Replies

1. 
   charlie_sullivan | Mar 23, 2021 07:41am | #1

   I would rate mastic as the best way to seal it well.

   But I would also suggest considering a heat pump dryer that doesn't need a vent.

1. 
   pnwbuilder | Mar 23, 2021 12:15pm | #3

   Thanks Charlie! I was leaning towards using the mastic too, but am wondering if it's lifespan would be limited in this application due to high temperatures. I am considering heat pump dryer as well, just don't want to limit my options. It is much easier to install duct now than do it later. Very "painful" experience though to drill a 4" hole in the exterior wall after spending a lot of effort on air sealing :)

2. 
   Patrick_OSullivan | Mar 23, 2021 09:07am | #2

   Good tape works well for sealing relatively straight and flat seams. Check for the appropriate UL ratings on the tape (https://tapeuniversity.com/products/foil-tapes/ul181ap-ul181bfx/).

   Mastic really shines for irregular shapes and corners.

1. 
   pnwbuilder | Mar 23, 2021 12:17pm | #4

   Patrick, thanks for the link. My main concern is the longevity of the method I use since the duct is going to be inaccessible.

3. Expert Member
   BILL WICHERS | Mar 23, 2021 12:49pm | #5

   I would use good quality foil tape here. I actually think it will hold up better than mastic in this particular application, and I've used both methods. I like mastic for sealing fittings and transitions, but foil tape is good on new, clean duct and linear seams as Patrick mentioned.

   I'd clean the duct with some isopropyl alcohol or acetone prior to applying the tape, this ensures there is no oil residue from manufacturing that will interfere with the tape's adhesive. If you want extra durability, put on a second layer of tape.

   Bill

1. 
   pnwbuilder | Mar 23, 2021 02:22pm | #6

   Bill, would you recommend any particular brands/types? Do you think foil backed mastic tapes like Nashua 360-17 would work better than the ones with acrylic adhesive? It seems that mastic tapes have thinner aluminum backing(2 mil) vs non-mastic tapes(4mil and higher).

1. Expert Member
   BILL WICHERS | Mar 23, 2021 02:58pm | #7

   I've used Nashua 324A for this type of thing in the past. I'm pretty sure my roll here is 324A, but when I checked, it appears they don't list a part number on the cardboard tube, and I don't have the packing wrap anymore. Home Depot carries the 324A though, so it's easy to get.

   You want the thicker backing if you're going to tape the duct then slide it into place. You need the thickness for tear resistance. I've never had a problem with the acrylic adhesive as long as the duct is clean first. If there is any oil residue (which seems to be pretty common on new duct parts), it's really important to clean that off first so that the tape will stick well.

   Bill

1. 
   pnwbuilder | Mar 23, 2021 03:34pm | #8

   Thanks Bill, 324A looks like the best option here.

1. 
   Patrick_OSullivan | Mar 23, 2021 05:41pm | #9

   I can vouch for that tape being pretty durable with a very aggressive adhesive.