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Community and Q&A

HRV exhaust problem

Bob Irving | Posted in Green Building Techniques on

We completed this home in 2009, using Hardiboard cement clapboards. The vent at the bottom of this wall (on the northeast) is the exhaust from the HRV. The section of wall above the vent, to the right of the window, has peeling paint and delaminating siding, which we believe to be a result of moisture from exhausting air. An infrared scan shows the plume of hot, moist air which has the same footprint as the siding problems. No other wall areas in the house have any issues.

So the question is: what is the solution to the siding issue? The vent could be relocated to vent under the screen porch (to the right of this area) or to the west under a deck, but I don’t want to create a new problem there.

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  1. Bob Irving | | #1

    more info: there is a 3/4" rain screen cavity behind the siding.

  2. Stephen Sheehy | | #2

    Where is the moisture coming from? The HRV exhausts just air from inside the house, supposedly with most of the heat removed. It shouldn't add any moisture unless you are taking a lot of long, hot showers or boiling a lot of water. I can't see why an HRV would exhaust moist air more than a dryer would. Have you tried the infrared camera when there is no active source of moisture?

    Could the problem be exacerbated by the fact that it's on the shady side of the house, under an overhang and it doesn't get a chance to dry in the sun?

  3. Jim Tyler | | #3

    If it were me, I would think about taking a half step and switching to a louvered style vent cover. Maybe this would allow the exhaust to travel out further from the house before it rises? It would be a cheap test at least before redirecting ducts.

  4. Charlie Sullivan | | #4


    The HRV doesn't add moisture, but it increases humidity by bringing the air temperature down close to or below the dew point of that air. Winter operation often involves condensation inside the HRV that can drain out. When that happens, you are reducing the absolute humidity of the exhaust air relative to the inside air, but you are then exhausting it at close to 100% relative humidity. When it cools further upon mixing with outside air, fog and dew result.

    It's no more than the moisture in dryer exhaust, but it's running 24/7. And the relative humidity in the dryer exhaust could be lower even if the absolute humidity is higher.


    It's not obvious what to do about it. As Stephen suggests, locating it on a sunny side could help. Bringing it up through the roof would work really well, as the hot exhaust would rise away from the building, and the roof materials could tolerate it better than the siding does...but we all love to hate roof penetrations.

    If you were to run a duct from the vent along the outside side of the house to some other location, much of the condensation would happen inside the duct (assuming it's uninsulated) so you'd want to be sure there's a provision for that to drain, and to use a rustproof material for the duct. But that might overall be a good thing, as you'd remove more moisture from the air before releasing it to waft against the siding.

    In brainstorming mode, expecting that others will point out why this is a bad idea, I'd be tempted to suggest runing a duct away from the house, perhaps buried six inches so that nobody trips on it, and then come up from underground to exhaust 6-12 feet away from the house.

    An ERV would keep more of the moisture inside the house, for better or worse.

  5. Bob Irving | | #5

    Good ideas; thanks. There are no sunny sides to this home, and I do not want to run it out the roof. But we have thought of running the duct out the end wall under an attached screen porch, running it the length of the porch (18') and terminating there.

  6. GBA Editor
    Martin Holladay | | #6

    You've gotten some advice already. I would be inclined to experiment with a different termination fitting -- one that blows straight out away from the building -- as Jim suggests.

    My guess is that your indoor humidity level is high. (I know that the best way to address high indoor humidity is to run an HRV, so this is somewhat of a circular observation.) Nevertheless, I wonder whether your home has any unusual indoor sources of moisture.

  7. Charlie Sullivan | | #7

    Here's a nozzle designed to form the airflow into a jet for long "throw"... 26 ft. (!) for 120 CFM in the 6" size, for example. I'm not sure how they define throw, but it would surely offer some improvement. Perhaps you can aim it at a garden and extend your growing season.

    It's not designed for outdoor use so you'd be on your own for figuring out how to prevent rain from getting in.

  8. GBA Editor
    Martin Holladay | | #8

    If you buy the stainless steel version and build a little cap with flashing to protect the top from the rain, that just might work. (Unless, of course, you require a backdraft damper. You would also need some type of insect screen to keep out the critters.)

  9. Hobbit _ | | #9

    You're exhausting the air straight down where it can roil
    around near the ground and stay right next to the wall,
    and eventually rise against it. Even getting the outlet
    six inches away and traveling away from the house would
    probably fix the problem completely. Here's how mine
    turned out; it's 6-inch PVC capped with a cheap rooftop
    exhaust cap modified for a larger opening and better

    Sort of surprising that the rainscreen didn't help...


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