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Sealing Duct from the Interior

Harry Thompson IVMarch 4th, 2019

And why it ensures the tightest seal...

Many Sheet Metal Contractors recognize that ALL of the Duct’s Pressure is from the inside…pushing out, and that sealing the lock’s interior is not only aesthetically better but also better from an engineering perspective. Many engineers don’t want to see mastic on the exterior and the pressure test will reveal any ductwork that is unsealed anyway. And some engineers spec cartridges of duct sealant on the interior of the ductwork rather than the exterior due to the benefit of extending the life of the seal upwards of 30%.

Arrows indicating the only point where leakage could occur. Sealing the Pittsburgh Lock on duct interior requires only a narrow bead of mastic. Shops that are aware of this use a mastic in a cartridge.

Good: Bucket and Brush

With a Brush and Bucket this is what sealing the interior duct Pitts seam often looks like.

Bucket and brush, though sometimes labor intensive is the most common method in HVAC Duct for Longitudinal Seam, Transverse Joints and Duct Penetrations.

Better: Caulking Gun

Cartridged Duct Sealant for a cleaner application. Ideal for the Longitudinal Seam; caulking provides small bead which can then be smoothed out with a squeegee.

Cartridge and Caulk Gun
Duct sealant applied by cartridge gun to Pittsburgh Lock on duct interior.

Best: Extrusion Gun Kit

Hardcast’s Cartridge Killer allows the shop to apply a bead of mastic the same way they would use a Caulk Gun and Cartridge of Mastic but instead with an apparatus that applies the bead of mastic using an applicator with extrusion tips and a wand extension connected to a hose, Graco 390 pump and 5 gallon pail of Duct Sealant.

For a contractor time is money and spraying is much faster than brushing. Most duct shops understand this because they used to brush on adhesive and now they spray it on. The same applies to duct sealant . With the Graco 390 Model, the contractor can use this 0- 3300 psi Pump to extrude a caulk bead of sealant or spray on the sealant . Adjust to a position point that’s a half dial (or approx 1500 psi) for Spraying. Then, for the Extruder, start at 0 and keep bumping up to the desired bead.