GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

High humidity in new construction – Mitsubishi Multi System playing a role?

benthrush | Posted in Mechanicals on

Hi Everyone – Longtime lurker and looking forward to being a member of GBA. Thank you in advance for advice.

Question/concern I have is high humidity in new home I’ve built for my family. Highest levels in upper level bedrooms. Project location is Western Maryland (Zone 5A I believe). Here are a few details about construction that might help:

Main Level – 1,223 sq.ft. – ( Dense packed cellulose)
Upper Level – 586 sq.ft – (Dense packed cellulose)
Basement – 1,179 sq.ft – (2 inch EPS Foam followed by 2×4 wall with unfaced fiberglass)
Attic – blown cellulose – R44
While not perfect I paid attention to proper air sealing, EPS foam in all rim joist areas with perimeter sealed with spray foam, caulked plates etc… No blower door test yet but overall I feel the house is very tight. Exterior wall profile is Cedar Lap, vertical felt strips for small air gap, tyvek, OSB, 2×6 stud wall with celllose, drywall, paint.

HVAC – Mitsubishi Multi System – H2i Outdoor Unit (MXZ-5C42NAHZ, 42,000 BTU 3.5 ton), Indoor Unit is a MVX-A36AA4 (3ton) and finally 2 Mini Splits (MSZ-FE09NA, 9,000 BTU). The Basement & Main level are served by the forced air system and the Upper Level consist of 2 small bedroom each with a mini split. Keep in mind bedrooms are only 150 sq.ft. and 200 sq.ft. with remainder of upstairs space being a small bathroom and hall/sitting nook. I do have a 6 inch fresh air duct tied into return with honeywell damper that is open about 25% and trying to consistently run upper level bath fan to help exchange air. I have a feeling this set up needs improved. ERV, HRV, more sophisticated bath fans??

My goal is to figure out if I’m simply over reacting to high humidity (65%-70%) and need to let the house go through a winter/heating cycle due to all of the moisture associated with new construction. I currently have 2 off the shelf dehumidifiers that keep things in the 50% range but not thrilled about having to run them all the time. Also thinking that the Minisplits may be oversized and simply overcooling the space vs conditioning the space, I installed access panels below each unit to check condensate drains and they seem to produce very little condensate which I thought was weird.

Anyway, I’ll stop rambling and looking forward to any suggestions…

All the best,

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. charlie_sullivan | | #1

    This has been a hot, humid summer and we are right in the middle of the worst of it. So it would be premature to worry too much, because dealing with the outside moisture load plus the construction moisture is a lot. If you have to run dehumidifiers some until the weather improves that's not surprising.

    Beyond that point, in a tight house, you will need ventilation in the winter to keep moisture down, but you need ventilation anyway. Any type of ventilation will work to control humidity, although HRV or ERV will allow you to lose less heat in the process.

    In the shoulder seasons, your air intake in your forced air system won't supply much air, so you'll be relying on the bathroom fan(s), and you may need to keep them on a lot to keep humidity down.

    In the summer, the more ventilation you have, the more humidity you bring in, regardless of what type of ventilation you use. So right now, to help control humidity, you would want to minimize the use of the bathroom fans and keep that damper closed more. You only want ventilation to make sure you have enough fresh air. ERV would bring in less moisture per CFM of air, but it still brings in moisture.

    Then we get to the question of whether your A/C is oversized. Does it run constantly, or cycle on and off? If it cycles on and off, that indicates that it's oversized.

    As a house becomes better insulated, the need for cooling goes down, but the need for dehumidification does not. So oversized A/C that isn't particularly good at dehumidification works out OK in a poorly insulated house. But once it's well insulated, the dehumidification becomes a bigger portion of the job of the A/C, and you can't tolerate oversizing as easily.

  2. GBA Editor
    Martin Holladay | | #2

    Charlie provided you a very complete and technically accurate answer. His advice is good.

    (Thanks, Charlie!)

  3. benthrush | | #3

    Charlie & Matin,

    Thank you for taking the time to respond. I've closed off the fresh air damper to see impact on humidity. I'll also continue to run dehumidifiers, they definitely help but also create a lot of heat/noise. Are there any stand alone units that you think are the best? Is a whole house dehumidifier worth considering at some point?

    One thing I've noticed with the Mitsubishi System is that it runs all the time, I believe this is by design as the whole system has modulating variable speed blowers along with the outdoor unit. Do you think the modulating abilities of the mini splits/forced air system provides a larger margin of error when it comes to sizing?

    Also with the fresh air duct closed more should I be concerned with IAQ or am I overestimating the amount of fresh air the home needs. I have 2 young kids so always overanalyzing things like this!

    Also what do you think the next step should be in regards to ventilation, if any? I originally skipped the ERV/HRV concept as I was trying to keep things simple. I was thinking a Panasonic Whisper Fan as an easy upgrade to the loud 70cfm Broan i have...

    Oh and one more thought, I've always been a bit worried about cellulose in walls still being damp, I waited about 10 days before drywalling as I was uncomfortable going with the recommended 2 day waiting period. That being said I think it was still damp towards the outside of wall. Could this be contributing to situation. Will it really ever dry out, science tells me it will but my nightmares are of never ending damp walls and mold ;).

    Thanks so much!

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Are there any stand-alone dehumidifiers that you think are the best?"

    A. I don't have a favorite brand. I would choose an Energy Star model if I were you.

    Q. "Is a whole house dehumidifier worth considering at some point?"

    A. I doubt it. Wait at least a year -- until you are sure that the construction moisture has dried out -- before making that decision.

    Q. "Do you think the modulating abilities of the minisplits/forced air system provides a larger margin of error when it comes to sizing?"

    A. Compared to what? Certainly, when compared to single-speed equipment, an inverter-based modulating minisplit will provide a larger margin of error when it comes to sizing.

    Q. "With the fresh air duct closed more, should I be concerned with IAQ or am I overestimating the amount of fresh air the home needs?"

    A. You can usually use common sense to determine your family's ventilation needs. If your house is smelly or it feels stuffy, increase the ventilation rate -- but don't overdo it.

    Q. "What do you think the next step should be in regards to ventilation, if any? I originally skipped the ERV/HRV concept as I was trying to keep things simple. I was thinking a Panasonic Whisper Fan as an easy upgrade to the loud 70 cfm Broan I have."

    A. The answer depends on your budget and expectations. Certainly an HRV will provide better (more uniform) ventilation, at a lower operating cost, than exhaust-only ventilation. The question is: Can you afford the installation cost? For more information, see Designing a Good Ventilation System.

    Q. "I've always been a bit worried about cellulose in walls still being damp."

    A. Absent any evidence of a problem, you should probably stop worrying. If you really want to know what's going on, cut an inspection hole in the drywall, scoop out some cellulose, and use your hand to feel the humidity level. Of course, that approach requires repair and paint touch up.

  5. benthrush | | #5

    Thanks Martin - I did cut inspection panels in a few closets this past weekend! I would say the insulation felt very dry in one and perhaps a touch damp in the other. I assume this does indeed have the ability to dry in closed walls as the Nu-wool literature suggest? Just for fun I taped some clear plastic over the inspection hole to see if it would gather any moisture. Not sure on the validity of this test but nothing happened. Again thank you for helping me brainstorm on this,

  6. charlie_sullivan | | #6

    I would interpret your exploratory probe of the insulation dampness as an indication that 1, it's drying out just fine, like it should, and 2, that has been a factor contributing to the humidity level in the house, so you can expect less difficulty maintaining low humidity next summer.

    Expanding on Martin's replies in comment 4, short of large circa $1k dehumidifiers, most energy star dehumidifiers are designed to just barely meet the energy star threshold, so they all have the same efficiency. The only concern I'd have in selecting one is that some have a fan that runs continuously even when the humidistat is satisfied. That can use a lot of unnecessary electricity if it is in that mode for a lot of the time.

    It sounds like your HVAC sizing is working out fine.

    If you decide to get a Panasonic Whisper fan, i recommend the "Whisper Green" model, because it has fabulously low electricity consumption, and isn't much more expensive than the other models.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |