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Community and Q&A

Double wrapping bathroom exhaust fan ducts in attic to increase R value

Otosan | Posted in Energy Efficiency and Durability on

I’m in climate zone 5a, in southern NH.

I have 2 baths that that I am planning to vent via 6 inch metal ducting and flexible ducting.  I am planning to vent with an in line Fantech 200+ cfm fan.

Previously there were 3 baths on this system but I’ve disconnected the longest run (45 feet) and will place a regular bath fan in that bathroom, vented on it’s own duct and termination.  This original set up was also vented with a Fantech inline fan.

This was installed in 1996 with very minimum fiberglass/vinyl duct insulation.  It looks like 1/2 inch or less.  The metal ducts were not air sealed either. The longest run is now 20 feet since I disconnected the very long 45 foot run. It was suspended above the ceiling insulation in an unconditioned attic and vented out of a roof jack due to having a hip roof.  A recipe for disaster, which over the years occurred.  So much condensation occurred in the 20 foot run from the inline fan to the roof jack that it leaked out of the unsealed galvanized ducting, soaking the insulation and rusting through from the outside in and dripped on to the ceiling drywall.  Condensation also occurred in the fan body which was uninsulated and was 45 feet from the far bath.  Some condensation also occurred in the end of that 45 foot run, just upstream from the fan.

My options as I see them are:
1. Plumb the remaining 2 baths with a similar set up of 6″ ducting and a new in line fan to the exterior (either roof or soffit (with a soffit vent designed to shoot the vapor away from the soffit air intake))  I would increase the duct insulation from the minimal R2(?) they have now to at least R8 and insulate the inline fan, which was not insulated originally.  I will still have a 19-20 foot run from the interior bath to the exterior.  The two ducts from the baths will join in a wye, then to fan, then to exterior.

Pros: a. One vent to exterior
          b. Can reuse existing infrastructure of bath ceiling vent and in attic      ducting.
          c. One fan instead of 2 fans.

Cons: a. Still have a long 20′ run from the interior bath.  The other run will be 10-12 feet.  I have this long run because that is the distance to the soffit or a reasonably accessible point on the roof decking.  I have a 9/12 roof and can’t reach the roof deck except near the eaves with out an ladder, which the tiny ceiling attic access and blown in insulation over the ceiling joists makes extremely difficult to use.

Option 2. Put in two regular ceiling bath fans connected to exterior via 4″ ducts.

Pros: a. Each of the 2 baths could be vented independently.  Not really a huge deal in my opinion.
          b. I could shorten the run for the exterior bath but not the interior one.

Cons: a. Two exterior penetrations instead of one. 
            b. Two additional ceiling/vapor barrier penetrations.

This is a very long winded way of asking for opinions, and advice.

Some more questions:
Can existing duct wrap, which includes a vapor retarding barrier, be over wrapped with additional fiberglass insulation, with it’s own vapor barrier, to increase R value?

In line with that, can unfaced fiberglass batts be wrapped over insulated ducts to increase R value.  I understand that the duct wrapping would have to be completely air sealed.

For my 3rd bath which will be vented with a Panasonic ceiling fan via a 4″ four to five foot run I was planning to use R6 flex duct due to obstructions.  Would I be better off to use R8 (if I can find it)?  I know that smooth wall ducting is preferable. 

Thanks for your patience and interest if you have read this far.

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  1. Expert Member
    BILL WICHERS | | #1

    The duct itself needs to be air sealed, the insulation wrap is much less critical. You want to try to pitch the ducts so that they slope back towards the fan a little (assuming a ceiling-mounted exhaust fan), which helps to prevent any condensate from pooling in the duct where it will eventually rust through and leak out onto the ceiling somewhere.

    I don't see an issue with a second layer of duct wrap as long as the innermost duct itself is air sealed. What you don't want is a situation with a leaky duct that is injecting moisture in between two layers of duct insulation that have seperate vapor barrier coverings. That would bring you back to the "water pools in the duct and leaks" scenario, with the leak this time from the duct wrap instead of the air duct itself.

    Rigid ducting is preferable. You can get it from commerical HVAC supply houses. You can insulate either with the usual "sock" style flexible insulation, or you can get split rigid fiberglass insulation. The more the merrier for insulation, but keep in mind that it's not going to be fun trying to apply a second layer of wrap in a very confined attic space.


    1. Otosan | | #2

      Thank you Bill. Any comment on using R6 for the 4-5 foot 4" duct? I guess I could always wrap it with more later if that proved inadequate.

      1. Expert Member
        BILL WICHERS | | #3

        R6 is probably the most common option, but more won't hurt. The primary purpose of this insulation is to keep condensation under control so that you don't have condensate running out of your vent fan.

        Be sure to insulate the housing of the exhaust fan too, which might just be burying it in loose fill insulation if that's what's already on the attic floor.


        1. Otosan | | #4

          Yes, that's what I've got, blow in loose fill.

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