Part VII: Seal Ductwork to Keep Moisture out of Your Attic

Series: Installing Blown-in InsulationPermalink

Save money by adding blown-in insulation to your attic.

Ducts that leak conditioned air into unconditioned spaces (such as your attic) can increase your heating and cooling bills.

Fear not, you can prevent that unnecessary waste by sealing and insulating your ducts with some inexpensive supplies and a little bit of effort.

Sealing Supplies Permalink

The supplies that I used were foil tape, mastic sealant, and a chip brush for applying the mastic.

It’s counterintuitive, but you should never use duct tape because it won’t last nearly as long as the mastic or aluminum foil tape.

If the joint or hole is larger than 1/8th of an inch it’s recommended that you use fiberglass tape

Apply Mastic to Duct Joints Permalink

I applied two coatings of the mastic on my bathroom exhaust vent and waited a day between applications.

You’ll notice that I unfortunately cut my rigid metal duct a little short at the bottom where it meets the elbow. I compensated by laying the mastic on thick. I hope that will sufficiently seal the exhaust duct so that moist air doesn’t enter the attic.

Wrap Duct with Insulation Permalink

Once the mastic dried I wrapped the duct with a layer of insulation. Had I more room I would have doubled up on the duct wrap. I used the aluminum foil tape to stop the insulation from unraveling.

We’re Ready for More Insulation Permalink

The only other metal duct that I sealed and insulated in my attic was the exhaust to my oven. This length of duct was considerably longer and had a greater circumference, but the steps I described above still applied.

Once the ducts were sealed and wrapped I was one step closer to blowing in more fiberglass insulation into my attic.

Leave a Reply