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ERV Ducts in Insulated ceiling

markgh1 | Posted in General Questions on

I have decided to build a passive house 4 plex utilizing a double stud wall assembly at r45 and ceiling insulation on the second-floor units at R80 insulation.  My question is this.  If I run my ERV ducts (flexible duct sealed to passive house standard) in the space directly above the bottom chord of the roof trusses, won’t the r80 act as if it was a conditioned space, or does the ducting have to be insulated flexible duct or do I need to get the rvalue to R100?  On the first floor, erv unit ducts run in the floor trusses of the next floor level.  I also have the choice of installing the ERV’s in the Closet ceilings or in the hallway ceilings external to the units themselves is either one better than the other?

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  1. Expert Member
    Akos | | #1

    Running any supply/returns ducts in the attic is asking for trouble no matter how much care is taken.

    You can run the outdoor connection ducting from ERV through the attic, but keep the rest inside.

    The supply ducting for a room is very small, you can use 2.5" or 3" un-insulted semi rigid aluminum ducting which is easy to run through interior walls. Simplest is to run all the ducting through the floor bellow the unit and up into the walls as needed. The ducting and registers meant for high velocity AC is great for this (not cheap though).

    ERVs need easy access for filter change, so try to locate them in a convenient spot. Mounted on one of the walls of the mechanical closet is probably the best spot.

    Since ERVs can be installed in any orientation, go for one of the slim ones meant for ceiling install but install it in a vertical orientation. This way the outdoor connections would be on the top for an easy run up and outside and the indoor connection on the bottom to run it into the floor joist space bellow.

    If you must run the ducts through the attic, it is the same as dealing with air handler ducting, follow the rules here:

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