Q&A session No. 231

Q: My air-conditioning person told me I should seal my ducts to keep insects and contaminants from getting into my ductwork. Was that just a sales pitch?

Sean M., Phoenix

A: The answer is, frustratingly, no and yes.

The No: The gaps in most duct leaks – especially in modern flexible ducts – don’t allow any but the smallest bugs to enter. In addition, most ducts are either elevated off surfaces on which bugs can crawl or are buried in insulation. So, while it’s possible, it is rare.

The Yes: Some bugs can get into ductwork, and contaminants absolutely get into ductwork directly or into homes indirectly, because of duct leakage:

• Underground returns – though the bugs usually come from inside the house and fall in, rather than coming into the duct from the earth

• Floor registers in mobile homes, old homes (1920s and earlier) and some custom homes.

A far more common occurrence is bugs and contaminants that make their way into the home because of duct leakage.

For example, supply-dominant duct leakage – wherein there is more duct leakage on the supply side (where the conditioned air comes out of the registers) than return duct leakage (between the filter and the air-conditioner – causes the air pressure in the home to drop. This, in turn, causes air to be sucked into the home through gaps that connect the attic to the wall cavities of the house; crawlspace penetrations and gaps; leaky windows and doors, etc.

If this is happening, then the next question is, “What is in the attic and crawlspace?” If you haven’t just eaten, take a look at the photos. Yes, that’s a dead dog, and yes, those are droppings from rodents that were eating the dead dog.

Also, dead birds, dead rodents, sometimes asbestos-laden vermiculite and other asbestos-containing objects, mostly in older homes. But there’s also dirt, mold, dander, and all other manner of detritus.

So, sealing your ducts is a no-brainer, right? The answer is, frustratingly, no and yes. If you seal ducts on a system that already has trouble breathing, you might choke it to death. So the real answer is, have a home energy audit done first. You may actually need duct modification or even replacement.