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ERV Ventilation System for Hot-Humid Climate

quantumgirl | Posted in General Questions on

My old 3500 sf house is slowly getting renovated and retrofitted with spray foam to make it air tight. I need a complete new AC system and the hvac contractor talked me into a ductless mini split system since I have limited space for ductwork (the remodel will take away all attic and crawl spaces and turn them into usable living space). I’m on board with that.
In addition, I was originally interested in installing an ERV for ventilation. The contractor says that’s a bad idea since I’m in Houston (very hot and humid). He thinks a 95 pint dehumidifier with mechanical damper and fresh air intake is a better option.

Does that sound correct? Will that give me enough air exchange to keep the house from depressurizing? Would other ways of ventilation, like bathroom exhaust fans for instance, still be necessary with that set up?

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Replies

  1. Charlie Sullivan | | #1

    And ERV doesn't give you as much dehumidification of the incoming air as an active dehumidifier does, but what it does give you, it gives you for free. So I would argue that it's more important in a humid climate than in a more moderate climate. You might also need a dehumidifier, but it won't need to run as much, especially given that you can run the minisplits in a dehumidification mode.

    If you aren't already doing so, please consider specifying HFO-blown foam instead of HFC blown foam to avoid the major climate impact of HFC gases (>1000X the effect of CO2).

    1. quantumgirl | | #4

      So an ERV with an additional stand alone dehumidifier would be a better choice then? He argues that the dehumidifier with fresh air intake and correctly sized bathroom exhaust fans will give me the same benefits for a fraction of the installation costs.

      1. Jon R | | #6

        This is a guess and deserves a proper analysis, but to give you an idea: an ERV might drop the cost of running the dehumidifier by $150/year.

        IMO, you should also have bathroom exhaust fans.

        1. Charlie Sullivan | | #11

          The savings through the use of ERV isn't just in dehumidification--it's also in heating and cooling.

  2. Steve Knapp CZ 3A Georgia | | #2

    Do you mean open cell foam? If so, managing indoor humidity becomes even more important in your climate. Read this article for more on why: https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing.

    1. quantumgirl | | #3

      Yes open cell. I have 3” of polyiso boards on the outside of my roof. I figured having another vapor barrier on the inside of the roof would be bad in case there’s ever a leak?

  3. Steve Knapp CZ 3A Georgia | | #5

    With that much rigid foam on the exterior, I would consider high-density fiberglass batt. You have about R-17 (maybe even R-24) now and could probably get another R-20 on the cheap. I'm assuming 2x6 rafters, but yours might be deeper. Just a thought...

    See here for more info: https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation.

    On the ERV vs. dehumidifier costs, dedicated ventilating dehumidifiers aren't cheap. And something like the Panasonic Intelli-Balance ERV isn't super expensive. The systems are probably pretty close cost-wise. Your HVAC person may be thinking of the Zender system, which can get pricey.

  4. Walter Ahlgrim | | #7

    First I am skeptical that a renovated home can be made tight enough to truly need a ventilation system. I say forget a ventilator unless you have a blower door test under 0.6 ACH50 seems very unlikely.

    If you lived in the great white north in a tight home with less than 500 sqf per resident I would be on board for a ventilator. With a 90° difference between inside and outside in a crowded house they need to get rid of moisture by bringing in dry air. You do not have any dry air to bring in.

    The way I see it your climate has a very different set of challenges than a northern house and the solutions they need are not your going to work for you. If you home gets a little stuffy crack a window the temp is not all that different.

    Do take the time to watch the YouTube videos by Matt Risinger his homes are in your climate and he seems to favor central dehumidification because so much of the time you need little heating or cooling if only the humidity were reasonable.

    Walta

    1. Eric Habegger | | #8

      I disagree. Standard thinking is that anything under 3-5 ACH 50 deserves an ERV/HRV. That is easy to get to even in a renovation. CO2 will quickly go to undesirable levels without one. I have personal experience with that after I renovated my house and got to a 2.25 ACH 50. Before an ERV was installed CO2 rose to undesirable levels.

  5. Steve Knapp CZ 3A Georgia | | #9

    Eric brings up a good point, Quantumgirl. Have you had a blower door test done? How tight is the house current, and have you been tracking your indoor humidity levels? So far, I think we have been offering advice without really know if there is a problem.

    1. quantumgirl | | #10

      Haven’t had a blower door test for the simple reason that the whole house is not yet insulated. I’ve got the roof and 2 major rooms/ add ons. I’m kinda going room by room with the renovation. With 3 kids and doing most of everything diy it’s just not feasible to rip open the entire house at once.
      The only reason I even want to tackle the whole hvac issue now is because my current unit is fixing to give out. Would I be better off installing the new mini split AC system (based on calculations of how tight I think the house will be one day), then tackling the ventilation/ dehumidification issue after everything is build and insulated and I’ve had a blower door test to see where I actually stand?

  6. Steve Knapp CZ 3A Georgia | | #12

    You could hire an engineer to perform an aggressive Manual J. That what GBA generally advises. Read this article for more information: https://www.greenbuildingadvisor.com/article/who-can-perform-my-load-calculations

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