I feel like duct leakage really isn’t on many people’s radar when it should be. Not the most interesting topic, but it’s a big deal.
The concept of leaky ducts contributing to an uncomfortable room is pretty easy to grasp. Think of a room far away from the air handler or furnace. Pretend you’re at the air handler, where you pick up a bucket of cold water so that you can cool the far away room. The problem is, your bucket has a bunch of holes in it! While carrying it, you run as fast as you can down the hall, up the stairs, down another hallway finally arriving at the hot room, but now there’s very little cold water left because it leaked out! Duct leakage is kind of like that. It’s not surprising that room is never comfortable.
Another very common scenario of duct leakage that leads to discomfort and increased energy usage is a little harder to grasp. Here in Virginia we have a lot of duct systems that are outside the thermal and pressure boundaries of the house. (Thermal boundary is a fancy term for insulation, pressure boundary is fancy for the air sealing layer, for example drywall) These duct systems are essentially outside of the house. Typically the supply duct system (positive pressure, delivering the conditioned air ) is more leaky than the return duct system (negative pressure, brings air back to the air handler/furnace). The supply ducts are leakier because there are more of them and have more connections, so there is more opportunity to leak.
This is where it gets interesting. Let’s say the air handler moves 1000 CFM of air (CFM stands for cubic feet per minute, a measurement of air volume), 1000 CFM goes in, 1000 CFM goes out. Now remember the ducts are outside. When the supply ducts leak air to the attic or crawl space, less air is delivered to the living space, let’s say 800 CFM are delivered and 200 CFM leak out. The air handler still pulls back 1000 CFM. Where did the extra 200 CFM come from?.... If you said outside, you’re right! Leaky supply ducts that are outside the pressure boundaries of the house will put the house into a negative pressure. The house can not stay in a vacuum, so outside air is brought in, equalizing the pressure.
In the scenario above, you just paid to condition the air, you then lost some of it to your attic or crawl space, then had to pay even more money to condition the hot humid, or cold dry air from outside. Ouch! Duct leakage is costly.
Even duct systems that are located in conditioned spaces (inside the house) benefit from not leaking. There is at least one study that shows there is an energy penalty for not delivering the air to its intended room.
So, what can you do about it?
There are plenty of Home Performance, and HVAC professionals out there that can evaluate your duct system. The best way is to perform some type of leakage test, with testing equipment. This is a bit of a process, that typically comes with a fee, but there is value in having an actual duct leakage number quantifying if investing in sealing them is worth it. If the ducts are exposed a visual inspection can reveal if they are exposed, just not how much they are leaking. (Sealing ducts didn’t become common practice until recently…odds are your ducts are not well-sealed)
If you find that the ducts are leaking there are a couple of different ways to do it:
Hand sealing. Expose the seams of the ducts, this typically requires removing insulation, mastic or tape the seams, put back the insulation (this is be a good opportunity to replace the insulation if it is damaged)
Aeroseal the ducts. This process seals the ducts from the inside. Think of it like fix-a-flat for your duct system. The benefits over hand sealing are, typically better sealing results, concealed ducts are sealed, insulation does not need to be removed, and a report is given showing the results.
Feel free to comment with any questions!