The Third Worst Place to Put a Duct
Ah, so many choices. If you want to design and install a duct system to create problems, the possibilities are endless. I've certainly written about a lot of them (as well as how to do it properly). Today, though, let's focus on one particularly bad place to put a duct. In fact, I think it's the third worst place to put one.
What spurred this article is that I recently moved from a condo to 1961 ranch house in Atlanta, and my new place has this problem. The photo above shows it but you can't tell for sure there. The photo below should make it clear, though. Can you tell what the problem is now?
The lead photo shows a low sidewall supply register. The second one shows the same thing but there's another clue here that makes the problem clear. Notice the window above the bedroom supply register. Yes, this supply vent is an exterior wall.
What's wrong with putting vents in exterior walls?
As it turns out, putting supply vents in exterior walls is bad for several reasons. Here's my list:
- Displaced insulation. Most homes have wall insulation in the cavities, not outside the framing. When you put a duct and a duct boot in the cavity, you don't have room for insulation. If the walls are framed with 2x4s, there's often not enough space even for a little bit of duct insulation.
- Duct losses to outdoors. Even if the duct and boot have some insulation, these components are rarely sealed. and the insulation is likely to be compressed and insufficient. Those two factors add up to warmer supply air being delivered in summer and colder air delivered in winter.
- Moisture problems. Putting cold air into an uninsulated piece of sheet metal in a poorly air-sealed enclosure is a recipe for condensation and mold for homes in humid climates.
The photo below, which I took at a gut-rehab in progress in Atlanta a couple of years ago, shows what it looks like inside the wall.
Basically, this configuration results in a wall that doesn't perform as well as it could, a duct that doesn't perform as well as it could, and the possibility of rot, mold, and indoor air quality problems. (An appropriate response here might be, "Other than that, how was the play, Mrs. Lincoln?")
It's mainly an older-house problem
I've seen one or two new homes with this problem in the past decade or so, but this is a practice that has mostly disappeared. That's the good news. The bad news is that if you have an older home with your supply vents in the exterior walls, you ought to plan some way of addressing the problem.
The best thing to do is open the wall and remove the duct and boot, do your air-sealing, and then insulate. That means finding another place for the supply vents, of course. In my case, I've got an air conditioner and furnace in the basement serving the whole house, and I'm going to rip it out.
The new systems will serve the first floor with two ducted units in our encapsulated attic so I'll put all the supply and return vents in the ceiling. Once I do that, then I can completely gut the basement, including the ductwork. Then later, when I work on the first floor walls, I can remove the vestiges of the old duct system from the walls and be done with this problem forever.
Another way to go is leave the sidewall vents in place and try as best you can to get them insulated and air-sealed. The photo below shows the basement duct going up into the wall and feeding that register in the second photo above. A previous improvement project included spraying foam on the band joist and trying to seal it up from below, but this doesn't solve the problems in the wall above.
Even if I opened the wall and sealed and insulated the boot as best I can, that thing is still displacing wall insulation. The only way to fix it fully would be to add exterior insulation to the wall. In my case, that's not going to happen because I'm not removing the brick veneer.
The only way to save those sidewall supply vents and eliminate all the potential problems is to seal and insulate the duct as well as possible and then add exterior insulation to the walls and band joists. Otherwise, you need to find another way to get heating and cooling to your rooms.
The first and second worst places for ducts
Now, since the problem I've just described is, according to me, the third worst place for ducts, you may be wondering what two places are worse. Right? The first one is easy. Can you guess what it is? (If not, just click that link in the previous sentence or the first article in the list below.) The second place is anywhere else in an unconditioned attic.
I'm curious what you think might be the fourth worst place.
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