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Community and Q&A

Insulating Ventilation Equipment Ductwork

jonny_h | Posted in General Questions on

Hi all, a couple questions about ductwork & ventilation equipment insulation practices & materials:

For reference, I’m in Ohio (zone 5).  All ducts in questions are fully within the conditioned space.

Situation 1:  I’m installing an inline filter box before my ERV and kitchen exhaust makeup air system.  The walls of the filter box itself are just metal, and it seems like a bad idea to have a 2+ cubic foot box of outside air inside my conditioned space 😉  I’m looking for ideas on how best to insulate (& provide a vapor barrier for condensation control.)  Possible options include:

– The duct insulation that seems to be most commonly available around here is R-6 FSK-faced fiberglass duct wrap (let’s not mention the bubble wrap stuff.)  R-6 seems a bit low for a box of outside air, when my walls are like R-30, and this stuff is also pretty flexible and might be difficult to wrap nicely around a filter box, but it is commonly available.

– I’ve found a source for higher-density FSK-faced rigid fiberglass duct insulation board, which gets me to R-8 or 9, but it’s not as available — it’d cost about $200 (with shipping, or $100 + 2 hours of driving to pick it up)

– I have plenty of leftover insulation, if any was usable:  2″ unfaced rigid rockwool board, 1.5″ foil-faced polyiso board, and 3.5″ or 5.5″ unfaced mineral wool batts.  The polyiso isn’t rated for being left exposed on the interior, so although it’d be great I don’t think it’s an option unless I build another box of some sort around it.  The various mineral wools would perform fine if I could supply an appropriate vapor barrier cover.  Should I go with a hybrid approach, like using my mineral wool board plus a cover of the R-6 duct wrap or the bubble wrap for a vapor barrier?  How do people insulate fresh air intake stuff, or do y’all just not worry about it?

Situation 2: I’m installing a ducted mini-split unit.  Although all ducts will be within conditioned spaces (mostly a small conditioned attic), I’ve seen some references suggesting that they should be insulated to some degree for condensation control during cooling season.  This does make sense — if the unit is capable of cooling below the interior ambient dew point, then logically the air coming out of it and thus the ductwork carrying that air could be below the dew point and condense water.  I assume that for condensation control inside conditioned spaces, the commonly available R-6 duct wrap would be sufficient — but before I go to the effort of this, can anyone confirm whether this is a real concern or if I’m overthinking this?


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  1. DennisWood | | #1

    We're in 7A, so quite a bit colder, but I just finished an update to our air exchanger system (to 6" wall penetrations). I used short lengths of 6" insulated flex duct for the connections to outside (with 6" rigid elbows for the bends), but then added an additional 8" insulated sleeve over that to take the R value to ~R9. Code minimum for duct insulation in this climate for HRV/ERV exterior supply/return is not enough in my opinion. Given that HRV/ERVs will have just an inch of insulation interior, you'd likely be fine insulating your filter box with 1 1/2" to 2" EPS on the outside to make it a bit easier to manage insulation and vapour barrier in one go.

    I have an inline filter in our system, but it's on the warm side of the heat exchanger specifically to address the insulation challenge. With outside temps as low as -30C in winter, having a filter box on the cold side of the exchanger would 100% require careful attention to insulation/vapour barrier as frost and water damage would be a certainty.

    On your mini-split in cooling mode, you might see air in the 55F to 60F range in the ducts. 70 degree air at 50% humidity has a dew point at 50F. 70 degree air at 60% humidity has a dew point at 55.5 F. Depending on your home's humidity control, temp set point etc. you may see conditions at times where the duct is around the dew point of your interior air, so R4 to R6 would be fine to ensure this does not happen.

    1. jonny_h | | #2

      Thanks for the reply! Nesting progressively-larger duct insulation sleeves is a clever idea.

      You mentioned "you'd likely be fine insulating your filter box with 1 1/2" to 2" EPS" -- I have plenty of 1.5" foil-faced polyiso around, which would be easy to work with, good insulation, and a great vapor barrier, but it's Johns Manville AP Foil Faced, which I think isn't rated for being left exposed -- I'd need to either build another box of something that counts as an ignition barrier around it, or try to find something like JM CI Max or Dow Thermax, which don't seem to be readily available / can only be special ordered in large quantities.

      1. Patrick_OSullivan | | #4

        > I'd need to either build another box of something that counts as an ignition barrier around it

        Speaking as someone who tends to follow the code pretty honestly, I think you're overthinking this. If you want to throw some leftover rigid foam on ducting/filter boxes, I say have at it.

        Even if it's technically not allowed, it's such a small amount of material in the grand scheme of things that it introduces no net new practical risk.

        Think of it this way, you can install the ducting without installation, be code compliant, and then immediately store a stack of this same insulation directly below your ducting and the building code can't regulate that.

        Similarly, you can have a king sized memory foam mattress in your bedroom and the building code can't do anything about that either.

        In my own home, I prioritize addressing practical risks above navigating every intricacy of the code.

  2. Expert Member
    Akos | | #3

    I'm in the north edge of zone 5 with pretty hot and muggy summers and ducting is almost always inside the house is never insulated.

    About the only time I've had ducts/air handler sweat is after major drywall work or when the place was left with the windows open in the middle of the summer with the AC off. Even then, the only sweating was for a short time until the AC managed to dry the air a bit and dewpoint dropped.

    Unless you have very major air leaks in your basement and your equipment is running properly, the indoor dewpoint will always be bellow duct surface temperature so the ducts won't sweat. The one spot some insulation makes sense is for flex runs to bedrooms, this helps a bit with sound.

    My filter box is insulated with foil faced polyiso and I loose no sleep over it. I guess you can wrap the rigid in drywall/plywood/sheet metal to comply.

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