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Community and Q&A

Air tight office humidity

jeuteneier | Posted in General Questions on

I built a tiny 7×9 office and while I haven’t done a blower door test I was extremely meticulous in air sealing. I live in Portland, OR and even though the temps are not really high (upper 60s) it’s quite humid inside. With windows open it’s better but I”m wondering…

Do I need a dehumidifier, A/C, both? Admittedly this part of building science is not my strong suit.

I’m sure I haven’t provided all the details necessary so wondering what would be helpful to know?


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  1. Expert Member
    DCcontrarian | | #1

    You will need ventilation air when it's occupied. I know it's counter-intuitive to build as tight as possible and then add ventilation, but doing it that way allows you to control the ventilation.

  2. jeuteneier | | #2

    Makes sense. Best practice to achieve that in very small space? Passive or active ventilation?

    Also, when I walk in after a day of not being used and it can feel quite humid.

  3. Expert Member
    Michael Maines | | #3

    One of building science guru Joe Lstiburek's sayings is, "build tight, ventilate right." This is a good place to start:

  4. walta100 | | #4

    I don’t think a dehumidifier is the answer to your problem in the summer time. A dehumidifier is a super-efficient heater with a COP well over 1.

    Do you require AC in the summer or would you be comfortable with open windows?

    In the winter if you crack open a window and run a heater that will likely vent enough moisture to keep the humidity under 50%

    Given you mild winters it seems very unlikely that an HRV/ERV would ever save enough energy to recover its costs. My guess is if you are occupying the space 6 hours at a time with a computer, printer and some lighting you will need open the windows or run the AC 10 months of the year.

    Consider a small heat pump like this 12K DIY miny split.


  5. jeuteneier | | #5

    Thanks for the link Michael. That article may not apply to my very tiny 7 x 9 space. An HRV is overkill for my situation. I just use it for office hours as Walta suggested.

    Walta I don't *need* a/c but obviously it's a nice to have. I'm perfectly happy to manage windows and fans and my windows happen to be double hung so leaving them cracked may not be a problem, though I do live in Portland so theft is a concern.

    I think I'll look for a small a/c unit and manage windows as a start.

    1. Expert Member
      Michael Maines | | #6

      Your call. Your dew points are rarely over 55 so ventilating, whether by fan or ERV, should be enough, without needing AC. I often use a Fantech EGO HRV for bathrooms about the size of your office. You may be less happy about fans and open windows in the shoulder seasons when the dew points are similar but the outdoor temperature is lower than it is now.

      1. jeuteneier | | #7

        I want to make sure I got this right.
        1. A small portable AC helps reduce moisture but doesn't really provide ventilation but does provide temp control.
        2. Windows open with fans provides ventilation so reduces humidity but little temp control.
        3. HRV provides ventilation and helps control humidity and if i'm using either AC or heater it will retain some of that temp.

        On the last one would a bath fan be acceptable (losing the retained temps) and if so I'll need an intake (either permanent or cracked window.

        I've got a small Envi heater for shoulder seasons.

        1. Expert Member
          Michael Maines | | #8

          1. Yes, though in such a small space, an AC will cool the space before much dehumidification occurs. But it will help reduce humidity to some degree.
          2. Yes.
          3. Ventilation yes, but an HRV only helps reduce humidity if the outside air has a lower dewpoint than the interior. I.e., the outside air is holding less moisture than the interior air. A bath fan would do the same thing, just less efficiently because it's not providing heat recovery. An ERV does more to preserve the dry air created by an air conditioner or dehumidifier but I don't know of any that are small enough for your space.

          A bath fan is a few hundred dollars, while a Lunos is somewhere around $1500, so if you're mainly running it in mild weather, a bath fan would probably be just fine. You'll have a few days a year that are more humid than ideal but it shouldn't be a lot of days.

          1. jeuteneier | | #10

            Very helpful Michael. If I went the bath fan route does that run full time? I have a humidity sensing fan in my bathroom but not sure if that's gimmicky. What is best practice for intake?

  6. Expert Member


    I built a 12 ft x 10 ft backyard office for a local house designer this spring here on Vancouver Island. Cooling isn't an issue as it's under a large tree and we get ocean breezes most afternoons. The heating load - especially when occupied - is so low an oil filled portable electric unit suffices. Ventilation comes from two operable windows on opposite walls.

    With such a small building I'd try to more diligently use the simple passive systems (windows) and see how they do before adding mechanical ventilation or de-humidification.

    1. jeuteneier | | #11

      Thanks Malcolm. Similar to my setup with opposing windows for that purpose. This being my first summer with it I intend to do the passive version with fans, then add AC if needed, then go mechanical if still needed which Michael is helping me to understand.

      If your climate was hotter how would you handle that in peak heat?

      Nice office btw!

      1. Expert Member
        MALCOLM TAYLOR | | #14


        I'm sorry - I have no good suggestions. These tiny buildings offer a big challenge in that all the good solutions are made for larger spaces and seem excessive in cost and complexity for such a small area.

  7. walta100 | | #12

    The way I see it.
    1 Humans have well-developed senses, if you need ventilation in your tiny space as seems likely, you will know it and open a window.
    2 HRV transfer almost zero energy when the difference between the indoor temp and outdoor temp is less than 15°. Given your mild climate that is most of the year.
    3 If you feel hot and or muggy in the summer time then you need an air conditioner 12k BTU or so.
    4 A heat pump will use a quarter of the energy consumed by the Envi heater while providing cooling.


    1. jeuteneier | | #15

      Thanks Walta. Appreciate the to the point info.

    2. Deleted | | #17


  8. Expert Member
    Akos | | #13

    There is a Midea 8k window mount inverter heat pump (MAW08HV1CWT). In your mild climate this will easily heat and cool the place plus these are WAY quieter than your normal window shakers. The 12k is pretty much the same unit and bit more readily available.

    In terms of ventilation, if you want something better then an open window or bath fan, you can look at a Panasonic WhisperComfort ERV or one of the other single room ERV/HRVs. I've used the Panasonic, pretty simple install if you combine it with their tandem vent hood.

    1. jeuteneier | | #16

      I'll look into those. Thanks!

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