Earlier this summer I went up into my attic to retrieve something, and was surprised to feel a cool draft. I discovered that a gap had developed between two portions of the big metal box that distributes cold (or hot) air throughout my house. (This box apparently contains both the blower, and the gas heater.) From the first picture below, you can see that a wire-mesh fabric had been pasted down with this white stuff that eventually cured to form a seal. But over time the seal here broke. In fact, it looks like some settling may have occurred, and the unit shifted about a centimeter to the right. (This is at what I'll call the "far left end" of the air exchanger.) Image of broken seal This was installed about 2 years ago, when my previous unit died. The installers placed it on some styrofoam blocks, which are themselves sitting on a sheet of plywood. However, at the far right-- the opposite end from the broken seal-- they missed the plywood; there the styrofoam blocks are sitting on top of blown-in insulation, which is pretty much worthless as a foundation. (See second picture.) However, this overhang of weight is so small-- it itself is just a styrofoam duct junction-- that I doubt that it caused the shift. (There are styrofoam blocks every 12 inches or so for the entire length of the unit.) Picture of styrofoam blocks

So, my question is: is this something that an inexperienced DIYer should tackle, or is there any reason why it would be best left to a professional? And if it is reasonable to DIY, what is that white material that failed? Should I buy more of it, or go with something else?

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    Actually with the horrible support job your contractors did, the constant vibration of the air exchanger when its running would probably be enough to cause the settling once the seal between the duct and the unit was broken. I'd be SCREAMING at the jack-holes who did this job to get out here and re-do it properly. Insulation is not a structural support!!! Commented Jul 1, 2014 at 10:02
  • I appreciate the advice you've given here. But could you confirm that when you say "horrible support job" and "insulation is not a structural support", you are referring only to the right-most styrofoam blocks? Commented Jul 1, 2014 at 16:22
  • (Note that the settling occurred on the left-most end, farthest away from the last picture.) Commented Jul 1, 2014 at 16:29
  • Well, I for one wouldn't use Styrofoam to support a heavy, vibrating object in any case, but some foams CAN be strong enough to take it, so I was staying clear of that. But frankly I think the idea was bad at its core. Commented Jul 1, 2014 at 19:52

1 Answer 1


Yes, you can fix it. First, fix the support for the unit.

The white stuff is a duct sealant. You can find it at pretty much any big box store in the furnace aisle. However, rather than messing with that - after you fix the support of the duct work and get it re-aligned, you can seal the gap with aluminum tape. Not "duct" or "duck" tape - that's not actually meant for ducts. You want the aluminum foil tape. Simply apply to completely cover the gap and press down on the tape everywhere with a rubbing motion in order to ensure a good seal.

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