Semi rigid duct straight couplers

[jayc89]

A couple of my duct runs are too short, at the distribution box end. I plan on getting a couple of straight couplers, like these - https://www.paulheatrecovery.co.uk/product/connector-for-90mm-semi-rigid-ducting/ - should I use anything else when using these; an acrylic sealant within the sleeve and airtight tape externally? 

[Conor]

I used both, sealant and tape. But try and find ones with rubber gaskets, so much handier.

[Savage87]

whenever I used couplers or elbows on semi rigid ducts, they came with rubber gaskets on the inside and I used acrylic sealant and aluminum foil tape, a bit of overkill but it works.