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Community and Q&A

HRV ducts in a attic

mfroberts | Posted in Building Code Questions on

I am building in Bellingham WA (zone 4, coastal) Our local building inspector is insisting that all the HRV ducts be lifted above the cellulose attic insulation.  He says the code requires it but I can think of no reason this would be a good idea.  We will be installing 18 inches of insulation in the attic.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Mike.

    HVAC ducts belong in the conditioned envelope of a home. When they aren't, there are big energy penalties. Burying ducts in insulation seems like an obvious next best scenario, and it may be, but doing it right is tricky. The current International Residential Code does allow it, but that doesn't mean that local building officials will. You can read more about it here: Burying Ducts in Attic Insulation.

  2. MattJF | | #2

    The ducts leading to and from the supply/exhaust registers in the home should be within the thermal envelope. At least bury those.

    The supply duct from the outside is outside air and should not be buried. The same applies to the exhaust duct connected to the outside. Both should have an insulation wrap to prevent condensation. Both of the ducts connected to the outside should be as short as possible.

  3. Expert Member
    Akos | | #3

    Air in the fresh air supply and stale air return of the HRV is pretty close to room temperature. It is not as cold as the supply lines from an AC unit.

    This means that in most moderate climates where the outside dew point doesn't often go above 75F, you can bury the ducting and not have issues with sweating.

    None the less, it is a good idea to insulate the ducting, even an R4 jacket buys you a fair bit of margin (R8 is better) if you are in a warmer climate.

  4. mfroberts | | #4

    Thank you for the responses. All the ducts are insulated with a R8 jacket (flex duct). The issue is that the inspector is requiring me to suspend the ducts out of the ceiling insulation and I am looking for a building code section I can use to justify leaving them in the insulation. The attic will be insulated with 18 inches of cellulose.

  5. Expert Member
    Akos | | #5

    This a good start.


    You can always hang the ducts and bury them afterwards.....

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