Many builders and do-it-yourselfers use flexible ductwork as an alternative to hard metal ducts. Flexible ductwork carries with it several advantages and disadvantages.
It's a cheaper product. When you use flexible ductwork instead of metal ducts, you will save some substantial money. Flexible ductwork is going to be cheaper than metal ducts in nearly every case. When you are trying to stick to a budget on the project, this savings can help you do so.
Labor is also cheaper. If you hire the job out, you will also pay much less for labor with flexible ductwork. This type of ductwork is simpler to install and therefore, requires less-skilled labor to install it. This represents another way to save on the job when compared to metal ductwork.
This type of ductwork can go in tight spaces that metal ductwork could not fit. If you need to go around a sharp turn or some other angle, this might be your only option.
While those that sell flexible ductwork may have you believe otherwise, it does not allow as much airflow as a similar sized metal duct. When you want to heat and cool the house efficiently, you need to allow as much airflow as possible through the ducts. Since it is so flexible, it is also easy to obstruct the airflow even more. If something is pushed up against it in the attic, for example, it could constrict to a smaller size than it already is. While you will save some money with flexible ductwork, you will have to sacrifice some airflow in the process.
Flexible ductwork is not as durable as metal ductwork. It will not last as long under similar conditions and it will not hold up to the wear and tear over the years. You are dealing with a very thin product that does not have much to it. After a few years, it has been known to become brittle and easily break if something touches it.
While you might save a little bit on the front end with a flexible duct system, you will likely have to replace it long before metal ductwork. This will add considerably to the overall price that you pay. Therefore, you can pay a little more upfront and be done with it or run the risk of replacing it a few times.
While it does not hold up over the long-term, it is also easy to damage it in the short-term as well. This makes installing it without damaging it very difficult. You could rip through it or damage it by stepping on it. Once it is installed, it is not uncommon to see where a mouse has actually eaten through it. Touching against any sharp objects could also rip it beyond repair. You have to be extremely careful while working with it.