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Question DetailsAsked on 5/19/2017

In AZ, metal ducts in dropped ceiling but open to attic. What is best practice to seal/insulate?

In Arizona, sheet metal ducts running in extremely hot attic. The sheet metal ducts are in a dropped ceiling that is open to the attic with only about 3 inches of blown cellulose over the sheet metal ducts.

Can the dropped ceiling be sealed off bringing ducts into conditioned space. If rigid foam is used for this, can it be left exposed in attic or does it need to be covered with drywall? Will I run into moisture problems if top of dropped ceiling is air sealed.

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Ok - A/C ducts in attic are bad practice as you appear to realize, but is very commonly done, especially in ranch story houses - unfortunately meaning probably most common in the hottest areas of the country where it has the most impact on your A/C efficiency. Sort of like running the cold air ducts through a big oven to preheat your expensive cool air - not a real bright solution.


On the spray foam, varies by local code - many areas are starting to ban ALL exposed foam insulation due to the fire hazard (to occupants in case of fire, and to firefighters) as the fumes o the burning foam are quite deadly, some (and most common national/international building codes) currently only prohibit it being exposed in living areas and allow it in exposed in crawlspaces and attics and such which are firewall protected from the house - and in some areas do not even require the firewall - just normal construction. Can do a job on a house - I have seen several buildings and also several house trailers/modular homes that burned very intensely because of exposed sprayed foam in a crawlspace catching fire from heat tape on pipes, buring through the flooring and coming up through openings in the floor (especially with plastic HVAC ducting) like a blowtorch.


Also on spray foam - if doing this yourself - a can of spray foam gaenerally produces about a cubic foor of foam, and commercially applied foam is more expensive than fiberglass batting by a factor or two or three commonly, maybe more for this small a job - so can get quite expensive for a small job like this.


Sprayed-in foam does encapsulate ducting well but causes a couple of problems which makes me recommend against it for that use - because it is in close contact with the metal but does not totally block out air movement, you commonly get condensation on the metal which makes them rust out quicker. Also makes it a pain and more expensive if the ducts ever need work on them because the tech has to dig out the foam, though that is a secondary consideration because metal ducts rarely require work, commonly lasting 50-100 or more years before needing replacement.


The dropped ceiling area can be and commonly is made part of the conditioned space - and should be if there are open penetrations like flush-mounted ("trougher") flourescent lights or non-airtight can lights because those let a lot of moist and temperature-controlled household air into the attic. Problem on existing dropped ceilings - you generally have to tear the dropped ceiling drywall AND joists out, put vapor barrier and drywall up on the underside of the attic joists or trusses like for a normal ceiling (and this should be painted to protect the drywall paper against condensation), then replace the dropped ceiling joists and drywall to do this, so a pretty major undertaking and quite rarely done - though converting the dropped ceiling to full height is very common, because low headroom clearance dropped ceilings are pretty much out of


Generally, if the dropped ceiling is underneath an "environmentally airtight" (vapor barrier/drywall) "ceiling" on the underside of the attic joists/trusses, it is recommended that the dropped ceiling void be ventilated to the inside space - commonly with louver vents (aka architectural grill) like these -


https://www.amazon.com/Architectural-...


https://www.amazon.com/gp/product/B00...


opening into the airspace through the side ofthe dropped ceiling. If there are underside openings like troughers, then side vents into the ventilated space on the side furthest away are commonly used. If there are no underside air openings of note, then commonly side vents are placed on two sides of the dropped ceiling as opposite each other as possible - like in the common kitchen application, on the dining room and kitchen door sides.


My recommendation, since the ducts are exposed in the attic and I am no fan of exposed foam insulation (having seen/heard of several nasty cases of fatalities from the fumes or rapid-spreading flames in house fires) - just wrap them as much as you can, given the available space, in unfaced fiberglass batting (to allow any moisture coming from below or from the ducting to escape) and then put another 6-12 inches or so of unfaced batting over the top. Some HVAC contractors and industry blogs recommend coating the ducting in a 1-2 inch layer of commercially sprayed on closed cell foam (which sticks MUCH better than foam-in-a-can) then batting around that for a cheaper solution than 100% foam. The energy code standard is for R6 minimum on heating/cooling ducts except in the colder parts of Alaska and Canada where it is R8 minimum - return ducts are required to have R3.5 insulation in vented attics. [Where heating and cooling has its own ducts the insulation requirements are lesser for supply ducts in some areas - lower for heating in warm areas, lower for A/C ducts in cooler areas - see ASHRAE 90.1]


It is generally recommended that if you are doing any sort of insulation around a duct that the seams be sealed first - though some feel commercial spray-on closed cell foam does the job fine. There is duct caulk for "open" seams, and clear and colored (to better see where you have done it) spray sealant for properly connected seams (which still leak some air unless they were assembled gasketed, which is rare in homes). You can also use duct sealing tape meeting UL181A or UL0181B certification (double tape layer required so there is a fullwrap of tape sticking to the first layer of tape, because it will eventually peel off the metal if not well stuck to itself). These products are also available in firestop versions where the code requires it because the duct is passing through a potential fire source area like a utility closet or garage - but I have never heard of that being required for attic ducting, though not a bad idea for safety purposes.

Answered 1 year ago by LCD




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