Air-Seal an Attic Series: Sealing Ductwork
Leaky ducts put heat, moisture, and dirt where you don't want it; here's how to seal them.
It’s a good idea to keep all ducts in conditioned space, but if you can’t, you should at least stop them from sharing air between your attic into your living space.
Here, we have housing for a ceiling light, a fan assembly in the bathroom below, and the discharge duct for the warm, moist air being exhausted from the bathroom. The trouble is, when this was installed, someone used regular duct tape. And over time, duct tape doesn’t seal well to the pipe. The adhesive gives up, deteriorates. Our goal is to make sure we don’t get any air leakage through these joints. We must remove this tape and reseal those joints, eliminating the potential problem of mold or condensation that can lead to rot in the roof rafters.
Get the right tape for your ducts
You can see when I pull back the tape, it easily comes off. And you can see the adhesive is sticking to the pipe, and not bonding to the tape anymore. I’ll pull all of the tape off at each of the joints. Because of the residue left by the old duct tape, I’m going to have to use a less than preferable method. If this were a new installation, I’d use aluminum tape that has an acrylic adhesive on the back side. It’s a lot more durable and long lasting than ordinary duct tape. Because there is contamination from the old duct-tape residue on the joints, I’m not going to be able to get a good bond between the aluminum tape and the ducts themselves.
Serious sealing materials
I’m going to use duct mastic, which is the messier method. It’s usually used with an HVAC system, but it’s also good to seal up ducts for exhausts venting from a bathroom fan. We have some joints that are weak, so to begin the process, I’m going to reinforce them by wrapping regular fiberglass tape around the joint. The tape will help reinforce the mastic so that the combination of the fiberglass tape and the mastic will hold it all together.
Mastic is essentially a paste. I could apply it with a paintbrush and sacrifice a brush, but I find globbing it on with my hands while wearing gloves gets a better contact and seal. Neatness doesn’t count. It’s just a matter of putting it on all the way around the joint. Mastic is latex-based, relatively nontoxic. Once this cures up, it’ll seal well with the pipe. One trick, if you’re going to apply the mastic with your hands, wear two pairs of gloves. When I’m done, I can remove the outer pair with the goop on them, and then I don’t get any of the mastic on my hands.
Keep flex-duct bends to a minimum
Even though we’re doing air-sealing, the customer mentioned that the bathroom fans weren’t drawing and exhausting the excess moisture from the bathroom. When we got up here and pulled the insulation back it was obvious why this fan is not exhausting. When it was installed, someone put this slinky hose on and gave it some radical bends. These flexible ducts usually come in lengths of 20 or 25 ft., but they only need to cover about 4 ft.. Instead of trimming it, the installer threw the whole length up there. When flexible ducts are not stretched out tight, there is very bad airflow. The customer is probably losing over 50% of potential airflow from the fan.
I’m going to cut it, shorten it, and reseal it to the housing. I need to give it at least 18 in. to 2 ft. of fairly straight length from the housing of the exhaust fan before it starts to turn. That way, there’s less of a restriction of flow. There’s no need to measure, just eyeball. The wire that forms the slinky needs to be cut; that’s where the tin snips come in handy. Once I get the insulation back underneath the housing, I can connect the duct. To seal this, instead of regular duct tape I’m going to use some acrylic-faced aluminum tape. I make sure to keep that insulation away from the adhesive face so I don’t contaminate it. It’s important to seal the exhaust fan well at the joint so the air doesn’t bypass or leak out of the joint. If it leaks, it can condense in the surfaces area around here and potentially lead to mold or rot. Before I put the insulation back, I’m going to establish that nice sweeping curve. Now there will be much better airflow out of the house.
Videos in this series:
- Sealing Wire & Plumbing Penetrations
- Sealing a Ceiling Electrical Box
- Sealing Wall Intersections
- BONUS: Materials Overview
- Sealing Ductwork
- Sealing Soffits and Chases
- Insulation Ventilation in Rafter Bays
- Improving Attic Knee-Wall Insulation