tape, to act as a vapour barrier. With care this approach will look great and work perfectly.