Q:  My wife wants me to call a pro to seal our HVAC ducts. What do you think?

A:  My question to you: do you really want to spend a day patching leaky ductwork while balancing on ceiling joists in your attic or crawling in your crawlspace where ducts are typically located? It’s not a fun job, but it is a very important part of making your home healthier and more energy efficient.  

The HVAC ducts are an integral part of any central heating and air system. The fan of the unit forces heated or cooled air into the branches of the ductwork to be distributed to the conditioned rooms in the house.

Today’s energy codes require that all duct connections be sealed. Unfortunately, it is very common to find leaks in the ductwork even in new homes because the connections either are not sealed at all or poor quality sealing materials have been used.
   
Leaks in the ductwork for forced air heating and cooling equipment are often underestimated in terms of the energy wasted if they don’t get properly sealed. And not only can leaky ducts add hundreds of dollars to your annual heating and cooling bill.

DUCT LEAKS CAN BE HEALTH HAZARDS
But also can leaks especially in the return ducts be hazardous to your health as contaminated air from the crawlspace, basement or attic is drawn into the living space. The potential health hazards include dust and mold spores, toxins from termite treatment, radon, and combustion gases from the furnace or water heater. Unwanted humidity can be drawn in as well.

Leaks in the supply ducts can contaminate the indoor air in a similar way by creating lower pressure in the house and as a result air from the crawlspace or attic are drawn in. In addition leaky supply ducts don’t distribute the air properly throughout the house and some rooms may end up too hot or too cold. Also the life of the HVAC unit may be shortened because the heating and cooling capacity is lowered by leaky ductwork.

WHAT A PRO WILL DO
If you hire a pro to come in and seal the ducts, the first step will be a pressure test. The registers get taped shut and a special fan is installed to pressurize the air distribution system. This will tell how badly and where the ducts are leaking.

The most common types of duct material used are rigid sheet metal and flexible nonmetallic duct also called flex duct. Often a combination of the two types is used.

The metal duct is very efficient because the smooth interior surface has very little resistance to air flow. However there are a lot of pieces that have to be connected to make up a metal supply or return duct and all connections have to be carefully sealed prior to wrapping the outside with duct insulation.

Flex duct on the other hand can be run in continuous lengths with very few connections. It has the disadvantage of getting kinked or compressed in addition to getting punctured since it only consists of a thin inner liner supported by a helix wire coil that is factory wrapped with insulation.

All joints and connections have to be sealed preferably by using duct-sealing mastic. Duct tapes loose their permanent seal because the adhesive tends to dry out. Mastic instead is a thick paste that gets brushed on and provides a permanent seal as it cures.
 
Remember that the insulation does not stop air leaks. In fact, dirt streaks in the duct insulation are a good indicator for where the leaks have been occurring.

TIP
If you are building a new home consider bringing the duct system into the conditioned space. By doing so a lot of the problems associated wit leaky ducts can be eliminated.