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Insulating ducts to exterior in CZ6

evanlebrun | Posted in General Questions on

I (very, very foolishly) didn’t plan for ERV ductwork before framing my home in climate zone 6. The biggest challenge for me is figuring out where to run the two exterior ducts for the unit I’m considering (Panasonic Intellibalance 100).

This is an 1800 sq ft. two-story home with 2 occupants. The ceilings are 8′ with a vented attic. I’m not planning on running any ductwork through the unconditioned attic.

A couple questions:

1. ASHRAE recommends R-12 insulation for exterior ducts in my climate zone, but I can’t find any R-12 insulated ducts or R-12 duct wrap or sleeves near me. I’m assuming that R-12 duct insulation exists given the recommendation, but where can I find it? 

2. The Intellibalance can use 4″ or 6″ ducts. I’m sure 6″ would be a better/more efficient size for the ducts to the exterior, but is there a severe energy or performance penalty for going with 4″ instead of 6″? If I need to insulate to R-12, a 4″ duct would be way less obtrusive.

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Replies

  1. plumb_bob | | #1

    Im in CZ7A and we insulate ducts with r6, I believe most of the major brands offer r6 insulation. I remember looking in our code and that level of insulation is compliant here.

    Where are you going to terminate the ducts at the exterior? The intake and exhaust must be separated so stale air is not brought back into the house, and the exhaust should not be directly adjacent to opening windows and doors. Typically these will get terminated at a gable, through the box joist, or through the roof.

    For what its worth, I never see 6" ducting for HRVs, just 4". I do not often see ERVs.

    1. evanlebrun | | #3

      Thanks! I’m still determining where to end the ducts at the exterior, but both of the easiest places to do it are on the eaves side — the second floor of the south side under a 2’ overhang, or the first floor of the north side under a shed roof where I’ll be keeping my firewood. I plan to keep the ducts 10’ apart per the IB100 install manual.

      I’m leaning toward the south side on the 2nd floor because it would be a lot easier to detail and also because I may want to close in the woodshed on the north side some day.

      Terminating through the roof would mean running the ducts through the unconditioned attic which I’d prefer not to. Is terminating under the eaves problematic if they’re under a large overhang or a roof?

  2. Expert Member
    Akos | | #2

    The easy way to get higher R value is to get a insulation sleeve for a larger duct. For example a 6" insulated duct would fit inside a 10" duct sleeve. This is somewhat annoying to do, so I would only do it if you have long runs. For a short run the standard R6 is good enough.

    As for the 6" VS 4". Short runs of 4" smooth pipe are borderline for 100CFM. It can work but will add to your restriction, you'll have to see how the rest of your ducting turns out if it can be made to work. You want to aim for the duct losses including both outdoor and indoor to be within the autobalance range of the IB100 which is 0.4" WG. A 6" flex is a less restrictive option and easier run.

  3. evanlebrun | | #4

    Thanks! One of the exterior ducts would end ~8’ past the unit and the other would be ~2’ most likely. If I go with a 4” rigid duct, would that be a little closer in efficiency to a 6” flex?

  4. DennisWood | | #5

    Use 5” duct hoods if you can, and 6” insulated flex to the unit. Yes 4” hoods will work, but 4” duct will be quite restrictive..using both will drop air delivery from the ERV considerably. Remember the ERV is rated to 100CFM at low static pressure, so if you add restrictions you will also drop output. For boost, you’ll want to max airflow as near to 100 CFM if you can.

    No one mentions this, but if you create a restrictive duct system, the ECM motors will need to use more power to meet your target, and will use this extra power 24/7. It may be only a 30 watt penalty on the IB100, but it will be there 24/7 for the life of the product.

    1. evanlebrun | | #9

      Thanks! This is helpful. Sounds like I should at the very least make my exterior runs 6". I'm debating whether to do those exterior runs in flex duct or metal. One of the runs will go through a bedroom which seems like an argument for flex if the static pressure drop from that isn't too substantial.

  5. onslow | | #6

    evenlebrun,

    If you are running both duct paths through conditioned space why do you need full run insulation? I am planning for a similar setup and, so far, only see a need for a bit near the exit/entrance points to avoid condensation on the exterior side of metal ducting. Am I missing something?

    1. evanlebrun | | #10

      I must have been unclear, but I'm not planning on insulating anything but the runs to the exterior.

  6. Expert Member
    Akos | | #7

    You have to be careful where you put your outside wall terminations. Not too close to the ground, away from a driveway where a car could idle, far from the neighbor that likes to smoke a cigar or barbeque all the time and not above any asphalt roof surface. Soffit is actually a good spot for these. I would prioritize clean air for the vent location even if it is harder to run, having to turn off the ERV each time your neighbor starts up a wood burning fireplace gets old pretty quick.

    4" duct is pretty hard at 100CFM. For a reasonable loss of say 0.15" WG for each outdoor run you have 20' of equivalent length. A wall cap with the damper removed is about 15' equivalent length, doesn't leave a lot and definitely no room for any bends. You can do it all in 5" hard pipe but 6" flex is a much easier option plus it connect straight to the unit.

    Only the outdoor connections (OA and EA) to the unit need to be insulated, the indoor connection (RA/SA) can be uninsulated as long as they run inside conditioned space. If you are insulating over hard pipe for the outdoor connections make sure the vapor barrier sleeve is sealed tight otherwise you will end up with condensation on the duct.

    An in-between option to hard pipe is to use semi rigid aluminum pipe (similar to dryer vent pipe). These are still easy to run, much lower loss than flex and can be insulated with a flex duct insulation sock.

    1. DennisWood | | #8

      Yes, just your OA and EA ducts need to be insulated and R6 (in zone 7A) is still standard.

      Akos’ comments on the fresh air intake are spot on…location is everything. In a cold climate, under an eave on the south side (where temps will be a few degrees higher) is a good bet. In a warmer climate try to locate the fresh air intake on the shaded side of your building.

      One comment on insulating aluminum flex vs using the insulated flex (plastic). The aluminum flows well, but also conducts cold more readily, so if your ducting outside air seal is not 100%, condensation will be worse on the aluminum. I’ve used both…

      1. evanlebrun | | #12

        Thanks, Dennis. I'll see how much insulation I can fit in the space, but I don't think there's anyway I can fit a 6" duct and R-12 insulation above the windows where I'm going to need to run a chase for one of the ducts to the exterior, so it's helpful to know that going below R12 still seems to work in cold climates.

    2. evanlebrun | | #11

      Thanks! Thankfully we live in a rural area and are building on a 10-acre lot so no neighbor smells to avoid. The tip about aluminum pipe is really helpful. I still have to calculate the static pressure drop for the runs I'm planning, but the more I research, the more it seems I should keep flex ducts to a minimum.

  7. DennisWood | | #13

    You're welcome Evan :-)

    R6 is standard, but like the NRC found in their Arctic studies, the amount of pre-heating that occurs in even relatively short run intakes and exhaust is surprising. I'm seeing about 3-5F in just six feet in my instrumented setup (gain) from ambient. In other words, although R6 is pretty standard for flex and will address condensation, in my opinion, it's not enough for cold climates. So if you can insulate to a higher level (particularly with longer EA/OE runs), do it.

    You can always limit CFM flow at a duct's exit, but it's pretty hard to increase duct size after! I've spent about a month rabbit holing on heat exchange, flow and efficiency and it's surprising how quickly flow drops off once you start reducing duct diameter, add bends, silencers, filtration etc.

    There was a 20 CFM gain in just freeing up a stiff exhaust flap so it would open fully.

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