Heating contractors love flexible, insulated, plastic ductwork. It is light weight, quick and easy to install and it is easier to comply with energy code requirements.
Metal type ducts take time, have lots of seams that have to be taped and then the ducts still have to be insulated.
The problem is, that in the long run, when it becomes necessary to clean the ducts, it may not be possible to clean either one—but especially the plastic ones. This might result in them needing to be replaced. Of course one can argue that replacement of the flexible ductwork will be cheaper than if the metal ones become contaminated and need to be replaced.
For my money, I think I would still go with the metal ducts—if I have to choose one or the other. They are much easier to support properly and air flow can be less restrictive than the flexible ducts. Flexible ducts may be OK in an attic where they can lie flat and fully supported (not to mention better insulated), but when they have to be strung across the bottom of joists in a crawl space they can develop sags and restrictions at the hangers—-unless there are a lot of them.
Too often the pipes are not adequately supported in crawl spaces and end up collapsed on the ground where they get crushed by workmen that don’t know any better or where vermin are attracted to sleeping on them. Crushed metal ductwork is also common when it has not been adequately supported or there is not enough room to get around the ducts without damaging them. Rodents love ductwork that is warmer in the winter—whether it is plastic or metal.
The plastic ones are more susceptible to damage from rodents and rodents can easily gain access to the interior of the ductwork. Rodents will of course get into the insulation around any kind of ductwork and keeping the critters out of house structures is critical.
The reality is that rodents will almost always sooner or later find a way into a crawl space.
Creating homes for them inside the crawl space, while not done intentionally, is very difficult to avoid. Once inside the ductwork they can of course pretty much go anywhere the ductwork goes—-making quite a mess along the way. The furnace blower can then blow odors from their activity throughout the house—no ones idea of an “air freshener.”
Here are a few pictures of plastic ductwork from several crawl spaces.
The next picture shows where an enterprising rodent has made their way into the insulated duct.
It seems like creating a network of tunnels for rodents to freely move about the house is not a good idea.
For my money, “quick, easy and cheap”—-may be more expensive in the long run.
Ductwork in our homes is not going away any time soon, although there are some alternatives such as the air to air heat pump/ac units that can be installed in each room of the home—eliminating the need for duct work. These systems also come with drawbacks—but duct work is not one of them. The less we need air conditioning the number of alternatives goes up, so creating energy efficient homes that require no, or very little air conditioning, must be pursued in the context of finding solutions to the heating and cooling of our homes.
Ultimately finding a way to eliminate ductwork in homes is appealing on many levels.
Charles Buell, Real Estate Inspections in Seattle
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