Sealing Furnace Ducts with Mastic

sealing furnace ducts with masticThe duct work in our house, at least in the places I could access, leaked a lot of conditioned air into non-conditioned spaces. For example, in the crawlspace and basement lots of air was escaping from the duct work where two pieces joined. I figured I was loosing quite a bit of energy by 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. Conversely, if you live in a warm climate and have an air conditioner, sealing the ducts will stop the cold air from cooling unconditioned space as well.

I purchased two tubes of mastic sealant used for sealing furnace duct work. It comes in a tube you place in one of those 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. All you do is squirt it out and smear it all around the joints in the duct work where warm air is leaking out.

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 just 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 razor 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. Again, clean up is very easy.

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 registers worth of air leaking into my basement and crawlspace. That’s quite a bit when you think about it! I then began the task of insulating the duct work in our ventilated crawlspace.

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