From Sherman, M. and Walker, I., “Can Duct Tape Take the Heat?”, lbl-41434, Energy Performance of Buildings Group, Lawrence Berkeley National Laboratory:
Popular culture abounds with uses for duct tape in duct tape calendars, 101 uses for duct tape, duct tape books, etc. Unfortunately, it appears that duct tape should not actually be used to seal ducts.
[Reprinted as an article in Home Energy Magazine Online, July/August 1998.]
[Report LBNL 53547, on revised testing methods, with excellent pictures, showing that UL-181B-FX, regarding flexible ducts, is not a good predictor of duct tape performance when it comes to sealing ducts.]
I’m working on a number of sealing and caulking problems around my house. It’s all crap, it all depends critically on exacting, nit-picky, fanatically clean installation techniques that cannot be achieved in the field (i.e. in a filthy dusty hot sweaty attic), the instructions never seem to apply to my situation, and it’s all required by code, which basically says, “do it” without really saying how.
By the way, I laughed when I saw the picture in the above report of the duct sealed with clear packing tape.
It was one of their top performers.
So, what am I supposed to do with this $15 roll of UL 181B compliant foil tape?
The one sealing solution I’ve found that actually seems to work is roof shingling, and there’s a simple reason why: its performance does not depend on adhesives and sealants, but on the physical integrity of the shingles, and on the straightforward installation principle that the higher row overlaps the lower row. There is some tar involved, but that’s an adjunct.
What opened this whole can of worms was reworking my inadequate and unsafe water heater exhaust duct. The new vent, using the double-walled pipe known as B-vent, was improperly installed where it went through the roof. I have pictures, which I’ll eventually get around to posting.
When I re-worked it, I discovered that it does not adhere to the shingle principle. It must be caulked or taped to seal the roof joint. Appalling. It will fail, eventually, and likely before the shingles do.
There’s a critical component called the storm collar, which exposes an upward-facing joint. I cannot find good clear instructions on sealing the damn thing. Apparently, what you do is run a thick bead of caulk (I used GE’s 50G.01 Silicone II Gutter and Flashing sealant, because it says not to use it on surfaces above 400 degrees, where everything else is limited to much less than 200) around the bottom of the pipe, then push the collar into that ring, then smear an additional bead of sealant around the joint. This joint is going to see a lot of movement, the application method seems contrary to everything I’ve read about good caulk joints, and it’s going to fail.
The original vent was unsafe, not intended for use as a combustion exhaust, but at least it kept the rain out without sealant.