Duct Sealant—Longitudinal vs. Transverse

Harry Thompson IVSeptember 25th, 2018

Longitudinal vs. Transverse

Sealing Rectangular Ductwork: Choosing a Sealant with Characteristics that Match the Seam/Connection and the Point of Application

Duct Sealant Characteristics

Longitudinal Seam: a narrow lock that usually is long and continuous and moves in one direction. Covering this lock with more than 1/2“ width of sealant makes for a messy appearance and a waste of mastic. It‘s often sealed in the fab shop before going out to the Job.

Transverse Joint: An end-to-end, four-sided Connection that will require a min of 2“ width of sealant if it‘s Slip and Drive and up to 5“ if it‘s TDC. It‘s almost always connected in the air on the job site and Sealed once it‘s hung and stationary.

Commercial Rectangular Duct has a Longitudinal Seam down the length and a Transverse Connection on each end.

Many sheet metal contractors use a different type of Duct Sealant on the Transverse Connections than they are using on the Pittsburgh Lock running down the length of the duct section. That’s because the Longitudinal Seam is a long and linear length, with a seam that has a narrow width and is often sealed in the fab shop. In contrast, the Transverse connection is four sided, much wider and almost always applied on the job site after the duct is hung.

Longitudinal Seam:

The ideal mastic for the three characteristics of this seam is one that’s very smooth , and thus easy to apply in a straight and narrow fashion but also, since it’s often applied in the Fab shop before transport to the Job Site, non-drying, which will give more flexibility during handling. In fact, Fiber free sealants like Hardcast’s 550 Flex Grip , Ductmate PROseal or anything sprayable remain flexible because any Prefabricated Duct work demands it. The idea of a Sealant that stays permanently flexible is similar to the type Ductmate has in every sleeve of their #35 Flange or that some shops are pumping into the Longitudinal seam during fabrication (aka the Pitts Seam Sealant).

Since this rules out all Fibered sealants as a good choice on the Longitudinal Seam because once they dry , they will crack if the Duct Section is handled or transported, the niche for Fibrous mastic is the Transverse.

Sometimes the Longitudinal Seam is sealed on the inside of Duct rather than exterior.

Hardcast 550 Flex Grip Fiber-Free Sealant on interior duct of Pittsburgh Lock in Atlanta. Contractors use a cartridge and caulk gun to apply a smaller bead, then smooth it with a squeegee.

On the narrow longitudinal seam, many shops choose to use a cartridge and caulk gun instead of a bucket and brush.

Transverse Connection:

Fibered Duct Sealants have a thicker consistency and are ideal on the Transverse connection that’s already connected and hung. Covering more width with every stroke of the Brush using a mastic like the Hardcast CCW181 makes it easier to ‘glob” the mastic on all four sides of the duct. Plus, since the ductwork is already connected end to end, the field crew is applying this mastic while up in the air, sometimes with a chip brush over the head. Just like peanut butter is more likely to stay on a utensil than margarine, thicker means better when avoiding any mastic falling into one’s face when brushing over the head.

CCW181’s fibrous consistency is ideal for covering a wider joint or avoiding mastic falling off when brushing overhead.

550 Flex Grip’s smooth fiber free consistency allows for a cleaner look on a narrow seam like the Pittsburgh lock and it stays flexible after drying which means shop application is no cause for concern during handling.

Using a fiber free Sealant in a cartridge allows for a clean narrow bead of mastic up the Longitudinal Seam. 5 ft lengths like these require sealing half way and then flipping over the seal the balance.

All of this fibrous sealant (CCW18 White) was applied after duct was hung and connected.

The Longitudinal Seam is very narrow and flows in one direction, requiring less than a .5″ width of sealant. The Transverse Connection is an end to end, four-sided connection that has a width spread of 3-5″ of sealant.

White Heavily Fibrous Hardcast CCW181 on Transverse Joint Connections in New Orleans.

The Future...

The increasing challenge of finding Field Help for Mechanical and Sheet Metal Contractors has bolstered the trend to do as much of the process in the fab shop rather than the job site. Straight Duct Sections sent to job site seamed down the length, ready to be hung and connected end to end is common; but increasingly, many contractors are going a step further and connecting and adding mastic to the Transverse connection in the shop too. This would mean an increase in the Fiber Free Flexbile when dry mastics as they would be used on both theLongitudinal Seam and the Transverse Connection.

Understanding how and at what stage a contractor seals rectangular duct is a priority for us in adding value as a supplier. Matching the characteristics of the sealant with the seam or connection. Years ago when most all duct was seamed and connected in the field, the choice was typically a thicker consistency. Painting duct mastic with a chip brush over the head meant that the more sealant the brush could hold, the less that might fall into one’s face when brushing onto Duct. To make it Thicker means adding more fibers, the more fibers, the thicker the product. Thicker means it can cover more width and since the Transverse Connection is about 5x the width of the Longitudinal Seam, this makes a difference.