Pre-formed duct sleeves are decent for insulation, but in my experience, they don’t really address leaks the way mastic does. I’ve used those wraps in a pinch, mostly to help keep the crawlspace ducts from sweating in the summer, but if there’s any air movement at the joint, they just don’t seal tight enough. If you’re after longevity and less hassle down the road, sticking with mastic (even if it’s a pain) seems to pay off. The “quick fix” stuff looks tempting but rarely holds up through a full season—at least not for me.
I’ve run into the same issue with those sleeves—they’re fine for condensation, but if you’ve got any kind of pressure at the seams, they just don’t cut it. Mastic’s messy, but it’s the only thing I’ve found that actually holds up over time. One trick I’ve picked up is using mesh tape with the mastic for bigger gaps; it’s a bit more work, but it really locks things down. Curious if anyone’s tried foil tape as a compromise? I’ve seen mixed results, especially in damp crawlspaces.
Honestly, I’ve never been a huge fan of foil tape in crawlspaces or anywhere moisture’s an issue. It looks tidy at first, but give it a season or two and you’ll see the corners start to peel—especially if there’s any flex or vibration in the ductwork. Mastic’s messy, yeah, but once it’s cured, it’s basically bulletproof. I get the frustration with sleeves too; they’re more for show than substance when it comes to sealing under pressure.
That said, for really tight spots where mastic just isn’t practical, I’ve actually had better luck doubling up with butyl tape under the foil tape. It’s not perfect, but it does buy you some time and holds up a bit better in damp environments. Still, nothing beats a proper mastic job if you can swing it. Just my two cents from crawling around too many basements...
