When we first bought our house, I found the ducting leaked a lot of conditioned air into non-conditioned spaces. We were loosing quite a bit of energy by unintentionally heating parts of the house no one lived in, and was probably making it comfortable only to the bugs and other critters that might like some warmth during the winter months. Same goes with an cool air conditioner (if we had one).
To solve this, I purchased a couple tubes of mastic sealant specifically created for sealing furnace duct work. The mastic comes in a tube you place in a squeeze-handle guns. It’s basically a medium colored grey goo that has fibers in it. It squirts out wet but dries pretty quick depending on humidity levels in the house. Wherever two pieces of sheet metal meet, you squirt the mastic on the seam and smear it around.
It’s very simple and no special tools are required. Therefore, there really isn’t any trick to this stuff and you can be as sloppy as you want. It cleans up very easily with water.
Don’t apply it sparingly, but rather apply it liberally. Squirt it out and make sure you put enough down to adequately fill the gaps where the air is coming out. In the tight or hard to reach spaces, I squirted it out onto my finger and shoved it in and around the gaps. If doing this, be careful as the edges of the sheet metal can be really sharp!
The tube says to wait 24 hours before starting the furnace so the mastic has time to dry, but I didn’t. In fact, I applied it when the furnace was running so I could tell if the air wasn’t leaking anymore. I suppose if there is a lot of air pressure coming out of the hole, it might keep pushing through the mastic, but I didn’t run into that problem. It took about 12 hours for it to completely dry but we live in a very dry climate.
In my case, the basement and crawlspace are now much cooler so I know that air is making it into parts of the house where it should be. After I plugged all the leaks I could reach, I figure I saved about 1.5 floor registers worth of air leaking into unwanted areas. That’s quite a bit when you think about it! It’s about a small bedroom worth of air.